Enhanced Leaf Cooling Is a Pathway to Heat Tolerance in Common Bean.

Chetan R Deva,Lenka Svitákova,Milan O Urban,Andrew J Challinor,Pete Falloon

doi:10.3389/fpls.2020.00019

Abstract

Common bean is the most consumed legume in the world and an important source of protein in Latin America, Eastern, and Southern Africa. It is grown in a variety of environments with mean air temperatures of between 14°C and 35°C and is more sensitive to high temperatures than other legumes. As global heating continues, breeding for heat tolerance in common bean is an urgent priority. Transpirational cooling has been shown to be an important mechanism for heat avoidance in many crops, and leaf cooling traits have been used to breed for both drought and heat tolerance. As yet, little is known about the magnitude of leaf cooling in common bean, nor whether this trait is functionally linked to heat tolerance. Accordingly, we explore the extent and genotypic variation of transpirational cooling in common bean. Our results show that leaf cooling is an important heat avoidance mechanism in common bean. On average, leaf temperatures are 5°C cooler than air temperatures, and can range from between 13°C cooler and 2°C warmer. We show that the magnitude of leaf cooling keeps leaf temperatures within a photosynthetically functional range. Heat tolerant genotypes cool more than heat sensitive genotypes and the magnitude of this difference increases at elevated temperatures. Furthermore, we find that differences in leaf cooling are largest at the top of the canopy where determinate bush beans are most sensitive to the impact of high temperatures during the flowering period. Our results suggest that heat tolerant genotypes cool more than heat sensitive genotypes as a result of higher stomatal conductance and enhanced transpirational cooling. We demonstrate that it is possible to accurately simulate the temperature of the leaf by genotype using only air temperature and relative humidity. Our work suggests that greater leaf cooling is a pathway to heat tolerance. Bean breeders can use the difference between air and leaf temperature to screen for genotypes with enhanced capacity for heat avoidance. Once evaluated for a particular target population of environments, breeders can use our model for modeling leaf temperatures by genotype to assess the value of selecting for cooler beans.

Highlights

Common bean (Phaseolus vulgaris) is the most consumed legume in the world (Araújo et al, 2015), and an important source of protein in tropical Latin America, Eastern, and Southern Africa (Beebe et al, 2011)
We have shown that leaf cooling plays an important role in heat avoidance
We have shown that heat tolerant varieties exhibited higher stomatal conductance and a greater association between VPD and leaf cooling

Summary

Introduction

Common bean (Phaseolus vulgaris) is the most consumed legume in the world (Araújo et al, 2015), and an important source of protein in tropical Latin America, Eastern, and Southern Africa (Beebe et al, 2011). Common bean is more sensitive to high temperatures than other legumes (Beebe et al, 2011), making breeding for heat tolerance an urgent priority as the climate continues to warm (Beebe et al, 2011). Some heat tolerant crops maintain photosynthesis under elevated temperatures by maintaining stomatal conductance (Porch and Hall, 2013). The magnitude of transpirational cooling has been used by plant breeders to screen for heat tolerance in spring wheat cultivars (Porch and Hall, 2013). We turn to the evidence on the contribution of leaf/canopy cooling to heat avoidance in important food crops and the links between heat tolerance and leaf cooling

Methods

Results

Discussion

Conclusion