胡杨的夜间蒸腾——来自树干液流、叶片气体交换及显微结构的证据

不完全的气孔关闭引起的夜间蒸腾在不同物种和环境中普遍存在，且其大小与水汽压差和土壤水分有效性正相关，这意味着荒漠河岸林是研究夜间蒸腾的理想区域。本文基于木质部液流、叶片气体交换、显微结构及环境因子测定证实了胡杨夜间蒸腾的存在:1)夜间叶片气孔是不完全关闭的，平均气孔导度为45 mmol/(m2·s)，远大于文献报道的杨属最小气孔导度(约为5 mmol/(m2·s))，平均蒸腾速率为0.7 mmol/(m2·s)，两者分别占白天的26%和17%，这表明高的气孔导度和蒸腾速率主要是气孔开放引起；2)木质部平均液流速率白天为31.3 cm/h，夜间为16.5 cm/h，约为白天的53%，无论是白天还是夜间，液流速率与水汽压差均呈显著的对数关系，水汽压差可以解释55%的夜间液流变化，这表明夜间液流由蒸腾和组织补水两部分组成，因此，如何将夜间液流区分为夜间蒸腾和组织补水还有待进一步研究。午夜后液流速率的增加与木质部水势和径向生长变化是一致的，而与水汽压差是相反的，说明午夜后液流速率的增加是组织补水而非蒸腾。 

Nocturnal transpiration of Populus euphratica authenticated by measurements of stem sap flux,leaf gas exchange and stomatal microsturcture

Nocturnal transpiration, as a consequence of incomplete stomatal closure, is prevalent across species and environments, and high nocturnal atmospheric vapor pressure deficit ( VPD ) along with high soil water availability are the most commonly reported environmental drivers of it, which is coincided with the conditions of riparian forest in extremely arid region. Based on the measurements of xylem sap flux, leaf gas exchange, stomatal microstructure and environmental factors, we confirmed the occurrence of nocturnal transpiration in P. euphratica from followings:1) incomplete stomatal closure was observed and mean stomatal conductance was 45 mmol/(m2·s), greater than the minimum stomatal conductance of Populus spp. reported in the literature ( approximately 5 mmol/( m2 ·s ) ) , and along with mean transpiration of 0. 7 mmol/( m2·s) , both accounting for 26% and 17% of daytime, respectively. This suggested that high stomatal conductance and transpiration are largely resulted from stomatal opening. 2) On average, sap velocity was 31. 3 cm/hour at daytime and 16. 5 cm/hour at nighttime, which accounting for 53% of daytime. Whether during daytime or nighttime, sap velocity was logarithmic positively related with VPD, and this could explain 55% of nighttime sap velocity change, suggesting that nighttime sap flow was composed of transpiration and tissue refilling. Thus, further research to distinguish the nocturnal transpiration and tissue refilling is needed. Noteworthly, the abruptly increased sap flow after midnight was synchronized with the stem radius change and water potential, but oppositely with VPD, showing that increased sap flow after midnight was induced by tissue refilling but not transpiration.