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

Abstract

Hymenaea courbaril, a widely distributed tropical leguminous tree, was used in two 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 five 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 one year under tong photoperiod treatments (LP) had an average stem length twice that of seedlings grown under an eight-hour photoperiod treatment (SP). In experiment 2, seedlings were grown under four photoperiods of nine hours 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 inctuded. STUDY OF THE TROPICAL LEGUMINOUS GENUS H3menaea (subfamily Caesalpinioideae) was initiated in our laboratory from a survey of resin-producing plan.ts through geologic time which showed that copious production of these compounds occurred predominantly in subtropical and tropical environments (Langenheim 1969, 1972, 1975). Because most of the current knowledge of resin-producing plants was from tem.perate-zone conifers, Hymenaea was selected as an example of a tropical angiosperm genus for an intensive ecological and evolutionary investigation of resin synthesis. This amphi-Atlantic genus, which includes 14 species (Lee and Langenheim 1975), seemed a highly appropriate choice because the Leguminosae are one of the most important resin-producing families and the subfamily Caesalpinioideae contains some of the most prominent components of New World and African rain-forest ecosystems (Langenheim 1973). Furthermore, it was evolutionarily interesting because of the occurrence of New World Hymenaea resins (am.ber) from the Tertiary epoch (Langenheim 1963, 1969) and ecologically because the species have radiated throughout the major tropical lowland ecosystems. One species, H. courbaril, has a remarkable distribution, not only essentially coincident with the New World distribution of the genus, but also occurring across the same range of ecosystems (fig. 1 ). Its occurrence in such a wide variety of habitats and considerable morphological and chemical variation led to the postulation of ecotype differentiation with respect to several environmental factors. Photoperiod is one factor in the physical environmen,t to which a widespread and variable species such as H. courbaril might become differentiated. Likewise, variation in resin chemistry between local populations led to consideration of possible ecological significance of this variation. Therefore, a comparative study of the effects of photoperiod on vegetative growth and resin chemistry of populations of H. courbaril was designed, and results emphasizing the chemical experiments have been reported previously (StLbblebine, Langenheim, and Lincoln 1976). In our collaborative work with Dr. Harry Borthwick, a pioneer authority on plant photoperiodism, we becaine aware of the importance of photoperiodism in tropical plants being generally dismissed in textbooks and reviews by many plant physiologists (Pauly 1958, Vaartaja 1959, Salisbury 1963, 1965, Salisbury and Ross 1969, Steward 1968, Mohr 1972, Meyer et al. 1973, Leopold and Kriedermann 1975); however, with notable exceptions (McClelland 1924, Avim 1964, Longman 1969, Longman and Jenik 1974). Also relatively few ecologists have pointed out its potential significance (see Daubenmire 1974: 233), and none of the above reviews or textbooks emphasizes the relationships between the physiological mechanisms of plant photoperiodism and the possible roles these responses have in tropical ecosystems. Therefore, here we are reviewing somne of the literature concerning the concepts of photoperiodism in plants, the physiological mechanisms of these responses, and their ecological roles under tropical conditions. The results of our experiments with H. courbaril are discussed in this context. 18 BIOTROPICA 10(1): 18-29 1978 This content downloaded from 207.46.13.14 on Sat, 16 Apr 2016 04:35:55 UTC All use subject to http://about.jstor.org/terms