Non-destructive phenomic tools for the prediction of heat and drought tolerance at anthesis in Brassica species

Abiotic stresses lead to an array of physiological and biochemical changes in crops which affect their growth and development leading to grain yield reductions. Oilseed Brassica species are vulnerable to heat and drought stress, especially in the early reproductive stage. We evaluated the use of several non-destructive phenomic tools to predict heat and drought tolerance at anthesis in three Brassica species, including five B. rapa, five B. napus and two B. juncea genotypes. Heat, drought or combined heat and drought treatments were applied for seven days from the beginning of anthesis, following which plants were returned to normal temperatures and well-watered conditions. Leaf and floral bud temperatures increased relative to ambient, and leaf conductance decreased in response to seven days of drought or combined treatments, but not to heat or control treatments. The drought and/or heat treatment significantly affected three photosynthesis-related parameters: the maximum carboxylation rate allowed by Rubisco (Vcmax), the photosynthetic electron transport rate and the rate of triose phosphate use. One day after the beginning of the heat and combined heat and drought treatments, Vcmax increased to more than double the basal level but remained low in the drought and control treatments. Vcmax on the first day of heat and combined treatments was positively correlated to the harvested seed yield of genotypes (r = 0.670 and 0.878, respectively), suggesting that a rapid increase in Vcmax in response to heat stress during anthesis was a positive predictor of final grain yield. The tissue volume of the whole plant, detected by plant growth imaging using a digital imaging system, was positively correlated with the above-ground fresh weight of genotypes after seven days in all treatments and was, therefore, an excellent surrogate for above-ground biomass of the whole plant. The volume of the yellow flowers after seven days in all treatments was positively correlated with the harvested seed yield of genotypes, and is a potential non-destructive phenomic screen for heat and drought stress tolerance in Brassica germplasm.