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DROUGHT (WATER DEFICITS)! · Cereal Killers

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Hi there, welcome back to Serial Killers, the podcast all about the pests.
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Diseases, weeds and challenges of arable crops.
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This episode is all about water deficits.
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I know I've titled the podcast as drought, however
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drought is very much a colloquial word and
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in science we tend to focus on water deficits because
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drought has connotations of being hot and
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dry and of there being like famine
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and things like this whereas water deficits the specific
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stress we're dealing with we're not dealing with heat stress here because
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you can have water deficits during the winter and so
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that's what we're going to focus on so a lot of my academic research
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back when i was a university phd student
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and a postdoctoral scientist was on water deficits however
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it was on horticultural crops because you could
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manipulate the plants with the
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use of irrigation and different irrigation techniques so you
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could actually produce a far better crop if you
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didn't give them all the water you needed and in
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fact a horticultural nursery will apply or certainly
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did at the time and i don't think things have changed much they
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would apply 400 times not 400
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times 400 percent of the water that the their nursery needed just to give the
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plants all the water they evaporate and transpire so you take the baseline 100%
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and that would be them transpiring all the water they needed and even then that's
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not optimum and then they put four times the amount of that so three.
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Three lots of that water were just going straight into the drain
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and was wasted now in the uk that's sometimes
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that's fine because there's plenty of water around and they've got
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reservoirs and captures and stuff but they were using tap water they
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would have been screwed and certainly would have been in the latest decades
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where they're paying a lot more for their their water So I focused on deficit
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techniques like regulated deficit irrigation and partial root zone drying and
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other techniques that tricked the plant into thinking it was stressed.
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And this produced ornamental plants with a lot more flowers per unit area and were more desirable.
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It was an alternative to use of growth regulators and also produce fruits with
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more phytochemicals, health-giving phytochemicals.
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However, in arable crops and field vegetables, manipulation of irrigation is
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not usually possible, not least because in the UK we tend to not irrigate many
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crops apart from potatoes.
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Any of this I can think of? Not many. And so
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water deficits are normally seen as deleterious and that's because you are going
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to have an impact on yield eventual yield so and biomass production so when
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we're talking about water deficits on this podcast we will be talking about.
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Negative aspects if you've found this podcast from receiving some liquid gypsum
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in the post from us or some clay
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drainer product and you're a gardener you might want to flip over to the,
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hydroponics daily podcasts that i do because a lot more of the topics will be
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focused on and more applicable in your garden than this one this one it all
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the knowledge i'm going to spit out now is going to be on for arable crops and
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field crops it's going to be a bit of a i'm going,
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to follow a logical path because I've done no research.
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I'm just going to check on my notes and see what interesting things I can talk
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about when it comes to soil water deficits.
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I think I'm doing this one now because we've had a period of prolonged dryness
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and dry weather, which a lot of people are happy about.
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And we've had rain, it seems, for years and years, probably about 18,
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two years of just continuous rains, it seemed.
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And then we've had this prolonged period of drought. and then within a couple
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of weeks we had a lot of the high profile farmers shall we say on the BBC news
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saying oh it's terrible there's a drought there's a drought and I was thinking
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and then you saw the pictures of their farms and there was a lot of grey soils a lot of very white.
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Yellowy soils and you're thinking well yes you
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are going to soon as any period of dry weather you're going
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to be in trouble because you've got no organic matter in your soil and
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I think the regenerative farms and those that are doing
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no-till or min-till will actually be the ones
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that are succeeding in this period of weather and i think it
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will be a boom year for regenerative farming because
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and anywhere where they're incorporating organic matter and crucially
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protecting the organic matter that's in the soil because they're
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just good it's just going to hold on to water better
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it's not going to crack it's not going to become hydrophobic and
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so they will have got a week
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two weeks three weeks four weeks more out of
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their soil than the conventional farmers who
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especially in high ph where the organic matter
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is being burnt off by the high ph and where
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they've plowed it and it's evaporated and it's holding less water it's drained
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faster and things like that so i think it's going to be a boom year for regenerative
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farming and you can thank this period long period of dry weather i did there
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was flex fertilizer based in denmark They sent out an email this morning saying,
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dry weather is the perfect time for foliar fertilizers.
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I'm not sure if that's true, because if the plant's not growing,
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then it's not going to be needing a lot of extra nutrition.
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But, and also, if we think about it as a drought and as hot weather,
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then obviously you need to be careful of scorch.
