Diurnal variability in chlorophyll fluorescence of soybean under field conditions: Implications for heat stress response

Abstract

Monitoring variations in photosynthetic efficiency throughout the day allows the variability of plant stress responses to be studied, helping to improve precision breeding and management. Chlorophyll a fluorescence (ChlF) was measured twice in the field during the flowering phase in nine soybean genotypes. The measurements were taken in the morning, which corresponds to optimal environmental conditions, and at midday, which corresponds to elevated temperatures that normally cause stress in the plants. Treatment, genotype and their interaction were highly significant sources of variability (p < 0.001) for all ChlF parameters tested. Genotypes differed significantly (p < 0.05) in their stress response, expressed as a percentage change in values from morning to midday. The largest average diurnal change was observed for the performance index on an absorption basis (PIabs, 63.8%) and the smallest for the electron transport flux per active reaction centre (ET0/RC, 6.5%). According to the results, heat stress affected both the efficiency with which the absorbed light is used in photochemistry (Fv/Fm) and the electron transport beyond the primary quinone acceptor (ET0/TR0). This was confirmed by the results of the energy fluxes per reaction centre, where ABS/RC, TR0/RC and DI0/RC increased as a result of the high midday temperatures, while ET0/RC decreased. This indicates that although the apparent energy input per active reaction centre increased, less energy was directed into productive photochemistry – that is, the overall photochemical efficiency was reduced. The average PIabs decrease confirms that the overall photosynthetic performance of PSII was impaired and less efficient at high temperatures, indicating a reduced capacity for photochemical conversion. Among the genotypes, photosynthetic efficiency was least impaired in line OS-922 and most impaired in OS-1522. The photosynthetic apparatus of line OS-1322 proved to be very stable under elevated heat, with PIabs remaining almost unchanged as a measure of photosynthetic performance compared to optimal conditions