土壤水分梯度对灰胡杨光合作用与抗逆性的影响

以塔里木盆地珍稀渐危种灰胡杨(Populus pruinosa Schrenk)幼苗为材料,采用盆栽方法研究土壤水分梯度对灰胡杨光合特征及抗逆性的影响。结果表明:(1)灰胡杨净光合速率(P_n)、蒸腾速率、气孔导度、胞间CO_2浓度和光能利用率均随土壤水分递减而降低,重度干旱比适宜水分依次降低了35.53%、25.32%、48.18%、15.62%和40.92%;而光合午休程度则明显增强,P_n下降主要是由非气孔因素限制造成。轻度干旱能够提高灰胡杨水分利用效率(WUE)3.05%,维持相对较高的P_n和WUE。(2)随土壤水分递降,灰胡杨光照生态幅缩窄,CO_2补偿点升高,RuBP再生受限,光与CO_2利用效率、Rubisco活性和光合效率降低。与适宜水分相比,中度与重度干旱下最大净光合速率(P_(nmax))、表观量子效率、光饱和点、羧化效率、光合能力(A_(max))、光呼吸速率、最大羧化效率、最大电子传递速率和磷酸丙糖利用率均显著降低(P0.05),其中P_(nmax)、A_(max)和生化参数分别降低了42.65%、38.26%、57.10%;63.01%、65.88%、73.43%。(3)土壤干旱显著降低了灰胡杨的枝水势和光合色素含量(P0.01),并且改变了光系统反应中心色素的组成比例,膜脂过氧化程度显著增强(P0.01)。灰胡杨主要通过积累大量脯氨酸和可溶性蛋白质参与渗透调节来减轻土壤干旱对光合机构的损伤。重度干旱对灰胡杨叶片光合系统造成了不可逆的伤害,严重抑制了其正常生长和光合作用。综上所述,塔里木干旱荒漠区灰胡杨生长适宜的土壤相对含水量为60%—65%符合极端干旱区植被恢复与高效节水的管理原则。

Effects of soil water gradient on photosynthetic characteristics and stress resistance of Populus pruinosa in the Tarim Basin, China

Water deficit is a major limiting factor in vegetation recovery and reconstruction in the Tarim Basin in the extremely arid desert region of northwest China. Populus pruinosa Schrenk is an ecologically important species growing in this region. However, the P. pruinosa population has been declining in recent years because of the low underground water table, mainly caused by increased human water consumption. Currently, the mechanisms underlying the decline in photosynthesis of P. pruinosa under soil water declines are not clear. The objective of our study was to investigate the effects of the soil water gradient on photosynthesis, and the relationship between photosynthesis and soil water content. Our study will enhance our understanding of the photo-physiological characteristics of P. pruinosa exposed to soil water declines and provide valuable information for the protection of this vulnerable species. Saplings of P. pruinosa were planted in pots under four different soil water gradients. The effects of the soil water declines on the photosynthetic characteristics and stress resistance of P. pruinosa were evaluated by analyzing gas exchange, photosynthetic light and CO₂ response curves, pigments, stem water potential, and osmotic adjustment substances. The results showed that:(1) The net photosynthetic rate(P_n), transpiration rate, stomatal conductance, intercellular CO₂ concentration, and light use efficiency of P. pruinosa decreased with a gradual decrease in soil water content by 35.53%, 25.32%, 48.18%, 15.62%, and 40.92%, respectively, under severe soil drought in comparison to normal soil water content(CK). However, the phenomenon of "noon break" was apparent, and non-stomatal limitation was responsible for a reduction in P_n. Water use efficiency(WUE) could be improved by 3.05% under mild drought conditions relative to the CK, and P_n was maintained at a high level. (2) Light ecological amplitude decreased, CO₂ compensation point increased, ribulose 1,5-diphosphate (RuBP) regeneration was limited, and light/CO₂utilization efficiency, rubisco activity, and photosynthetic efficiency of P. pruinosa decreased with a decrease in soil water content. Compared to the CK, light response parameters(maximum net photosynthetic rate[P_nmax], apparent quantum efficiency[AQY], light saturation point[LSP], carboxylation efficiency[CE], Photosynthetic capacity[A_max], photorespiratory rate[R_p], maximum carboxylation rate[V_cmax], maximum electron transport rate[J_max], and triose-phosphate utilization rate[TPU]) significantly decreased(P < 0.05). Notably, P_nmax, A_max, and biochemical parameters of photosynthesis decreased by 42.65%, 38.26%, and 57.10% (moderate drought) and 63.01%, 65.88%, and 73.43% (severe drought), respectively. (3) Stem water potential and pigment content were significantly reduced(P < 0.01), the pigment composition proportion of the light reaction center was altered, and membrane lipid peroxidation(MDA) was significantly enhanced(P < 0.01) with decreasing soil water content. P. pruinosa mainly accumulated proline and soluble protein to eliminate reactive oxygen and alleviate the impairment of the photosynthetic apparatus under soil drought. Irreversible damage was caused to the photosynthetic system of P. pruinosa, and normal photosynthesis was seriously inhibited under severe drought. Therefore, declines in soil water content resulted in drought stress and reduced photosynthetic ability. This indicated that the decline in the P. pruinosa population might be caused by the lower soil moisture. In conclusion, the soil water content for maintaining higher P_n and WUE in P. pruinosa forestlands should be approximately 60%-65% of the field capacity in the Tarim arid-desert region to adhere to the management principles of efficient water conservation and vegetation restoration in extremely arid areas.