Assoc. Prof. Walid Sadok, University of Minnesota, USA

Improved oat yield under heat stress: a promising role for canopy cooling traits as breeding targets

Assoc. Prof. Walid Sadok, University of Minnesota, USA

ABSTRACT

José R. López1, Bishal G. Tamang1, Daniel M. Monnens1, Kevin P. Smith1 , Walid Sadok1

1 Department of Agronomy and Plant Genetics, University of Minnesota Twin Cities, 411 Borlaug Hall, 1991 Upper

Buford Circle, St. Paul, MN 55108, sadoklab.cfans.umn.edu, E-mail: msadok@umn.edu

Summary

Heat stress is negatively impacting oat yields in the U.S. Midwest, making it critical to breed for more adapted varieties. To this end, we conducted a 3-year field experiment where we subjected 30 oat lines from 9 breeding programs to a “heat tent” treatment during reproductive development and measured various yield and canopy cooling traits using proximal and remote-sensing techniques. We found that heat stress-tolerant genotypes minimized yield losses thanks to cooler canopies, which can be detected rapidly using drone-mounted thermal imaging, making it possible to fast-track breeding towards more resilient oat varieties.

Introduction

Oat yield potential in the Upper Midwest has been declining over the last decades, particularly on days where temperatures exceed 28°C during reproductive development (Klink et al., 2011). This is due with the fact that gametogenesis and flowering are particularly sensitive to heat stress (HS), typically leading to lower seed set and yield (Prasad et al., 2017). In addition, HS also causes leaf photosynthetic penalties that lead to a reduction in photoassimilate availability for reproductive sink organs (Way et al., 2011). Here we tested the hypothesis that enhanced heat stress tolerance in north American oats from booting to heading could be achieved by breeding for genotypes that maximize canopy latent evaporative cooling arising from increased transpiration, which would both protect photosynthesis (source) and reproductive development (sink).

Methods

Genetic material and planting

A diverse breeding panel of 30 lines that flowered on the same time was planted in a split plot design with 4 replications, with HS treatment as main plot and line as subplot at the U of MN’s St. Paul field plots during the 2018 and 2019 growing seasons (240 plots of 1.5m rows, 30.5cm apart). The HS treatment was imposed for 12d from booting to heading by means of 4 “heat tents” where all key environmental conditions (T, RH, VPD, PAR) were continuously recorded every 15 min.

Proximal (physiological and yield) measurements

During the HS treatment, two plants per plot were evaluated for leaf gas exchange, flag leaf morphology, pollen fertility, and percentage of filled florets. Gas exchange measurements were collected using 3 LI-6800 Photosynthesis Systems, around solar noon on the middle of the flag leaf blade, with chamber conditions matching ambient conditions. Florets from the two marked plants per plot were collected and pollen viability was evaluated following Kearns and Inouye (1993). The percentage of filled florets was evaluated manually a month after the end of the HS treatment. Yield was determined for each experimental unit, and seeds were examined for protein, oil, and beta glucan content in the grain using a near infra-red (NIR) analyzer.

Remote-sensing measurements and data analysis

Nine lines selected from the 2-yr heat stress trial for their contrasting HS tolerance were planted in larger (six-row) yield plots in 2020 at the same location. Plots (0.72m by 1.2m) were machine-planted in a randomized complete block design with three blocks. Aerial thermal images of the nine lines were collected

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around solar noon with a thermal camera mounted on an unmanned aerial system using a specialized gimbal. Linear mixed models, heritability estimates, trait correlations and physiological path analyses were conducted to identify the physiological basis of heat stress tolerance, identify promising proxy traits and genotypes, and develop the basis of a phenotyping pipeline to support a breeding effort (not all results are shown).

Results

Heat stress has consistent effects on source and sink drivers of yield, except on pollen viability

The control and heat tent temperatures for 2018 and 2019 averaged 25.5/29.4°C and 23.3/30.3°C respectively. For all genotypes, the HS treatment had a negative and consistent effect on yield and its components across years (P < 0.001), resulting in yield penalties averaging 44%. This was driven by a 61% decline in grain number and 55.5% reduction in floret fertility, but surprisingly, not in pollen fertility. The treatment increased grain protein content by 24%, and reduced oil and beta-clucans 30% and 8% respectively. Net CO2 assimilation rate (A) declined by 17.5% as a result of the HS treatment. Significant genotypic variability was identified for all these traits.

Canopy cooling associated with improved yield performance across trials

In both years, leaf transpiration rate (E) increased by 56% as a result of the HS treatment, which was mirrored by a higher latent cooling as indicated by leaf temperature depression (LTD) which averaged 1.7 °C in both years. Under the HS treatment, grain yield was strongly and positively correlated with canopy cooling traits, namely E and LTD, and these positively correlated with the number of florets and number of grains per plant (Fig.1A). Consistently with this, the physiological path model showed that LTD has a stronger impact on the percentage of filled florets (PV = 0.27) than A on yield (PV = 0.017). In further confirmation of the positive role played by canopy cooling, the remote-sensed canopy temperature in the contrasting lines evaluated in the 2020 trial was negatively correlated with filled florets (R2 = 0.52, P < 0.05, Fig.1B) and yield (P < 0.01).

FIGURE 1. Trait correlations under the HS treatment (A) and between floret fertility under the HS treatment and canopy temperature in an independent trial (B). In (A), the size and colour of circles is indicative of the absolute value of Pearson’s correlation coefficients, where positive/negative correlations are represented with blue/red circles respectively

Conclusion

Collectively, these results indicate that pollen viability is not main driver of oat yield declines in response to heat stress, that canopy transpirational cooling plays a potentially major role in protecting reproductive tissues and yield from excessive heat and that it is possible to identify genotypes with improved HS resilience using thermal imaging based on remote sensing.

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