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Anywhere where the leaf temperature can rise you always need to be careful of
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scorch so i always say with like things like our liquid gypsum product never apply them on hot sunny,
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days because the leaf temperature will be high and
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then if you consider that there's a on if
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it's a hot sunny day now because there's less water in the
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soil there'll be less evaporation the stomata will be closed
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and so if you've got less evaporation you've got
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a higher leaf temperature anyway so the danger of scorch increases so
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i know flex fossilizers have got some interesting products and so maybe they've
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got products that will reduce scorch but anything with calcium or sulfur in
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you need to be careful and and there are plenty of other select nitrogen you
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need to be careful of scorch as well you can set your crop back significantly if you if you scorch it,
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unfortunately calcium is the one nutrient that is
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severely limited underwater deficits so if you've ever grown tomatoes you'll
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notice you'll get blossom end rot when you let the soil dry out too much and
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you can water them again but if you've a couple of periods of very dry soil
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and you get blossom end rot and it's a bit like frostbite and it affects anywhere where the.
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Newly grown tissues are growing so in lettuce you get blackened to the heart celery the same,
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and so the new developing meristematic tissue is
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severely affected now in cereals you might not get blackened tissue because
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grasses tend to grow in a different way and require a bit less calcium as well
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but you'll still get if you're going to have an issue under water deficits i
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would have thought the first nutrient to be affected will probably be calcium
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and you'll get calcium related issues where the physiology is affected,
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And calcium is needed for good stomatal aperture and closure as well.
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So it's a sort of feedback, positive feedback, which you don't want.
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When it comes to measuring soil water deficits or drought, it's actually quite difficult.
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And you know when a plant's under heat stress because we're under a period of,
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you know, the weatherman will tell you it's going to be, or the weather lady,
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sorry, will tell you it's going to be like 32 degrees Celsius.
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And you know when the light levels are going to be high and things like this.
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But knowing your soil is dry is difficult because you don't know where your
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roots are in the root in the soil profile and soil might become lighter when
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it's drier but it might not and unlike a plant pot you can't just pick up your
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arable crop and weigh how how much it is,
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how how much it feels like so it's difficult to to determine and just putting
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a sensor in there and saying this is a water deficit is is quite difficult it
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needs a lot of you need the knowledge about what's right in your soil.
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And even times, some soils can hold onto lots of water but not deliver it to the plant.
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And so you might think there's water there when there's not actually enough
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for the plant to actually take it out.
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You can get electrical sensors, they're called dielectric sensors,
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that measure the electrical conduction through the soil and can give you a measurement there.
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They're accurate, but they only measure one type of soil deficit.
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You can get also get a potentiometer which is like a clay porous tube that's
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filled with distilled water and that tells you sort of the pressure pull from
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the soil and they're good for measuring how much you know the ability,
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almost like a plant root so they're good at the sort of sensing
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the actual stress from the plant root however until recently
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it's been difficult to to hook those up to an
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electrical sort of readout that you could put to like feed
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into a computer either remotely or through bluetooth and things like
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that so they are getting better but they also require a lot of maintenance because
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there's obviously that distilled water that gets evaporated and pulled out so
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potentiometers and electrical sensors can give you some sort of gauge on on
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how stressful your situation is and on some farms they They bury them and know
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where they are and monitor them regularly just as they would the weather.
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You could measure the stomatal conductance. That's a measure of how much water
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is flowing out of the leaves per unit area.
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It's measured in micromoles per metre squared per second.
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And to measure stomatal conductance you need a piece of kit called a perometer.
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And perometers are not cheap. They used to sell for about £2,000 when I was a scientist.
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And that was for the cheap models. So you'd really want to, you know,
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have a justifiable case for buying a parometer.
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It's not sort of a kit that you just play around with.
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Certainly, if you are partnering with a university for any trial,
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then a parometer would be a good thing to see if they had.
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And you could monitor the stress of your crops. There is just a new funding
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call for on-farm trials, isn't there? It's just been released today or this week.
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What's it called? Is it called Adopt? Yeah, full Adopt grants.
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They've got £2 million. Yeah.
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Initially and then 20 million pounds for farming growing
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forestry businesses based in england that can
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adopt that can apply to for a share of up to two million
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pounds for on-farm trial and demonstration projects to
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improve adoption of new ideas or solutions
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in the agricultural sector so if you've got an idea for a project you want to
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try some new technology then you could apply for the new government adopt program
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and get some of that money back from the governments and they've taken off you
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recently so that's interesting let's have a look at that that just came through
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from the boffin group you've heard of them they've been at the trade shows recently,
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so where was i parometers yeah they're
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very useful and they they tell you how stressed your crop is
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obviously you need to know how much the stomatal conductance
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is when it's healthy and plenty of water but you can you could probably water
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some of your crop and then on a small area and see the difference between the
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watered and the unwatered see how stressed it is so that's something you can
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get sap flow gauges but they're more for tree crops they're routine in large.
