Photosynthetic Profiling of Cotton Leaves

At the full boll-setting stage under sunny and cloudless weather from 9:00 to 10:00 AM, the photosynthetic parameters of fully expanded leaves (fourth main-stem leaf from the apex) were determined using three randomly selected plants in the central two rows of each plot. SPAD-502 readings were used as a proxy of the chlorophyll content of leaves (SPAD-502, Minolta, Japan). A Li-6400 portable photosynthetic apparatus (LI-COR, Lincoln, NE, United States) was also used to measure the net photosynthetic rate (P_n), stomatal conductance (G_s), intercellular carbon dioxide concentration (C_i), and transpiration rate (T_r). The fluorescence parameters of chlorophyll, including the primary light energy conversion efficiency (F_v/F_m), non-photochemical quenching coefficient (q_N), photochemical quenching coefficient (q_P), and effective quantum yield of PSII photochemistry (ΦPSII), were measured by an FMS2 portable fluorescence system (Hansatech instruments, King’s Lynn, Norfolk, United Kingdom). The F_v/F_m determination required leaves to be dark-adapted for half an hour (Song et al., 2019 (link)).