Effects of irradiance and spectral quality on leaf structure and function in seedlings of two Southeast Asian Hopea (dipterocarpaceae) species

We studied the development of leaf characters in two Southeast Asian dipter ocarp forest trees under different photosynthetic photon flux densities (PF D) and spectral qualities (red to far-red, R:FR). The two species, Hopea he lferi and H. odorata, are taxonomically closely related but differ in their ecological requirements; H. helferi is more drought tolerant and H. odorat a more shade tolerant. Seedlings were grown in replicated shadehouse treatm ents of differing PFD and R:FR. We measured or calculated (1) leaf and tiss ue thicknesses: (2) mesophyll parenchyma, air space, and lignified tissue v olumes; (3) mesophyll air volumes (V-mes/A(surf)) and surfaces (A(mes)/A(su rf)); (4) palisade cell length and width; (5) chlorophyll/cm(2) and a/b; (6 ) leaf absorption; and (7) attenuance/absorbance. at 652 and 550 nm. These characters varied in response to light conditions in both taxa. Characters were predominantly affected by PFD, and R:FR slightly influenced many chara cters. Leaf characters of H. odorata were more plastic in response to treat ment conditions. Characters were correlated with each other in a complex fa shion. Variation in leaf anatomy is most likely a consequence of increasing leaf thickness in both taxa, which may increase mechanical strength and de fense against herbivory in more exposed environments. Variation in leaf opt ical properties was most likely affected by pigment photo-bleaching in trea tments of more intense PFD and was not correlated with A(max). The greater plasticity of leaf responses in H. odorata helps explain the acclimation ov er the range of light conditions encountered by this shade-tolerant taxon. The dense layer of scales on the leaf undersurface and other anatomical cha racters in H. helferi reduced gas exchange and growth in this drought-toler ant tree.