Night Temperature and Light Intensity Regulate the Physiological, Photosynthetic, and Molecular Responses of Off-Season Grown Watermelon (Citrullus lanatus)

Abstract

Night temperature and light intensity are crucial for off-season production of watermelon ( Citrullus lanatus ) while temperature is very low sometimes minus and lower intensity of natural light. They regulate carbon assimilation, energy conversion efficiency, and overall plant productivity. However, their interactive effects of watermelon during off-season production remain unclear. To address these issues the present study was investigated under controlled hydroponic conditions during the winter season (Off-season). A completely randomized designed was used for the evaluation of three environmental treatments: T0 [control; low temperature and low light: day/night 22.76/13.30 °C, photosynthetic photon flux density (PPFD) 492.03 μmol·m −2 ·s −1 ]; T1 (comparatively high temperature and high light: 30.30/21.66 °C, PPFD 929.71 μmol·m −2 ·s −1 ) and T2 (high daytime and low night temperature with high PPFD; 33.53/15.00 °C, PPFD 1235.57 μmol·m −2 ·s −1 ). The results revealed that among the treatments T1 had significantly higher chlorophyll content, photosynthetic rate ( Pn ), transpiration rate ( E ), and stomatal conductance ( g s ) compared with control and T2, chlorophyll a and b increased by 63% and 61% in T1 and 57% and 22% in T2, respectively. The effective quantum yield of Photosystem II (ΦPSII) and electron transport rate (ETR) also improved in T1, suggesting enhanced photochemical efficiency under elevated temperature and light conditions. Leaf Ca 2+ and Mg 2+ concentrations increased by 21.76% and 220% in T1, respectively, compared with control, suggesting enhanced nutrient assimilation. Morphological traits, including leaf area, plant height, and number of nodes, increased by up to 85% in T1, leading to earlier flowering and a higher number of female flowers. The anther dehiscence and many pollen observed in T1 like optimum growing season indicating easy pollination, which is the main obstacle during off-season cultivation. At the molecular level, rbcL gene expression, a key photosynthetic gene encoding the catalytic large subunit of Rubisco, was downregulated in control and T2 plants whereas T1 showed upregulation indicating the optimized light and temperature under T1 enhances photosynthesis. Overall, increases in temperature and light intensity synergistically promoted photosynthetic efficiency, nutrient uptake, and reproductive capacity, providing insights for optimizing off-season watermelon cultivation under controlled environments.