So, yes, as introduced, I'm here today to talk about what we know about heat stress
in wheat and how that complements what we know from the previous talk, talking about
the frost, and what we'll hear about later in the day from James Hunt relating to time
of sowing.
Very Good. I've got some control now. So I've been charged with running a couple of projects
over the years based at Roseworthy, and primarily that has been a SAGIT funded project. That
the South Australian Grains Industry Trust, which is a voluntary levy group in South Australia
that then independently funds R and D work. And I have also last year finished working
on a smaller GRDC project but the data I'm talking about today is primary out of the
SAGIT funded project. And that is primarily using a controlled environment
assay to phenotype in a detailed way the heat stress tolerance of current varieties, recent
varieties, exotic introductions from overseas, so landraces from ICARDA and whatnot. And
also using mapping populations to develop genetic tools for selection in breeding programs.
Very good. Okay. Moving on.
So this photo here is the fancy contraption that we use to screen in controlled conditions
the heat stress of our wheat lines. And we plot our experiments. We then monitor independently
each plant, and ten days after anthesis we put the plants into the heat chamber for three
consecutive days at 36 degrees with 40 kilometre per hour winds for eight hours each day. So
quite a significant stress. And hopefully it's replicating, at least in South Australia
where we get these cycles of hot north wind days that build up to mid-thirties, even into
the 40 degrees and then crash back down to even low to mid-twenties the following day.
And these plants are also watered to make sure we are not getting too much confounding
heat effects. And then we compare - we take a whole lot of measurements on the plants
and compare them to an unstressed control. So we measure leaf chlorophyll, leaf damage,
so senescence and hanging off. We measure spikelet fertility in a very similarly fashion
to what was talked about in the frost talk previously, and also thousand grain weight
to see what our affects are on grain size.
As we heard in the very kind introduction to this talk, we know about heat stress. We
know it is a significant issue, and there has been a number of studies over the years
that have proven this very point. But our research, in terms of where we are up to and
what we know about it, is still a fair way behind frost and drought. And we're doing
our best to catch up as quickly as we can. And I've included a couple of quotes from
studies that have started to indicate what the levels of damage may be. And the bottom
one there is 190 kilograms per hectare for every one degree rise in temperature during
grain filling.
So before we get into too much detail, heat stress is a very complex beast. It affects
a number of processes within the plant. And they all ultimately reduce or play in to reduce
grain yield. So during the growing season we have some more visual elements such as
leaf senescence and hanging off. But this damage leads to reduced photosynthetic rate,
photosynthetic capacity due to reduced photosynthetic area. In the head, depending on when the stress
occurred, whether it's during flowering or at the end of flowering into the start grain
filling, we can get different levels of sterility. If heat stress occurs during the grain fill,
we end up with a reduced grain fill duration, reducing grain size, all of these factors
compounding to reduce grain yield.
And then to complicate things even further, what about the environment - that hot environment
that is actually causing the damage? Is it heat itself, and there's plenty of evidence
to suggest it is. Is humidity or vapour pressure deficit playing into that to either compound
the effects or reduce the effects? Mechanical damage from wind directly damaging the plants
or changing the microclimate and the humidity around the canopy of the crop to again either
increase or perhaps alleviate some of the stress. And also when the stress occurs is
very, very important. And what we see as the heat stress damage out in the field is a complex
interaction of all of these factors, which makes my life very difficult. And also the
prospects going forward of being able to develop some genetic tools and understand it fully,
it is a complex process and going to be time consuming.
But, we already know that heat stress is an important issue, and if you've been listening
to our climate scientists or Dr Karl yesterday, we're going to be moving into a hotter environment.
