Water is one of the major limiting factors in vegetation recovery and reconstruction in the extremely arid desert region of northwest China.We established five moisture gradients to simulate a desert habitat,and measured the photosynthetic light responses of two-year-old Populus euphratica seedlings under drought stress using the Li-6400 portable photosynthesis system.The resulting data were fitted and analyzed using rectangular hyperbola,non-rectangular hyperbola,exponential and modified rectangular hyperbola models.This study aimed to optimize these models,investigate the adaptability of light-response models to different levels of drought stress,and understand P.euphratica adaptation to arid desert conditions.The results showed the net photosynthetic rate (Pn) decreased with increased drought stress,and the total Pn declined under the same photosynthetically active radiation (PAR).Relatively high photosynthesis rates and reduced photo-inhibition were observed in P.euphratica under conditions of low-moderate drought stress (relative soil water content (RSWC) ＞ 45％).When models were used to express this,the exponential,rectangular hyperbola,and non-rectangular hyperbola models fit well with the Pn and PAR response process,but the maximum net photosynthetic rate (Pnmax) were higer than the measured values and the light saturation point (LSP) were lower than the measured values (P ＜ 0.01).When the RSWC was lower than 45％ (where seedlings would experience greater drought stress),and when an obviousphoto-inhibition and significant decrease in Pn were observed under strong light,LSP and Pnmax were also significantly decreased.Under these circumstances,only the modified rectangular hyperbola model could fit the light response processes and light response parameters.Overall,the goodness of fit of the four light response models to the observed data was as follows:modified rectangular hyperbola model ＞ exponential model ＞ non-rectangular hyperbola model ＞ rectangular hyperbola model.The adaptability of the four models to different levels of drought stress was variable.The modified rectangular hyperbola model perfectly simulated the light response process and fitted photosynthetic parameters under all water conditions.It was especially suited to severe drought stress and extremely arid desert conditions,whereas the other three models were only suitable under high moisture conditions.The response threshold of the light response parameters differed with the changing levels of drought stress.The apparent quantum yield (AQY),Pn,P and LSP ofP.euphratica decreased gradually with increasing levels of drought stress,but its dark respiration rate (Rd) and LCP increased significantly under extremely severe drought stress.P.euphratica showed higher AQY,P and LSP under RSWCs greater than 45％,while its P and LSP decreased significantly when RSWC was less than 45％.The photosynthetic efficiency of P.euphratica decreased during drought stress,as did its light tolerance range.Under severe drought stress,photo-saturation and photo-inhibition increased significantly,while photosynthetic capacity and normal seedling growth were substantially inhibited.P.euphratica was sensitive to small changes in drought severity and decreased its light use efficiency and respiration to adapt to desert conditions.We conclude that to enhance species conservation and restore vegetation in extremely arid desert regions,the soil water content should be maintained at about 50％ of the field capacity,which would optimize desert plant growth and water management in the Tarim Desert area.%以塔里木干旱荒漠区2年生胡杨幼苗为试材,盆栽模拟荒漠生境5种水分梯度,利用Li-6400便携式光合作用系统测定胡杨在干旱胁迫下光合作用的光响应过程,并采用4种光响应模型对其进行拟合与比较,以期优选出适用于干旱荒漠环境的光响应模型,阐明胡杨光合作用对干旱胁迫的响应规律与适应机制.结果表明:胡杨净光合速率(Pn)随干旱胁迫加剧呈下降趋势,同一光强(PAR)下Pn降幅增大.中度干旱胁迫以下(土壤相对含水量,RSWC＞45％)胡杨在高PAR下仍能维持相对较高Pn,光抑制程度轻;直角双曲线、非直角双曲线和指数模型均可较好地模拟Pn-PAR响应过程,但最大净光合速率(Pnmax)、光饱和点(LSP)拟合值与实测值差异极显著(P＜0.01).中度干旱胁迫以上(RSWC＜45％)胡杨Pn随PAR升高而显著下降,LSP与Pnmax极显著降低,光抑制现象明显;仅直角双曲线修正模型拟合的胡杨光响应过程、光响应参数与实际情况较吻合.4种模型模拟效果顺序:直角双曲线修正模型＞指数模型＞非直角双曲线模型＞直角双曲线模型.4种光响应模型对干旱胁迫具有不同的适应性,直角双曲线修正模型适用于各种水分条件,尤其适用于干旱荒漠生境,其它3种模型适用于水分条件较好的生境.光响应特征参数对干旱胁迫的响应阈值不同.随干旱胁迫加剧,胡杨表观量子效率(AQY)、Pn、LSP与Pnmax持续降低,严重干旱胁迫下暗呼吸速率(Rd)、LCP反而明显增大.RSWC＞45％胡杨仍能保持较高的AQY、Pnmax、LSP,RSWC＜45％其Pnmax、LSP显著降低,干旱胁迫显著抑制了胡杨光合进程和光强耐受范围,降低了光合效率,严重干旱胁迫严重影响胡杨苗木的正常生长和光合作用.干旱荒漠环境下,胡杨采取缩窄光照生态幅、降低光能利用率和减少呼吸消耗来积极抵御荒漠干旱逆境伤害的生态对策.因此,从极端干旱荒漠区种群保护与植被恢复角度来看,胡杨林土壤水分应维持在RSWC 50％左右,符合干旱缺水地区植物生长和高效用水的管理原则.
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