民勤绿洲荒漠过渡带植物白刺和梭梭光合特性

研究了民勤绿洲荒漠过渡带两种优势植物白刺(Nitrariatangutorum)和梭梭(Haloxylonammodendron)气体交换特征参数和叶绿素荧光参数的日变化,结果表明:梭梭同化枝日均净光合速率(Pn)、蒸腾速率(E)、水分利用效率(WUE)和早晨PSII最大光化学效率(Fv/Fm)明显低于白刺叶片;同时它们在中午都呈现明显的光合“午休”现象。通过气孔限制值(Ls)、胞间二氧化碳浓度(Ci)和叶绿素荧光参数(Fv/Fm)变化趋势分析表明:梭梭同化枝净光合速率的下降在9:00~12:00主要是由于气孔关闭引起的,而在12:00~15:00主要是非气孔因素光抑制的影响;而白刺叶片光合速率的下降主要是气孔关闭引起的。同时,梭梭在一天中其同化枝的温度(Tleaf)高于大气温度尤其在中午差值更大,而白刺叶片的温度却低于大气温度,因此说明梭梭同化枝在中午不仅遭受着干旱和强光的胁迫而且面临严酷的高温胁迫。总之,在绿洲荒漠过渡带生境中白刺比梭梭具有更好的适应性,这可能是民勤绿洲荒漠过渡带人工梭梭林衰退的重要原因之一。

Photosynthetic characteristics of Nitraria tangutorum and Haloxylon ammodendron in the ecotone between oasis and desert in Minqin, Region, Country

Nitraria tangutorum and Haloxylon ammodendron are dominate the ecotone between oasis and desert in Minqin. The artificial H. ammodendron communities have degenerated where the water table declined after heavy pumping of the groundwater for agriculture. The diurnal variations of gas exchange and chlorophyll fluorescence of green branches in H. ammodendron and leaves of N. tangutorum were investigated in the ecotone. Net photosynthetic rate (P_n), Transpiration rate (E), water use efficiency (WUE) and predawn photochemical efficiency of PSII (F_v/F_m) in H. ammodendron were lower than in the leaves of N. tangutorum, while both species showed midday photosynthetic depression. Stomatal limitations (L_s), intercellular CO_2 concentration (C_i) and F_v/F_m showed that the midday (9:00 to 12:00) photosynthetic depression in H. ammodendron was predominantly due to stomatal limitation. By contrast, afternoon (12:00 to 15:00) depression of P_n was caused by non-stomatal limitation, namely, photoinhibition. Photosynthetic depression of N. tangutorum was predominantly due to stomatal limitation at all periods of the day. The temperatures of assimilating branches (T_(leaf)) of H. ammodendron were higher than the air temperatures (T_(air)) especially at noon, while the T_(leaf) of N. tangutorum were lower than T_(air). So the assimilating branches of H. ammodendron not only suffered from the drought and strong-light stress but also from the rigorous high temperature stress compared with N. tangutorum at noon. In conclusion, the results of physiological research on photosynthesis suggested that the N. tangutorum adapt to the environmental conditions of the ecotone between oasis and desert more than H. ammodendron, and this may be one of the important reasons of why the artificial N. tangutorum communities in the ecotone between oasis and desert degenerated.