Effect of Low Temperature Stress and Acclimation on Growth and Photosynthetic Parameters in Bitter Gourd Seedlings

This study investigated the impact of low temperature and acclimation on growth and photosynthetic traits in two bitter gourd genotypes: Punjab-14 and PAUBG-56. Fifteen-day-old seedlings were exposed to 5°C for seven days, either directly or following acclimation (a gradual reduction from 20°C to 5°C). Recovery was assessed at 25°C over seven days, with samples collected on the 1^st, 3^rd, and 7^th days post-stress, and on the 14^th day post-recovery. Acclimation resulted in 100% survival for both genotypes, compared to a 10.7% higher survival rate in non-acclimated PAUBG-56 seedlings versus Punjab-14. Acclimated Punjab-14 seedlings showed significant increases in root (16.45%) and shoot length (73.62%), whereas PAUBG-56 exhibited increases of 69.81% and 36.43%, respectively. Non-acclimated seedlings experienced reductions in these parameters. Photosynthetic pigments showed varied responses: total chlorophyll increased by 1.82% on the first day post-stress, while chlorophyll a, b, and carotenoids decreased by up to 20% in Punjab-14. PAUBG-56 showed less pronounced reductions. Acclimated seedlings had smaller decreases and better recovery. The Fv/Fm ratio decreased by 12.50% in both genotypes. Electrolyte leakage increased significantly in non-acclimated Punjab-14, peaking at 93.80% by the 3^rd day, while PAUBG-56 reached 67.94% by the 7^th day, with recovery values nearing controls. Acclimated seedlings had lower electrolyte leakage. Correlation analysis revealed significant positive correlations between total chlorophyll content and chlorophyll a (0.887), chlorophyll b (0.484), carotenoids (0.614), and the Fv/Fm ratio (0.491), and a negative correlation with electrolyte leakage (-0.498). In conclusion, acclimation improves the physiological performance of bitter gourd seedlings under low temperature stress, enhancing growth, photosynthetic efficiency, and membrane stability.