Vegetative Response to Photoperiod in the Tropical Leguminous Tree Hymenaea courbaril L

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Abstract
H. courbaril, a widely distributed tropical leguminous tree, was used in 2 experiments to compare the effects of photoperiod on vegetative growth and leaf resin chemistry and to examine the role of photoperiod in ecotypic differentiation. Seedlings grown from seeds collected in 5 different populations which span the latitudinal range of the tropics were used in both experiments. In experiment 1, using natural light in the greenhouse, seedlings grown for 1 yr under long photoperiod treatments (LP) had an average stem length twice that of seedlings grown under and 8 h photoperiod treatment (SP). In experiment 2, seedlings were grown under 4 photoperiods of 9 h of daylight in the greenhouse plus 50 ft-c light extensions. Average total stem lengths were nearly identical for plants in all treatments. In LP plants growth nearly ceased during the winter but resumed in the spring. A LP/SP ratio based on average stem length for each population was correlated with latitude of seed origin suggesting ecotypic differentiation. Leaf resin composition was unaffected by photoperiod. Ecological implications of these responses are discussed. Also because the importance of photoperiodism in tropical plants generally has tended to be dismissed in reviews and textbooks, a literature review emphasizing physiological mechanisms of response and ecological roles under tropical conditions is included.

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