Kosugi Yoshiko, Takanashi Satoru, Matsuo Naoko, Nik Abdul Rahim
Graduate School of Agriculture, Kyoto University, Kyoto 606-8502, Japan.
Tree Physiol. 2009 Apr;29(4):505-15. doi: 10.1093/treephys/tpn041. Epub 2009 Jan 30.
We observed diurnal and seasonal patterns of leaf-scale gas exchange within the crown of a Dipterocarpus sublamellatus Foxw. tree growing in a lowland dipterocarp forest at Pasoh, Peninsular Malaysia. Observations were carried out nine times over 6 years, from September 2002 to December 2007. Observation periods included both wet and mild-dry periods, and natural and saturated photosynthetic photon flux density (PPFD) light conditions. In situ measurements of the diurnal change in net photosynthetic rate and in stomatal conductance were carried out on canopy leaves of a 40-m-tall D. sublamellatus tree, which was accessed from a canopy corridor. A diurnal change in electron transport rate was observed under saturated PPFD conditions. The maximum net assimilation rate was approximately 10 micromol m(-2) s(-1). There was a clear inhibition of the net assimilation rate coupled with stomatal closure after late morning and this inhibition occurred year-round. Although the electron transport rate decreased alongside this inhibition, it sometimes followed on. Numerical analysis showed that the main factor in the inhibition of the net assimilation rate was patchy bimodal stomatal closure, which occurred in both mild-dry and wet periods. The midday depression occurred year-round, though there are fluctuations in soil moisture during the mild-dry and wet periods. The magnitude of the inhibition was not related to soil water content but was related to vapor pressure deficit (VPD): that is, whether the days were sunny and hot or cloudy and cool. On cloudy, cool days in the wet period, the net photosynthesis was only moderately inhibited, but it still decreased in the afternoon and was coupled with patchy stomatal closure, even in quite moderate VPD, leaf temperature and PPFD conditions. Our results suggest that patchy stomatal closure signaled by the increase in VPD, in transpiration and by circadian rhythms, was the key factor in constraining midday leaf gas exchange of the D. sublamellatus canopy leaves.
我们观察了马来西亚半岛帕索低地龙脑香林内一株狭叶坡垒（Dipterocarpus sublamellatus Foxw.）树冠内叶尺度气体交换的昼夜和季节模式。观测在2002年9月至2007年12月的6年时间里进行了9次。观测期包括湿润期和轻度干旱期，以及自然光合有效辐射（PPFD）光照条件和饱和PPFD光照条件。通过树冠走廊对一棵40米高的狭叶坡垒树的冠层叶片进行了净光合速率和气孔导度昼夜变化的原位测量。在饱和PPFD条件下观测了电子传递速率的昼夜变化。最大净同化率约为10微摩尔·米-2·秒-1。上午晚些时候后，净同化率明显受到抑制，同时气孔关闭，且这种抑制全年都会发生。尽管电子传递速率随着这种抑制而下降，但有时仍会持续。数值分析表明，净同化率受到抑制的主要因素是斑驳的双峰气孔关闭，这在轻度干旱期和湿润期都会出现。尽管在轻度干旱期和湿润期土壤湿度存在波动，但中午光合抑制全年都会发生。抑制的程度与土壤含水量无关，而是与蒸汽压亏缺（VPD）有关：也就是说，无论天气是晴朗炎热还是多云凉爽。在湿润期的多云凉爽日子里，净光合作用仅受到适度抑制，但下午仍会下降，并且即使在相当适度的VPD、叶片温度和PPFD条件下，也会伴随着斑驳的气孔关闭。我们的结果表明，由VPD增加、蒸腾作用增加和昼夜节律发出信号的斑驳气孔关闭是限制狭叶坡垒冠层叶片中午气体交换的关键因素。
