Leaf Anatomical Plasticity of Congeneric Rain Forest Tree Species in Relation to Light Acclimation

Abstract

Leaf structure of a seedling is an important indicator of the level of shade-drought-tolerance and the use of carbon assimilates of a species. The more shade-tolerant and conservative species have greater anatomical plasticity compared to more shade-intolerant and exploitive species. This study examined seedling leaf structure variation of four Syzygium species in the family Myrtaceae to different combinations of light environments. These species are usually found in the canopy/sub canopy in lowland rain forests. Six combinations of irradiance and spectral quality that found within the rain forest understorey to full open were used for this investigation. Two treatments exposed seedlings to different duration of direct sunlight (2h and 6h suntlecks). One treatment had the forest understorey light quality and quantity (50 Jlffiol m' S-I) and a control treatment exposed seedlings to full sunlight (1600 Jlmol m-2 S-I). Seedlings of Sifirmum, S. makul, S. operculatum, S. rubicundum were grown for two years within 24 environmental shelters. At the end of two years, leaves were sampled from each species in each light treatment. Subsequently, leaf blade thickness, stomatal frequency, and thickness of upper and lower epidermal and palisade mesophyll cell layers were measured. Analysis tested for differences and interactions among species and 22 Singhakumara, Gamage, Ashton light treatments. In general, leaf dimensions and cell layers of all species increased with increase in amount of light and were greatest in the full sunlight treatment (1600 J.I.molm·2S-1) S.firmum had the thickest leaf anatomical attributes while it was lowest for S. rubicundum. Greatest densities of stomata were measured for S. operculatum followed in declining order by S. makul, S.firmum and S. rubicundum. S. makul exhibited double rows of cells within the palisade mesophyll and lower epidermal layers while S. firmum exhibiited double rows of cells only within the Palisade mesophyll. S. firmum, with the thickest cuticle and leaf blade in full sun seems that it has a conservative carbon economy. The thinner leaf anatomical attributes in S. rubicundum when growing in the sun suggest that it is more exploitive compared to others but less water wasteful due to its lower stomatal frequency. S. operculatum is the most drought susceptible species due to its highest stomatal frequency. S. makul is moderate in terms of drought- tolerance and the use of carbon assimilates. The results demonstrate a close relationship between species leaf anatomy and their efficiency in use of water and carbon assimilates that had allowed them to occupy different sites within the rain forest environment.