Response of seedling growth and photosynthetic characteristics of Sapindus mukorossi to different habitats in low mountainous upland region of Chongqing, southwestern China

ObjectiveIn this paper, the growth, photosynthetic characteristics and the main environmental factors of Sapindus mukorossi under different habitats at low mountainous upland region of Chongqing were explored in order to provide a reference for seedling cultivation and intensive planting of Sapindus mukorossi in southwestern China.

MethodThe growth indexes, photosynthetic response curves and diurnal variation characteristics of the three site conditions including low mountains, high hills and low hills were measured in summer. Besides, path analysis method was adopted to figure out the direct and indirect effects of environmental factors on net photosynthetic rate (P_n).

Result(1) The diurnal variation of P_n and transpiration rate (T_r) in the three kinds of site showed a bimodal curve, revealing a distinct " noon depression”. The average daily P_n and T_r of Sapindus mukorossi in low hills were the smallest, and the difference between high hills and low mountains was not obvious. The order of daily average of light utilization efficiency (LUE) and water use efficiency (WUE) was both showed as low hills > high hills > low mountains. (2) Compared with before transplanting, the height of Sapindus mukorossi in low mountains, high hills and low hills increased by 1.16, 1.39 and 0.63 times, respectively, and the ground diameter increased by 2.41, 1.97 and 0.75 times, respectively. The dry mass of leaves obeyed the order of low mountains > high hills > low hills. And the total dry mass of roots and stems obeyed the order of high hills > low mountains > low hills. (3) The decision coefficient of the environmental factors including photosynthetically active radiation (PAR), atmospheric CO₂ concentration (C_a), air temperature (T_a) and air relative humidity (RH) in the low mountains followed the order of R_PAR² > R_Ta² > R_Ca² > R_RH², high hills followed R_PAR² > R_Ta² > R_Ca² > R_RH², low hills followed R_PAR² > R_Ca² > R_RH² > R_Ta². (4) Chlorophyll-a and chlorophyll-b content of Sapindus mukorossi obeyed the order of low hills > high hills > low mountains, the content of carotenoids obeyed the order of low mountains > high hills > low hills. (5) The photosynthetic response curves were fitted to obtain the maximum net photosynthetic rate (P_nmax) of Sapindus mukorossi, and the result followed the order of low mountains > high hills > low hills. The apparent quantum yield (AQY) of leaves in low mountains was the smallest, while there was insignificant difference between high and low hills.

ConclusionIn low mountains, Sapindus mukorossi can obtain more light than high hills and low hills, and the ability of using low light was lower than under the other two site conditions because of long term adaptation. PAR is the main driving factor of P_n in three kinds of site conditions. RH is the biggest limiting factor of P_n under the conditions of low mountains and high hills. The main limiting factor of P_n in low hills is T_a. The high temperature in summer of the low hill site limited photosynthesis of Sapindus mukorossi, resulting in the average daily P_n and growth of plants was the slowest in the three kinds of site conditions.