Photosynthetic and physiological characteristics for seedlings of Calotropis gigantea provenances in a hot-dry valley

To understand the photosynthetic and physiological characteristics of the seedling stage for different provenances of Calotropis gigantea in a hot-dry valley area, changes in daily mean values for net photosynthetic rate （Pn）, stomatal conductance （Gs）, intercellular CO2 concentration （Ci）, transpiration rate （Tr）, water use efficiency （WUE）, light use efficiency （LUE）, carboxylation efficiency （CE）, and stomatal limitation （ls） of C. gigantea seedlings from Yuanjiang, Honghe, Gejiu, Jianshui, and Yuanyang provenances were compared and analyzed using multiple comparison analysis of Duncans test. Results of diurnal dynamics for the net photosynthetic rates from all provenances showed bimodal curves, but there were different times associated with the values. C. gigantea has strong adaptability to high temperature, which can improve the photosynthetic assimilation efficiency by increasing the stomatal conductance and transpiration. The extreme high temperature in hot-dry valleys is not the limiting factor to influence photosynthesis and carboxylation reaction of C. gigantea seedlings, however, the high light radiation at midday significantly inhibits their photosynthesis and water utilization. The daily mean values for photosynthetic and physiological parameters showed that Honghe provenance had a significantly higher （P ＜ 0.05） net photosynthetic rate, transpiration rate, light use efficiency, and carboxylation efficiency than the other four provenances. Also, Gejiu provenance had a highest （P ＜ 0.05） water use efficiency and a high-level net photosynthetic rate, light use efficiency, and carboxylation efficiency. Consequently, Honghe and Gejiu provenances are considered superior for planting in hot-dry valleys having high-level photosynthetic production and water utilization potential.［Ch, 3 fig. 1 tab. 43 ref.］