Fig. 6

Photosynthetic light response curves of rice varieties (Purnendu, Swarnaprabha, IR64, and IR8) under normal light (NL) and simulated low light (LL) during the reproductive stage (Kharif 2022 and 2023). (a–d) Net CO₂ assimilation (PN) was measured in flag leaves across a gradient of increasing photosynthetic photon flux density (PPFD; 0–1600 µmol photons m⁻² s⁻¹) at a leaf temperature of 25 ± 1 °C and reference CO₂ concentration of ~ 400 µmol mol⁻¹. LL-treated leaves were measured under their native light conditions, without pre-exposure to high light, to preserve in situ low-light physiological state. (e–h) Apparent quantum yield (initial slope of the light response curve) was derived from the linear range (0–200 µmol photons m⁻² s⁻¹). Data represent means ± SE (n = 5 plants per variety and treatment, averaged across two seasons). Asterisks indicate statistically significant differences between NL and LL within each genotype (*P < 0.05; **P < 0.01; ***P < 0.001, by t-test).