Canopy Nutrient Allocation in Relation to Incident Light in the Tropical Fruit Tree Borojoa patinoi (Cuatr.)

The relationship of incident photosynthetically active radiation (PAR) and mineral nutrient allocation was evaluated in canopies of Borojoa patinoi (Cuatr.) growing in the Choco rainforest of Colombia, South America. Allocation of P in the canopy was positively correlated with incident PAR, principally because of increased leaf frequency (number of leaves per unit volume of canopy) brought about by local branching, with a smaller contribution from increased specific leaf weight (SLW, leaf dry weight per unit leaf area). The chemical fractionation of P within leaves did not respond to incident PAR. Canopy N allocation also was positively correlated with incident PAR because of increased leaf frequency and SLW. The N partitioning to soluble protein rather than chlorophyll was positively correlated with incident PAR. The allocation of K, Ca, Mg, S, Mn, and Cu also was positively correlated with incident PAR primarily because of increased leaf frequency and secondarily because of increased SLW. The area of individual leaves and the concentration of nutrients in leaf dry weight were not important in determining nutrient allocation responses to incident PAR. Our observations suggest that leaf frequency caused by local branching, followed by changes in SLW, are the primary determinants of canopy nutrient allocation in this tropical fruit tree. An understanding of the physiological factors influencing the efficiency of nutrient acquisition and use by fruit trees may contribute to the sustainability and productivity of fruit production in the United States, where intensive fertilization of orchard crops is of increasing concern as a source of groundwater contamination (Weinbaum et al., 1992) and in the tropics, where tree fruit production is often limited by low soil fertility (Opeke, 1982). The efficiency of nutrient use by plants has been defined in many diverse ways, as discussed recently by Clark (1990). In the present context, we will consider nutrient efficiency in terms of the economic analogy of resource allocation (Bloom et al., 1985), in which an efficient plant allocates limiting resources in a way that optimizes growth-limiting processes, as typified by photosyn- thetic C acquisition. In this study, we examined the role of canopy nutrient allocation as a part of overall nutrient efficiency of a tropical fruit tree native to low-P soils. Leaf N content often is correlated closely with leaf photosyn- thetic capacity, because both the light-harvesting apparatus and the CO2-assimilating enzymes have high N contents (DeJong, 1982, 1983). It has been proposed that C gain within plant canopies would be maximized by preferential partitioning of N to leaves receiving higher levels of light (Field, 1983; Mooney and Gulmon, 1979). This hypothesis has been supported by observation of the correlation of light and leaf N content within canopies (Charles- Edwards et al., 1987; DeJong and Doyle, 1985; Hirose and Werger,