Zweifel R, Böhm J P, Häsler R
Institute of Plant Sciences, University of Bern, Altenbergrain 21, CH-3013 Bern, Switzerland.
Tree Physiol. 2002 Nov;22(15-16):1125-36. doi: 10.1093/treephys/22.15-16.1125.
We used local microclimatic conditions and twig sap flow rates to interpret midday stomatal closure in the canopies of two 250-year-old Norway spruce (Picea abies (L.) Karst.) trees at a subalpine site in the Swiss Alps (1,650 m a.s.l.). Both trees showed midday stomatal closure on most clear summer days, despite the permanently wet soil. We used a modified Penman-Monteith formula to simulate potential transpiration of single twigs (ET(T)) based on high-resolution temporal and spatial microclimate data obtained both inside and outside the crowns. Comparison of calculated ET(T) values and measured twig sap flow rates enabled us to pinpoint the occurrence of midday stomatal closure and the microclimatic conditions present at that time. We found that vapor pressure deficit (and for upper-crown twigs, ET(T)) largely explained the timing of initial midday stomatal closure but gave no explanation for the different patterns of stomatal behavior after initial closure in upper- and lower-crown twigs. After the initial stomatal closure, upper-crown twigs maintained high transpiration rates by continuously regulating stomatal aperture, whereas stomatal aperture decreased rapidly in lower-crown twigs and did not increase later in the day. Midday stomatal closure in lower-crown twigs occurred on average 1 h later than in upper-crown twigs. However, the microclimate at the time of initial stomatal closure was similar at both crown locations except that lower-crown twigs received significantly less solar radiation than upper-crown twigs both at the time of initial stomatal closure and afterwards. High rates of sap flow in twigs did not always lead to stomatal closure and therefore could not explain the phenomenon. We conclude that stomatal conductance can be modeled accurately only when both local microclimatic conditions and tree water status are known. Further, we hypothesize that both the quantity and quality of light play an important role in the reopening of closed stomata during the day.
我们利用当地的小气候条件和嫩枝液流速率，对瑞士阿尔卑斯山一个亚高山站点（海拔1650米）的两棵250年树龄的挪威云杉（Picea abies (L.) Karst.）树冠中午气孔关闭现象进行了解释。尽管土壤常年湿润，但在大多数晴朗的夏日，这两棵树都出现了中午气孔关闭现象。我们使用修正后的彭曼 - 蒙特斯公式，根据树冠内外获取的高分辨率时空微气候数据，模拟单个嫩枝的潜在蒸腾量（ET(T)）。通过比较计算得到的ET(T)值和实测的嫩枝液流速率，我们能够确定中午气孔关闭的发生情况以及当时存在的微气候条件。我们发现，水汽压差（对于树冠上部的嫩枝而言，还有ET(T)）在很大程度上解释了中午气孔最初关闭的时间，但对于最初关闭后树冠上部和下部嫩枝气孔行为的不同模式却无法给出解释。在最初的气孔关闭之后，树冠上部的嫩枝通过持续调节气孔孔径维持较高的蒸腾速率，而树冠下部嫩枝的气孔孔径迅速减小，且在当天晚些时候并未增大。树冠下部嫩枝中午气孔关闭平均比树冠上部嫩枝晚1小时。然而，除了在最初气孔关闭时以及之后，树冠下部嫩枝接收到的太阳辐射明显少于树冠上部嫩枝外，两个树冠位置在最初气孔关闭时的微气候是相似的。嫩枝中较高的液流速率并不总是导致气孔关闭，因此无法解释这一现象。我们得出结论，只有在同时了解当地微气候条件和树木水分状况时，才能准确模拟气孔导度。此外，我们推测光的数量和质量在白天关闭的气孔重新开放过程中都起着重要作用。
