| The purpose of this study was to clarify the change rules of photosynthetic and fluorescence characteristics of different leaf positions under drought stress, and then select typical leaves to accurately measure photosynthetic and fluorescence parameters of plants under drought conditions, to achieve rapid evaluation of photosynthetic capacity of plants under drought conditions. The photosynthetic and fluorescence characteristics of pepper seedlings at different leaf positions under different drought treatments (Sufficient water supply (CK): 75%~85% field water capacity; Moderate stress (MS): 65%~75% field capacity; Severe stress (SS): 55%~65% field water capacity)were compared and analyzed by pot experiment. The results showed that, with the increase of the leaf position, the gas exchange parameters (net photosynthetic rate, transpiration rate, stomatal conductance), chlorophyll content, leaf area, chlorophyll fluorescence parameters (photochemical quenching coefficient，PSⅡmaximum quantum yield of photochemical reaction) first increased and then decreased and reached the maximum at 3rd leaf under sufficient water supplies. Under drought conditions, the parameters of each leaf position decreased gradually with the increase of water stress degree at 1st to 3rd leaf position, and the parameters increased first and then decreased at 4th to 6th leaf position, but all reached the maximum value at 4th leaf position(except Ci, F0, Fv/Fm, NPQ). Under MS and SS, the Pn of the third leaf position decreased by 14.05% and 55.91% compared with CK, respectively. Pn of the fourth leaf increased by 25.20% in MS compared with CK, and decreased by 15.42% in SS compared with CK. The third and fourth leaf positions were the main contributors to the growth of pepper seedlings. However, the variation coefficients of photosynthetic parameters, leaf area, chlorophyll concentration and chlorophyll fluorescence parameters at the third leaf position were larger than those at the fourth leaf position under water stress,especially for Ci, Chlb and F0, the variation of photosynthetic parameters, chlorophyll parameters and chlorophyll fluorescence parameters was the largest, and the coefficient of variation of the third leaf position was 133.33%, 180.00% and 200.00% higher than that of the fourth leaf position, respectively. The third leaf position was more sensitive to water stress. Therefore, the third leaf position should be selected to measure photosynthesis and fluorescence parameters of pepper seedlings in drought stress experiment. |