贝壳堤岛旱柳光合效率的土壤水分临界效应及其阈值分级

为阐明黄河三角洲贝壳堤岛旱柳(Salix matsudana)叶片光合效率对土壤水分的适应机制, 明确其水分阈值效应, 以二年生旱柳为材料, 采用人工给水与自然耗水相结合获取系列水分梯度的方法, 测定分析贝壳砂生境下旱柳叶片光合效率参数对土壤水分的响应特征及其生产力分级。结果表明: 旱柳叶片净光合速率(P_n)、蒸腾速率(T_r)、水分利用效率(WUE)及光合光响应参数具有明显的水分临界效应。(1) P_n、T_r、WUE和潜在水分利用效率均随土壤水分的降低先升高后下降, 但各指标水分临界值表现不同步, 其中P_n水分气孔限制转折点和水分补偿点分别出现在相对含水量(W_r)为42.9%和14.4%时; P_n和T_r的水分饱和点为73.1%和68.9%, WUE水分高效点为80.1%; (2)水分胁迫下旱柳叶片具有明显的光抑制现象, 可通过减弱对光的利用来适应水分逆境。随土壤水分的增加, 表观量子效率(AQY)、光饱和点(LSP)和最大净光合速率(P_nmax)表现为先升高后降低, 但光补偿点(LCP)相反。P_n、AQY、LSP、P_nmax和暗呼吸速率(R_d)均表现为渍水胁迫明显高于干旱胁迫。W_r为69.1%时, LCP达到较低值(18.6 µmol?m ^-2?s^-1), AQY最高(0.05), 利用弱光能力较强。W_r为80.9%时, LSP达到最高(1775 µmol?m ^-2?s^-1), 光照生态幅最宽, 光能利用效率最高, 水分对光强的补偿效应显著; (3)采用临界值分类法确定出贝壳砂生境下旱柳光合效率的5级水分阈值, 73.1%_r<80.1%范围内为高产高效水, 此时旱柳具有较高的光合能力和高效生理用水特性。贝壳砂生境内旱柳表现出一定的耐水湿而不耐干旱的适应特性, 在干旱缺水的贝壳堤岛滩脊地带栽植时需充分考虑其水分环境。

Critical effect of photosynthetic efficiency in Salix matsudana to soil moisture and its threshold grade in shell ridge island

Aims Shell ridge islands are distinctive shell sand deposits lying on the upper surface of tidal flats where shellfish grow in abundance and fresh water discharge is minimal. The objective was to elucidate the critical effect of photosynthetic efficiency parameters in leaves of Salix matsudana on soil moisture, clarify the threshold range of photosynthetic efficiency to soil moisture, and define the water adaptability in shell ridge islands of Shandong Province, China, in the middle of the Yellow River Delta.
Methods Two-year-old S. matsudana grown on shell ridge islands was selected as the experimental material. Soil water gradients were obtained by providing water and by natural water consumption. ACIRAS-2 portable photosynthesis system was used to measure the photosynthetic efficiency parameters under different soil water conditions. The light response curves of net photosynthetic rate (P_n) and the water response curves of gas exchange parameters in leaves of S. matsudana were fitted and analyzed.
Important findings The P_n, transpiration rate (T_r), water use efficiency (WUE) and photosynthetic parameters of light response in leaves of S. matsudana had significant critical effects on soil moisture. P_n, T_r, WUE and intrinsic water use efficiency first increased and then decreased with decreasing soil water, but their critical values were different. The critical value of relative soil water content (W_r) from stomatal limitation to non-stomatal limitationof P_n was 42.9%, and the water compensation point of P_n was 14.4%. The water saturation points of P_n and T_r were 73.1% and 68.9%, respectively, and the water efficiency point of WUE was 80.1%. Salix matsudana appeared to have photo inhibition under drought stress and had the physiological strategy of weakening light utilization to counter stress. With increasing soil water, the apparent quantum yield (AQY), light saturation point (LSP) and maximum net photosynthetic rate (P_nmax) first increased and then decreased, while the light compensation point (LCP) first decreased and then increased. The values of P_n, AQY, LSP, P_nmax and dark respiration rate (R_d) under water logging stress were higher than under drought stress. When W_r was 69.1%, LCP reached a lower value with 18.6 μmol·m^–2·s^–1, and AQY reached a higher value with 0.05, indicating that S. matsudana had strong ability to utilize weak light. When Wr was 80.9%, LSP reached the highest point with 18.6 μmol·m^–2·s^–1, indicating that S. matsudana had wide light ecological amplitude and high light utilization efficiency. The compensatory effect on light intensity of soil water was significant. The soil water content was divided into five threshold grades by critical values to maintain photosynthetic efficiency of S. matsudana at different levels in shell sand soil. W_r of 73.1% to 80.1% was classified as high productivity and high efficiency; in this range, S. matsudana had high photosynthetic capacity and efficient physiological characteristics for water consumption. In conclusion, S. matsudana had the typical characteristics of water tolerance and no drought stress in shell sand, thus plantings should give full consideration to the soil water environment in shell ridge island.