Dynamics of leaf antioxidant enzymes, physiological traits, and photosynthetic pigments for climate-resilient bael [Aegle marmelos (L.) Correa] genotypes in response to seasonal variation in temperature

Bael [ Aegle marmelos (L.) Correa] is a hardy subtropical fruit crop that is sensitive seasonal variations in low temperature, particularly low temperature, which severely impacts productivity through the burning of leaves and twigs, fruit cracking, fruit drop and even plant mortality. To address these challenges, a study was conducted at Chaudhary Charan Singh Haryana Agricultural University, Regional Research Station, Bawal, from 2024–2025, to explore the climate-resilient genetic potential of bael genotypes, viz., NB 9, NB 5, NB 17, NB 16, CISHB 1, CISHB 2, Pant Aparna and Pant Sujata. This study focused on the activity levels of key antioxidant enzymes and physiological parameters, namely, superoxide dismutase (SOD), peroxidase (POD), ascorbate peroxidase (APX), catalase (CAT), proline content, ascorbic acid, RWC, MSI and photosynthetic measurements (Chl a, b, total Chl and carotenoids), to assess stress-responsive mechanisms across different seasons, i.e., the autumn season (normal growth conditions), the winter season (low temperature conditions) and the summer season (high temperature conditions), with sampling dates of August 4, 2024; January 2, 2025; and April 7, 2025, respectively. The results revealed that antioxidant enzyme activities, as well as biochemical and physiological parameters, changed in response to seasonal temperature variations. The maximum values of SOD (211.4 unit/g FW), CAT (3.079 unit/min/g FW), APX (0.570 unit/min/g FW), POD (3.67 unit/min/g FW), AsA (24.2 mg/100 g FW) and proline content (5.42 nmol/g FW) and minimum Chl a (0.943 mg/g FW), Chl b (0.238 mg/g FW), total Chl (1.180 mg/g FW), carotenoid (0.234 µg/g FW) MSI (28.58%) and RWC (79.3%) were recorded during the winter season (p ≤ 0.05). However, these values exhibited the opposite trend during autumn under normal growth conditions. The interaction of genotype × season highlighted NB 16 as the genotype exhibiting the highest enzymatic activities of SOD (264.4 unit/g FW), CAT (3.735 unit/min/g FW), POD (4.10 unit/min/g FW), APX (0.678 unit/min/g FW), AsA (28.3 mg/100 g FW) and proline content (5.94 nmol/g FW) during winter season-associated low temperature conditions. Similarly, the first three principal components (PCs) accounted for 87.72% of the total variability in the dataset. Among these, PC1 accounted for 62.24% of the total variance, followed by PC2 (17.14%) and PC3 (8.33%). These findings suggest that, compared with the other genotypes, the NB 16 genotype exhibits favourable physiological and biochemical responses under low temperature winter season conditions; therefore, it may serve as a valuable candidate genotype for future breeding programs.