宁夏东部荒漠草原猪毛蒿光合特征和水分利用效率对降水变化的响应

荒漠草原植物的生长和生存受降水影响很大，本研究以宁夏东部荒漠草原猪毛蒿种群为研究对象，2017-2020年连续4年监测了降水改变条件下土壤水分特征，测定了猪毛蒿净光合速率、蒸腾速率、气孔导度和胞间CO₂浓度等光合特征，并对其内禀水分利用效率和瞬时水分利用效率进行了分析。研究表明：4年中无效降水事件（<5 mm）发生次数较多，占当年降水频次的60.00%~82.54%；大雨（≥25 mm且≤49.9 mm）和暴雨等级（≥50 mm）降水事件较少，平均单次降水事件的降水量仅为3.3~6.5 mm·次^-1。不同处理各土层的土壤含水量存在较大差异，表层（10 cm）土壤含水量较次表层（20 cm）和深层（60 cm）的变幅大；次表层（20 cm）土壤平均含水量低于表层，与小降水事件发生频次较高无法入渗补给以及植物吸收消耗有关；深层（60 cm）土壤含水量变化小，较稳定。净光合速率随着光合有效辐射的增加而增大，当光合有效辐射在400~1600 μmol·m^-2·s^-1时，增雨处理提高了猪毛蒿的净光合速率；在光合有效辐射>1600 μmol·m^-2·s^-1时，猪毛蒿净光合速率有下降趋势，且增雨处理下降幅度最大。光合有效辐射在>400 μmol·m^-2·s^-1时，胞间CO₂浓度呈直线下降，达到1600 μmol·m^-2·s^-1时，胞间CO₂浓度最小。蒸腾速率和气孔导度随着光照强度的增加未有显著变化，但增雨处理下猪毛蒿的蒸腾速率和气孔导度均大于其他处理。气孔限制值随着光合有效辐射的增加呈上升趋势，同时降水的增加减小了气孔限制值。猪毛蒿的内禀水分利用效率和瞬时水分利用效率均在较低的光照强度时（<400 μmol·m^-2·s^-1）随着光强的增加而迅速增大，之后增速减缓。各处理中猪毛蒿最大内禀水分利用效率和最大瞬时水分利用效率在1600~2000 μmol·m^-2·s^-1时达到最大。随着干旱程度的增加，猪毛蒿最大水分利用效率对应的光照强度由2000减小至1600 μmol·m^-2·s^-1。在荒漠草原，当光照强度>400 μmol·m^-2·s^-1时，适度干旱提高了猪毛蒿的水分利用效率。

Response of photosynthetic characteristics and water use efficiency of Artemisia scoparia to rainfall changes in Eastern Ningxia desert steppe

The growth and survival rates of plants are greatly affected by rainfall in desert steppe.This research investigated the soil water fluctuations after rainfall events for four consecutive years from 2017 to 2020 in an Artemisia scoparia population in an Eastern Ningxia desert steppe. We determined the effects of several simulated rainfall patterns （decreased by 30%， unchanged， increased by 30%） on the photosynthetic characteristics and water use efficiency of A. scoparia. The photosynthetic characteristics of A. scoparia including net photosynthetic rate， transpiration rate， stomatal conductance and intercellular CO₂ concentration were measured， and the intrinsic water use efficiency and instantaneous water use efficiency at leaf level were analyzed. It was found that trivial rainfall events （<5 mm） occurred frequently， accounting for 60.00%-82.54% of the rainfall frequency from 2017 to 2020. Heavy rain events （≥25 mm and <50.0 mm） and rainstorm events （≥50 mm） were few， and the average rainfall in a single rainfall event over the four years was only 3.3-6.5 mm. There were large differences in soil water content in different soil horizons under different treatments， and the variation of soil water content in the surface horizon （0-10 cm） was larger than that in the subsurface horizon （10-20 cm） and deep layer （20-60 cm）. The average soil water content in the subsurface layer （20 cm） was lower than that in the surface layer （10 cm）， and this reflected the high frequency of small rainfall events， the inability of water to infiltrate and penetrate， and the absorption and consumption of water by plants. The water content of the deep soil horizon changed little over time. The net photosynthetic rate increased with increase in photosynthetically active radiation. At photosynthetically active radiation levels of 400-1600 μmol·m^-2·s^-1， increase in soil water had a great influence on the net photosynthetic rate of A. scoparia. At photosynthetically active radiation levels higher than 1600 μmol·m^-2·s^-1， photosynthesis tended to be suppressed by increase in soil water and the rainfall treatment exhibited the largest decline. At photosynthetically active radiation levels above 400 μmol·m^-2·s^-1， the intercellular CO₂ concentration decreased linearly with increase in radiation， and at 1600 μmol·m^-2·s^-1， the intercellular CO₂ concentration reached its minimum. Transpiration rate and stomatal conductance did not change significantly with increase in light intensity， but the transpiration rate and stomatal conductance of A. scoparia under the rainfall enhancement treatment were higher than those under other treatments. Stomatal limitation increased with increase in photosynthetically active radiation， and decreased with increase in rainfall. Intrinsic water use efficiency and instantaneous water use efficiency of A. scoparia increased rapidly with increase in light intensity at light levels less than 400 μmol·m^-2·s^-1， but at higher light levels the growth rate slowed down. The intrinsic water use efficiency and instantaneous water use efficiency of A. scoparia reached their maxima at photosynthetically active radiation levels of 1600-2000 μmol·m^-2·s^-1. With increase in drought severity， the light intensity at which the maximum water use efficiency was observed decreased from 2000 to 1600 μmol·m^-2·s^-1， and when the light intensity was above 400 μmol·m^-2·s^-1， moderate drought improved the water use efficiency of A. scoparia.