LEAF THICKNESS AND UV-B ABSORBING PIGMENTS OF PLANTS IN RELATION TO AN ELEVATIONAL GRADIENT ALONG THE BLUE MOUNTAINS, JAMAICA

Terrestrial plane species vary widely in their adaptation to (increasi ng) solar W-B radiation. Among the various responses of higher plants to enhanced UV-B are increasing leaf thickness and increasing concentr ations of W-B absorbing compounds. In some (W-B resistant) plant speci es increased leaf thickness and W-B absorbance may form part of mechan isms protecting plants from UV-B damage. However, in UV-B sensitive pl ant species leaf thickness and UV-B absorbance may increase as well wi th enhanced UV-B radiation. In the latter case however, this response cannot prevent plant damage and disturbance. In the present field stud y the relationship between these plant parameters and a natural elevat ional UV-B gradient on the tropical island of Jamaica was described. F our plant species of the Blue Mountain Tropical Montane Forest, occurr ing on open forest sites along the roadside and paths were studied alo ng an elevational gradient. Plant species studied are Redbush (Polygon um chinense), Wild ginger (Hedychium gardneranum), John Crow Bush (Boc conia frutescens) and White clover (Trifolium repens). The elevational sites were at 800, 1000, 1200, 1400 and 1600 m above sea level. Leaf thickness was measured of leaves of intact plants around midday in the field. Leaf disks (5 mm) were sampled and extracted with a methanol/H Cl mixture. UV-B absorption of these leaf extracts was measured spectr ophotometrically. For all species leaves from higher elevations were t hicker than those from lower elevations. In addition, the absorption o f W-B of leaf extracts increased with increasing elevations. It is ass umed that the calculated gradient of the UV-B-BE from 800 m above sea level: 9.45 kJ m(-2) day(-1) to 9.75 kJ m(-2) day(-1) at 1600 m is rel ated to the measured increase of leaf thickness and W-B absorbing comp ounds. The responsiveness of these plant parameters to the elevational gradient does not necessarily imply that the plant species are W-B re sistant. One possibility is that the species studied, which are growin g on open, disturbed sites on the forest floor and along mountain-road s, are relatively sensitive to UV-B. In addition to clear sky conditio ns, mist and clouds occur frequently in this tropical mountane forest at Jamaica. Also, the low nutrient status of the soil (low pH, nutrien t deficiency) and the high content of polyphenols in leaves of many pl ant species of the tropical montane rain forest may relate to the mark ed response of the species studied with increasing elevation.