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Orchards you know commercial orchards however probably the best way now is to
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use leaf temperature so back when
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i was a scientist that was an academic scientist anyway back in around.
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2008 they we were looking
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at um thermal imaging cameras and that
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was state-of-the-art then we had a thermal imaging camera that you
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couldn't actually take out of the country because it was considered military
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technology so we couldn't take it to iran and places like that
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got a lot of listeners in iran actually shout out to iran but
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now thermal imaging cameras are not 10 a penny but they're cheap you can get
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mobile phones that have got thermal imaging cameras on them and so you can you
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can water a few plants at the soil base in your crop and then you could probably
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pick out the difference between the watered and the unwatered.
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Probably take a five litre watering can into the middle of your field,
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water some plants, come back a few hours later and take a thermal image of the plants.
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Obviously don't wet the leaves because that's going to show up as cool but you
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could do a lot of messing around with a thermal imaging camera to see how stressed your plant is.
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So yeah that'd be interesting these are these are you talking hundreds
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of pounds rather than thousands of pounds for these now this
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is not just a small difference if you take a plant like gourds or pumpkins between
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watered a well-watered and a stressed plant the difference in leaf temperature
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can be up to seven degrees celsius and that's massive that is the difference
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between that's also going to
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lead to heat stress isn't it you know if you think a plant being suddenly.
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27 degrees celsius going from a nice 25
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degrees celsius when it would be healthy and happy to
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32 when it would be severely stressed possibly losing dropping
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flowers because of that and a broad leaf tree sorry
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its leaf temperature can reach up to 50 degrees
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celsius if it's stressed which is just crazy there
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are other ways to measure leaf temperature like thermocouples but
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thermal imaging cameras are now pretty standard for
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that and you can also you can
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also play around with them you know lords and ladies or arum lily
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that actually heats up and the flower heats
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up when it when it's releasing its pungent chemicals so
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you could look at that as well there are a number of different chemicals that
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do that so that's something you can look at there would there was a lot of research
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especially in in israel on leaf thickness and little sensors you could put on
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the leaves to measure the leaf thickness and that was a a good measure of how
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stressed the plant was due to water deficits.
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However, I've not seen any reference to them in many and many a year,
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so I don't think they've been commercialised much.
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A lot of cereal plants will give physical symptoms that they're water stressed,
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so obviously slower growth, chlorosis, but also curling up of the leaves to protect the stomata.
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That's another option in certain cereals or certain grasses, not all of them.
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Other things scientists use to measure, soil water deficits,
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evaporation pans, artificial leaves so they have like a.
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One thermometer that's wet and one thermometer that's dry and
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then measure the difference to see how much the evaporative demand
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is in the field that day you can literally
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measure the water content of the soil by taking it out
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and then weighing it and then putting it in the oven drying it
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out completely and that tells you how much moisture physically how
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much moisture there is in there so that's that's
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potential and also people have been looking at hormone biosensors
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could you use an electric chip and get some leaf juice
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out i don't like to say leaf sap i don't
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like the word the word sap analysis because it's not sap it's juice
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and so you could potentially pick some sap out and then measure it for abscisic
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acid straight away or abscisic acid is the main stress hormone you could measure
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so these are not available currently you normally when you're measuring abscisic
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acid or aba as it's better known you actually you actually take it to a lab to measure it.
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So ABA is the hormone that travels from the roots to the shoots and closes the stomata.
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And it prevents massive water loss due to just the plant dehydrating.
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If the plant just dehydrates and then the stomata closes, then it's a worst case scenario.
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And then ABA also has a role in maintaining root growth.
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So it stops shoot growth, it closes the stomata, but it's actually required to.
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Allow the roots to keep growing and that's important so that you have a good
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root system so it can access as much water as possible so it has a double edged sword so on roots it.
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Allows growth and shoots it stops growth another one
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for trees is you can actually detect the number of embolisms
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that are occurring in the tree as in its droughted conditions
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and you can get a stethoscope and then
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put it on the tree and then hear the embolisms and
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then more frequent under dry conditions than they
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are under well water conditions
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and that was one of the very first things i did when i was working at nest gardens
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part of university of liverpool was was was measuring
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these tree crops you can also sort of do
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this thing where you you do like a quick heat treatment
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and then that measures the sap flow as well and then
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you have a little thermometer above it and that measures how quickly the water
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is transporting there's lots of different things you can do in trees that you
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can't do on field crops and it's actually thought that water deficits are is
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the number one stress in crops worldwide and it causes more crop losses than
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all the abiotic stresses combined.
