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Abstract This study was conducted to investigate the effects of different foliar-applied boron doses on the physiological parameters of soybean at different growth stages. The experiment was carried out during 2020–2021 using a split-split plot design with three replications. The main plots were assigned to irrigation levels (I100: non-stress, I66: moderate stress, I33: severe stress), subplots to growth stages (G1: vegetative stage to beginning of flowering, G2: beginning of flowering to full flowering, G3: full flowering to pod formation), and sub-subplots to boron doses (B0: soil NPK only, B1: B0 + foliar 50 g da⁻ 1 boron, B2: B0 + foliar 100 g da⁻ 1 boron). The results indicated that water stress and foliar boron application at different growth stages had significant effects on Photosystem II (PSII), stomatal conductance, chlorophyll content and concentration, proline accumulation, and leaf boron content. Drought stress in 2020 and 2021 reduced stomatal conductance by 65% and 50%, PSII by 11% and 9%, chlorophyll content by 10% and 11%, and chlorophyll concentration by 36% and 32%, respectively. Conversely, proline concentration increased with increasing water stress, reaching maximum values of 50.41 µmol g⁻ 1 (2020) and 117.56 µmol g⁻ 1 (2021) under severe drought stress, and minimum values of 31.90 µmol g⁻ 1 (2020) and 77.51 µmol g⁻ 1 (2021) under non-stress (I100) conditions. Leaf boron content increased with boron doses, with the highest values recorded under non-stress conditions (I100; 21.22 ppm in 2020, 25.71 ppm in 2021) and the lowest under severe drought stress (I33; 20.03 ppm in 2020, 24.49 ppm in 2021). Among the treatments, foliar application of 50 g da⁻ 1 boron (B1) during the vegetative and early flowering stage (G1) had the most favorable effect on soybean physiological performance under water stress conditions.
Candemir et al. (Fri,) studied this question.