Effects of groundwater depth on the gas exchange and chlorophyll fluorescence of Populus euphratica in the lower reaches of Tarim River
Effects of groundwater depth on the gas exchange and chlorophyll fluorescence of Populus euphratica in the lower reaches of Tarim River
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作者：
Fu Aihong
为揭示地下水埋深变化对干旱区胡杨气体交换和叶绿素荧光的影响，对胡杨气体交换日进程、光响应曲线、光合作用对CO2浓度的响应进行了测定和比较研究（PN-Ci曲线）、叶绿素荧光参数、叶水势和叶绿素含量结果表明，塔里木河下游4.91m、6.93m和8.44m三个海拔高度的植被净光合速率（PN）均高于塔里木河下游4.91m、6.93m和8.44m三个海拔高度的植被。随着地下水埋深从4.91 m增加到6.93 m和8.44 m，最小荧光（F0）、最大荧光（Fm）、PSⅡ的实际光化学效率（ΩPSⅡ）、表观电子传递速率（ETR）、非光化学猝灭系数（NPQ）和中午测定的叶水势都发生了明显的变化，其中NPQ增加了109%~ 127%。说明胡杨可以通过NPQ过程增加对多余激发能的热耗散，避免光合机构受到破坏，但ΩPSⅡ下降了24%~ 29%，说明地下水埋深的增加导致了光适应叶片的光化学变化。F0下降12.2%~ 18%，Fm下降7%~ 15.5%，ETR下降17%~ 22%，中午降水下降31.6%~ 45.6%，说明随着地下水埋深从4.91 m增加到6.93 m和8.44 m，泽兰遭受的干旱胁迫程度增加。当地下水埋深从6.93 m增加到8.44 m时，凌晨和中午测定的Fm、FV/F0和F0变化不明显，说明胡杨在此条件下能够保持PSⅡ的光化学活性，PSⅡ反应中心没有受到破坏，同样，中午测定的Fm、Fv/F0和ETR也没有明显变化，结果表明，地下水埋深在6.93 ~ 8.44 m的树木，其表观量子效率（φ）变化不大，光合结构受干旱胁迫的程度也很小; Rubisco羧化最大速率（Vcmax）、最大电子传递速率（J）和磷酸丙糖利用率（TPU）变化不明显，说明胡杨在一定程度上保持了电子传递活性和磷酸丙糖输出量;同时，随着地下水埋深的增加，光化学猝灭（qP）的变化不显著，说明不同埋深的胡杨PSⅡ反应中心的开放程度基本相同。光系统Ⅱ的最大光化学效率（Fv/Fm）在黎明前和中午有较高的值，分别在0.85 ~ 0.87和0.81 ~ 0.82之间，说明干旱胁迫对泽兰PSⅡ的原始光化学效率没有影响，光合结构没有受到破坏，总体上，泽兰在地下水4.91-8.44 m深度范围内所受的干旱胁迫程度没有超出树木的耐受范围，即即使地下水埋深增加到8.44m，泽兰的光合能力也没有受到不可逆的损害。
To reveal the effects of groundwater depth change on gas exchange and chlorophyll fluorescence of Populus euphratica grown in arid region,we measured and conducted comparative studies among the daily course of gas exchange,light response curve,photosynthetic response to CO2(PN-Ci curve),chlorophyll fluorescence parameters,leaf water potential,and chlorophyll content of P.euphratica grown at different groundwater depths(4.91m,6.93m and 8.44m) in the lower reaches of Tarim River.The results showed that the net photosynthetic rates(PN) measured at 10:00,the minimal fluorescence(F0),the maximal fluorescence(Fm),the actual photochemical efficiency of PSⅡ(ΩPSⅡ),the apparent rate of electron transport(ETR),the non-photochemical quenching coefficient(NPQ) and the leaf water potential measured at noon(Ψmidday) were obviously changed with the groundwater depth increasing from 4.91m to 6.93m and 8.44m.Among them NPQ increased by 109%-127%,suggesting that P.euphratica could increase the thermal dissipation of excess excitation energy by NPQ process to avoid suffering the damage of photosynthetic apparatus.However,ΩPSⅡ decreased by 24%-29%,indicating that the increasing groundwater depth resulted in the photochemical change of light-adapted leaves.Moreover,F0 decreased by 12.2%-18%,Fm decreased by 7%-15.5%,ETR decreased by 17%-22%,and Ψmidday decreased by 31.6%-45.6% respectively,showing that the degree of drought stress from which P.eupuratica suffered would increase when the groundwater depth increased from 4.91m to 6.93m and 8.44m.Additionally,when the groundwater depth increased from 6.93m to 8.44m,the Fm,FV/F0 and F0 measured at predawn and noon did not change obviously,showing that P.euphratica could hold the PSⅡ photochemical activity and the PSⅡ reaction center was not damaged under this condition.Similarly,Ψmidday,ΩPSⅡ and ETR also did not change significantly,showing that trees grown at groundwater depths between 6.93m and 8.44m likely suffered from the same degree of drought stress.When the groundwater depth ranged from 4.91m to 8.44m,the apparent quanta efficiency(φ) changed slightly,with minimal damage to photosynthetic structures;and the maximum velocity of Rubisco for carboxylation(Vcmax),the maximum potential rate of electron transport(J) and the triose phosphate utilization rate(TPU) did not change markedly,suggesting that p.euphtatica can keep the activity of electron transport on a certain extent and the output quantity of triose phosphate from leaves;simultaneously,the photochemical quenching(qP) changed insignificantly with the increasing groundwater depths,indicating that the opening degree of PSⅡ reaction center of P.euphratica grown at different groundwater depths was approximate the same.Moreover,the maximal photochemical efficiency of PSⅡ(Fv/Fm) measured at predawn and noon possessed the higher values ranging from 0.85 to 0.87 and from 0.81 to 0.82,respectively,indicating that PSⅡ original photochemical efficiency was not influenced and the photosynthetic structures were not damaged by the increasing drought stress.Overall,the drought stress degree P.eupuratica suffered from within the groundwater depths 4.91-8.44 m was not beyond the tolerance of trees,that is,the photosynthetic capability of P.eupuratica did not get an irreversible harm even if the groundwater depth increased to 8.44m.