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And that's because plants are made of water and 95% of a herbaceous plant.
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However, even though it's the number one abiotic stress, there is still very
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little plant breeding that occurs to produce plants that are extremely water
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deficit resistant or drought resistant.
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That's because it's a lot easier to breed for pest and disease resistance or
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tolerance because their pest and diseases are going to be there every year and
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unlike water deficits where it might be you might get flooding another year
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or low light levels or cold treatment or.
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Some other abiotic stress you know it's very rare that you get consistent a
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stress consistently every year normally there's a difference and so that's why
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there's not much breeding for water deficits except in extreme environments so um like if you were,
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there's certain non-governmental organizations who will breed for sub-Saharan
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African crops and things like this.
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And there they will obviously look for improved drought tolerance or water deficits tolerance.
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They'll look for improved route architecture, a higher route to shoot ratio.
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Cropping before drought stress comes in.
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So in certain countries, the water appears,
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the weather you know the climate is that
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you know you get monsoons and then it dries up like rice you
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know paddy fields that dry up very quickly so having
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a crop that fits into that cropping cycle and things
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like that is is going to be crucial but for
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the uk i'm not aware of anyone who's breeding for drought
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resistance if you know differently please let me know so plants
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need the water for a lot of things you know they need it
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for photosynthesis photosynthesis is you
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know co2 plus water plus light equals carbohydrates
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and oxygen and so they're going to need water there
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but that's not a massive amount of water required
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there they need it for metabolism you know creating reactions but
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again not much they need it just to dissolve nutrients
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and sugars and hormones to get the sap moving around and they
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need it for structure to keep themselves turgid they
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don't have a skeleton and so if the cells
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aren't turgid full of water then they will wilt
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and that's what wilting is it's a lack of water and that is
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the worst case scenario plants don't want to wilt
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and they'll close the stomata to make sure they don't they need it for growth
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you know that that turga that pressure is what drives cells.
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To expand and the the enzymes
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loosen the cell walls and then they expand through the
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increased water tension pressure in the
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plants however again that's not much water they need
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it for thermal regulation and for
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moving things like calcium up there so I
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did say sap yeah so sap and temperature regulation I would say were probably
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the main two reasons plants lose water
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and then well and then this one
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is to absorb co2 so they
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need to absorb co2 they can't just have open
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stomata and dry mesophyll cells
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they have to be wet just like your lungs have to be wet to
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absorb the co2 so co2 absorbs into
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the water just like it would a waterfall and the higher the
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surface area the better so the stomata sort of limits that that loss they can
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close and then they open and so without wet mesophyll cells they would be able
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wouldn't be able to absorb co2 and so they can't just close the stomata they
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can't just have dry cells and so that's why they're losing all this water and
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that's why even the the most.
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Drought tolerant plant always loses water when
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it's growing so and that's why in the
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summer you have to water your cacti even though in the winter you
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don't want to do that unfortunately very dry
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soils are known to be less stressful to drought
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tolerant weeds so in lawns
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that's dandelions white clover and other grasses so
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in in arable crops that will be all
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your grass weeds will be more tolerant to
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the to the water deficits than your
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actual crop itself and so you might get an abundance of things
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like blackgrass rat's tail fescue bromes.
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Wild oats probably say yeah wild oats and
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things like that probably really robust ones but even things
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like power annua and what's the one i can't
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think of it like no i'll carry on unfortunately this
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period of drought stress has been during spring and
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so spring crops will be hit particularly hard and that's
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because seedlings are the stage of
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a plant cycle is most susceptible to a lack of water they're
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made entirely of tender herbaceous cells with no strong
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cell walls they haven't got any silicon in them yet and
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things like this to build up resistance and so
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that's why why it's so
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bad that you've got water deficits in spring and we've had i've
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just had a farmer on the phone whilst i've been recording this podcast asking
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for 200 liters of seaweed extract and just to get this the the crop away for
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his spring crops other things you could put in there are things like phosphites
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and seaweed extracts humic acids just to get a good root system amino acids
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they'll all be good to stimulate good root growth.
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And then in the long term you know increasing the organic matter content of
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the soil that's not something you can change overnight, but that's going to help with, you know,
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making sure the water stays, as much water as possible stays in the soil in
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a healthy way, not in a waterlogged way.
