Impact of varying shade on CO₂ assimilation, carboxylation efficiency, thylakoid electron transport and water use efficiency in Sesamum indicum L.

Important photosynthetic traits of sesame (Sesamum indicum L.) were studied under different regimes of shade (33%, 50%, and 75% of incident sunlight) with control (open sunlight). Significant reductions were observed in the rates of CO₂ assimilation, Carboxylation efficiency, Thylakoid electron transport and Water Use Efficiency with increase in shade intensity. Maximum reductions in the photosynthetic traits were noted in 75% shade, followed by 50% shade, while a moderate reduction was recorded in 33% shade in comparison to open grown plants. Shadeinduced adaptations to low light environment have been reflected in the functioning of photosynthetic apparatus through changing their light requirement to saturate the rate of CO₂ assimilation. Light saturated rate of CO₂ assimilation (A_max) was obtained at photosynthetic photon flux density (PPFD) of about 1400 μ mol m⁻²s⁻¹ in both open and 33% shade grown plants, whereas, these were about 800 μ mol m⁻²s⁻¹ for the plants grown in 50% or 75% shade. Shade-induced reductions in rate of CO₂ assimilation has been corroborated with the reduction in the carboxylation efficiency, photosystem-2 (PS2) activity and efficiency of photochemical reactions. The results would be useful in developing or selecting shade-tolerant crops for semi-arid climate with resource utilization efficiency.