Tree Physiology Advance Access published online on January 30, 2009
Tree Physiology, doi:10.1093/treephys/tpn041
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Midday depression of leaf CO2 exchange within the crown of Dipterocarpus sublamellatus in a lowland dipterocarp forest in Peninsular Malaysia
1 Graduate School of Agriculture, Kyoto University, Kyoto 606-8502, Japan
2 Corresponding author (ykosugi{at}kais.kyoto-u.ac.jp)
3 Forestry and Forest Products Research Institute, Tsukuba, Ibaraki 305-8687, Japan
4 Graduate School of Bioresources, Mie University, Tsu, Mie 514-8507, Japan
5 Forest Research Institute Malaysia, Kepong, Kuala Lumpur 52109, Malaysia
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Abstract |
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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 ~ 10 µmol 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.
Keywords: electron transport rate, net assimilation rate, patchy stomatal closure, stomatal conductance, tropical rainforest
Received October 6, 2008; Accepted November 23, 2008
Supplementary data for this article are available at Tree Physiology Online.