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Then if that wasn't bad enough, even when the rain returns, the plants will still be weak.
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Their food reserves will have drained. And at this time, the plants will be
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more susceptible to attack by pests and disease if they weren't attacked already.
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So mildews, especially powdery mildew, thrives under dry conditions,
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especially if you've got a high planting density.
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So dry soils, high humidity caused by high planting density,
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you're going to be really susceptible to powdery mildew there.
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If the soil was wet and it was high humidity, then you would be looking at downy
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mildew, which is even worse.
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So thank you, lucky stars on that one, really. And the plant will
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take some time to return to its ability to
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use as much water as it was before the drought so
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plants have these water channel chemical water
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channel channels proteins called aquaporins and
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they take a while to to reactivate after a
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drought so you can't just the crop won't just grow straight
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away after the after the rains return in
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potatoes irrigation is a bit more common and that's
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not because of drought the plant's particularly
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prone to drought although it is fairly prone to drought
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it's more about making sure that the tube is
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set correctly and they don't form lots of little tiny potatoes and
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also preventing a scab disease scab thrives in
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dry soils and that's the primary reason that potato
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crops irrigated the good news is that once the
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plants received a period of water deficits it's sort
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of primed to it remembers
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that it sort of chemically remembers it and then far
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more resistant to subsequent periods of droughts if
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you get another period of drought later in the growing season that the crop
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will be more resistant so that's good news as i
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said on my other podcast hydroponics daily there is a lot of interest in growing
24:04
wheat hydroponically now obviously a lot of companies do that for the headlines
24:09
and you know it makes good headlines to say people are growing wheat hydroponically
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i don't think anyone's actively looking at commercializing it but nasa did do some research in the.
24:21
Utah in the 1980s and they bred a
24:25
hydroponic a wheat variety that went
24:28
to yielded very quickly and went
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to harvest very quickly and used far less water so hydroponics use a fraction
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of the water that you need to grow crops but don't think that's coming to arable
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crops anytime soon they are certainly using hydroponics to speed breed arable
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crops so they they grow if you're if the trait you're looking for,
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the feature you're looking in your plant isn't going to be affected by whether
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it's grown in a grow room or in the field, you can actually get four to six life cycles,
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four to six harvests in a year in hydroponics.
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So that speeds up normal breeding where if you're a small breeder,
25:05
you have one chance a year to do a cross and to pick out the progeny that are the ones you want.
25:11
And then if you're a large grower, you normally have two bites of the
25:13
apple because you can then have a research station down in
25:16
the southern hemisphere where you can grow another arable the same
25:19
arable crop for the second generation however in hydroponics you
25:22
can do four to six and obviously hydroponics you
25:26
know you're talking five percent of the water use that
25:29
you would in a in a field at least so that's been
25:32
serial killers for today soil water deficits hopefully
25:35
you've learned something interesting bit light on the practical uses
25:39
really it's just a case of choosing your seed
25:42
wisely hopefully you know and and
25:45
then the actual techniques for mitigating it are
25:48
hopefully you're a regenerative farmer so you're not disturbing the
25:51
soil much mycorrhizae if you've got mycorrhizae there
25:54
in your crop because you because you are min tilling
25:57
or no tilling then they will help access water and give
26:00
it to the plants under drought stress and then
26:03
maybe applying a stress-resistant chemical
26:06
compound bioactive like seaweed extract humic.
26:10
Acids and or amino acids into that i probably go for
26:13
a seaweed extract i would have put
26:16
amino acids on to stimulate the root system early
26:19
on but now you're into the drought stress and you're
26:22
trying to get out of that drought stress especially once it
26:25
starts raining again i think seaweed extract is probably the one to go
26:28
for so yeah thank you very much for listening for
26:30
two serial killers apologies that the episodes have
26:33
not been as often as previous growing
26:37
seasons it's because i'm focusing on
26:40
hydroponics daily that podcast because that's in
26:43
2025 i'm doing an episode every day and that's
26:46
taken up a lot of time i've been up bitten off
26:49
way more than i can chew i don't know why i was listening or i
26:52
decided i would record two podcasts but if you're
26:55
interested in things like hydroponics or you're just
26:58
interested in things like all the different additives i do an episode on a different
27:02
additive every now and again a different crop every now and again definitely
27:06
check out hydroponics daily that's been it for today thank you very much for
27:09
listening click the follow button to be notified next time I release a sporadic
27:14
episode. Thank you. Goodbye.