Studies on variation and mechanisms of terminal heat tolerance in faba bean (Vicia faba L.)

Abstract

Faba bean is highly sensitive to heat stress during reproductive growth. Yield losses of up to 50% can be realised in Australia due to terminal heat stress; therefore, improvement for tolerance will help mitigate the losses occasioned by heat stress. This study evaluated 48 faba bean genotypes for heat tolerance in the field in northern NSW. Assessment for pollen thermotolerance using a newly developed impedance flow cytometry (IFC) protocol was also carried out. Two genotypes contrasting in their heat responses were further evaluated in field heat chambers to identify possible mechanisms of heat tolerance. Terminal heat stress effected by late sowing reduced the yields by 49–62% compared to early May sown crop, which averaged 3.65 t/ha. Lower yields in the heat-stressed crop resulted from shorter growing duration, decreased biomass production and partitioning, and lesser yield components. Podding and grain filling were more sensitive to heat stress than flowering; therefore, they will be critical in improving heat tolerance in faba bean. Pollen viability assessed through IFC was high at temperatures above 25 °C and could not be linked to lower pod set. Moreover, a study on faba bean pollen storage revealed that, pollen stored at 4–5 °C could keep for 28 days without significantly losing its viability. Low assimilates production was associated with high pod and flower abscission, which reduced yields. On the contrary, high yields were associated with taller plants, high PAR interception and normalised difference vegetation index, and better photosynthetic efficiency. Adaption to heat stress in northern NSW will be driven by maintaining a longer grain filling duration, early biomass production and high efficiency in partitioning to pods and seeds.