So I have made this small tutorial here to demonstrate how this could impact on our temperature
environment. So if we consider this bell curve. To the left our current environment. When
we get below a certain temperature we run into frost potential damage. When we get above
a certain threshold we are going to run into heat stress. And if over time our average
temperatures increase, this bell curve is going to shift more towards the higher temperatures,
which will open up the window for heat stress more frequently. And it would seem that perhaps
we might get less frost events, not that I'm trying to put you out of a job, Ben, because
if we have a hotter environment, chances are rainfall variation will increase, clearer
skies, more frost events. So therefore this happy window in the middle is going to be
less common, which just isn't a very positive outlook for agriculture going forward.
And also to complicate things further, we have at least anecdotal evidence that there
is different types of heat stress. So in South Australia and over here, heat shock is the
big one. We have at least moderate temperatures and then we have a period - a few days, a
day - of really high temperatures that come in and just obliterate everything. But more
common up north is they have a month or so of stable but hot temperatures of in the low
to mid thirties. And at least anecdotally we have evidence that there's genetic differences
in response to these two stresses. My research with the heat chamber is largely focused on
the heat shock, but that's not to downgrade the importance of extended heat stress, particularly
in the northern region.
To start to quantify the effects of heat stress, Dion, one of our breeders based at Northam,
put this data set together at the beginning of the project a few years ago. And it's just
a simple linear regression between average site yield at 600 NVTs all across the southern
region of Australia and some average climatic parameters during flowering and grain fills.
So on the right we have the effects of the climatic parameters. And to give an example,
we have every day over 30 during flowering on average within the range of average site
mean yields within the data set. For every one day over 30 degrees, we had a decline
of 380 kilos per hectare. And over 800 kilos per hectare for every day over 35.
So you can see that very quickly we could start to have some very serious, very dramatic
yield decreases with not very many heat events. But thankfully during flowering, generally
- I say generally because it is not impossible - but at least generally we are going to have
less heat events during flowering. But during grain fill, we still have very significant
effects on yield. There are a lot less than during flowering, but the chances of experiencing
heat stress during grain fill to some extent is very likely. So we are using these climatic
parameters to not just use, say, every day over 30 as defining a heat stress, but to
try and characterise the whole environment to: such as, average daily temperature, average
daily maximum, and how the interaction between these variables may explain the heat stress
environment, and how that may explain interactions with yield.
Moving on to the sort of analysis that we're looking at now. We have screened a whole lot
of varieties through the heat chamber to try and understand how varieties may vary in a
genetic response to heat stress. And so we focused our attention on two key yield determining
parameters of grain size and grain number. So grain size being thousand grain weight,
and grain number is a fertility index. And we can see that there is varietal variation
in response to these two traits. So for example, Yitpi has really good grain number stability,
but grain size really declines. And because we target a specific growth stage, it's irrespective
of maturity, it has nothing to do, well, at least up front it has nothing to do with the
fact that Yitpi is a later variety.
But things like Ellison and Halberd and also Mace to a slightly lesser degree, but Halberd
is very stable for both grain size and grain number, and is about the most heat stress
tolerant that we've screened through our heat chamber. So if you want to grow something
purely heat tolerant Halberd is the go, but it's not going to win you any yield contests.
So we've found variation. We are not - at least at the moment we don't have enough data
and we're looking really looking to develop heat rankings, such as is going on with frost
at the moment. We don't have enough data. But understanding the relationships between
the trends and the responses we see in the heat chamber to what we see in our field experiments
is very important to insure that what we're measuring is not a complete waste of time
and is relevant under field conditions. So I'll glaze over this reasonably quickly, but
the point of it is that a significant amount of the variation - 24 percent of the - sorry.
Fertility and grain size scores from the heat chamber accounted for 24% of the yield variation
across 64 location by year experiments in '08 and '09. So 24% of the yield variation
was accounted for by the fertility and grain size. Very, very significant, given that of
course all of your mainly rainfall is going to be having a very big impact as well.
Something I alluded to before is the effect of growth stage and the extent of damage.
