Reproductive stage-dependent heat stress responses from perspectives of photosynthesis, yield, and grain composition in contrasting Lupinus angustifolius varieties

This study aimed to elucidate heat resistance in Lupinus angustifolius L. using gas exchange and chlorophyll fluorescence methods and to examine relationships between photosynthesis, yield, and grain nutritional composition. For this we exploited six contrasting varieties: 'Iris', 'Jenabillup', 'Mandelup', 'Mirabor', 'Primabella' and 'Primadonna' selected based on physiological, yield, and grain nutritional performance. Plants were subjected to three day/night temperature regimes (23/18 °C, 30/25 °C, and 38/33 °C) for five days during one of two reproductive stages: flower spike emergence and grain filling. At the flower spike emergence, the 38/33 °C decreased the photosynthetic CO₂ assimilation (A_n) in all varieties, with notable differences observed among them. 'Mandelup', 'Iris', and 'Jenabillup' maintained the highest A_n. At grain filling, the effects of 38/33 °C were more pronounced, resulting in a greater reduction in A_n. Notably, 'Primabella' and 'Primadonna' exhibited the most substantial decrease in A_n. The investigation of chlorophyll quenching parameters revealed significant impacts of temperature treatments on the maximum quantum efficiency of PSII (F_v/F_m), with notable varietal distinctions particularly during the grain filling stage. We found strong correlations between photosynthetic parameters and yield traits, as well as grain nutritional composition. These relationships were also strongly dependent on stage of development at which the heat stress was imposed, which underlines the critical importance of timing in phenotyping. Our findings provide a foundation for future molecular, metabolomic, and physiological studies on heat tolerance in lupins and promote adoption of heat-tolerant varieties in European cropping systems in the face of worsening effects of climate change and increased demand for plant protein.