With fertility, the big issue is the effect of heat stress on pollen viability, and this
graph demonstrates that really quite nicely, in that with the red line we have the heat
stressed response in regards to growth stage, and that is compared to the control. And at
the earlier growth stages - so during head emergence and into early to mid-flowering
- a number of grains per spikelet or per head is significantly decreased compared to the
control. But conversely to that, with thousand grain weight, when we have less grain set
due to the effects on pollen viability, we get huge grains. But then later on when we
have all the grain set and we have a heat stress event, we really see a reduction in
grain fill duration and an increase in small grain in screenings.
And to put up some pretty pictures …
… I ran a more targeted experiment to see how the growth stages interacted, and I was
targeting as early its growth stage 45, which is booting, but is associated with pollen
formation roughly. And we can see that when a head was stressed at growth stage 45, there
was about four or five grains in that head. At growth stage 57, not a whole lot better.
And by early flowering there is quite a few more grains in there. So if we did have a
very serious 36 degree day during growth stage 45, that's the sort of thing you would expect
your crops to look like. Thankfully that doesn't happen very often.
And this roughly shows what I had two slides ago. So I will jump over it.
So to sum up those points, heat stress has a far greater potential for impact at earlier
growth stages, but thankfully the chances of getting an insanely hot day during grain
filling is far less than during grain filling where the impacts are lower in terms the impact
on yield, but due to the frequency of exposure, the impact on the economics will be far greater.
So to understand all of this a little bit more and to also set the scene for the thousands
of mapping lines I've been screening through the heat chamber and will be screening in
the field this year, we ran a set of 24 diverse genotypes in 2013 at six locations, and seven
locations in SA and another three locations in WA last year.
And these were selected to have a variation for both the thousand grain weight response
and for fertility response. And this analysis is still ongoing, because I'm still getting
the data in terms of quality in my hands. But for a very simple look at it, I ran essentially
a site mean regression to understand what the impacts of yield or what heat stress was
on yield. And we saw some very significant interactions with yield, and not too dissimilar
to what I showed in the earlier table. And most interestingly, for every one degree in
average maximum temperature during grain filling, we had a nearly 450 kilogram per hectare decline
in yield. But that explained 59% of the yield variation across these experiments. So demonstrating
that heat is incredibly important, and, well, we need to do what we can to understand it
further and improve our varieties.
This is that same result. So this is the two years. We have 2013 and 2014. And to demonstrate
this heat stress, as I was alluding to before, is a very complex beast, and it's not necessarily
just heat. There's also potentially acclimation going on within the field, and the slopes
in response to average maximum daily temperature during grain filling are very different. On
average, through the middle that explained 59%. But from 2013 to 2014, both significant
effects on yield but very different responses. So going forward these interactions are what
we are trying to understand a little bit more.
So to sum up, I don't have a magic bullet. I'm not likely to in the near future, or even
the more distant future perhaps. But we do see variation within our varieties, and generally
the varieties that we have coming out of the southern region - so Roseworthy and even here
at Northam - generally they have a pretty decent base level of heat stress tolerance.
Nothing is rubbish. But we're trying to understand the genetics so we can go forward and hopefully
we will be able to produce some incremental improvements for heat stress tolerance. But
the key message that both Ben Biddulph and James will also make sure, I'm sure, in his
talk, is that optimising the timing of the flowering window to minimise your risk for
both frost and heat is the key point to make sure you're not going to get too many severe
impacts on grain yield. And that is really the take home message from this talk. Hopefully
in the future we will be able to have something a little bit more groundbreaking.
And to touch very quickly, last year through the controlled environment assay, I put through
4,140 something plants through that heat chamber. It was a ridiculous amount of work, and I'm
sure my assistant wanted to stab me in the back. But anyway, we got there. And this is
primarily eight double haploid populations. If you don't understand what that is and you
want to, ask me. It's not really perhaps that interesting. But we are using these tools
- 1,500 of these mapping lines to screen them through the heat chamber. We are going to
be screening them in the field this year at a number of locations, trying to understand
the same responses that we were talking about before with the varieties, and hopefully we'll
be able to identify some genetic regions associated with improved trait performance, so we can
use those as selection tools within the breeding program.
And that's me.