According to the theory of optimal nitrogen partitioning within a leaf, the chlorophyll (Chl) a / b ratio is expected to increase when leaf N content decreases. Here, we report the first empirical support for this prediction. The Chl a / b ratio increased while Chl content decreased in response to N limitation in photosynthetic cotyledons and leaves of seedlings of four tropical woody species in the Bignoniaceae. The responses of all four species were in the same direction, but differed in magnitude. For Tabebuia rosea , the species that exhibited the greatest increase in Chl a / b ratios (up to values of 5.9), detailed photosynthetic characteristics were also examined. Light and N availability were positively correlated with the light-and CO 2 -saturated photosynthetic O 2 evolution rate, as well as with leaf carboxylation capacity ( V cmax ) and electron transport rate ( V j ). Severe N limitation and high light did not cause chronic photo-inhibition (i.e. no change in quantum yield or in dark-acclimated F v / F m ). The observed change in the ratio of V cmax to leaf N in response to N availability was consistent with likely functional reasons for change in the Chl a / b ratio. Adjustment of the Chl a / b ratio was apparently an integral feature of acclimation to high light conditions and low N availability.

Seedlings of nine tropical species varying in growth and carbon metabolism were exposed to twice the current atmospheric level of CO for a 3 month period on Barro Colorado Island, Panama. A doubling of the CO concentration resulted in increases in photosynthesis and greater water use efficiency (WUE) for all species possessing C metabolism, when compared to the ambient condition. No desensitization of photosynthesis to increased CO was observed during the 3 month period. Significant increases in total plant dry weight were also noted for 4 out of the 5 C species tested and in one CAM species, Aechmea magdalenae at high CO. In contrast, no significant increases in either photosynthesis or total plant dry weight were noted for the C grass, Paspallum conjugatum. Increases in the apparent quantum efficiency (AQE) for all C species suggest that elevated CO may increase photosynthetic rate relative to ambient CO over a wide range of light conditions. The response of CO assimilation to internal C suggested a reduction in either the RuBP and/or Pi regeneration limitation with long term exposure to elevated CO. This experiment suggests that: (1) a global rise in CO may have significant effects on photosynthesis and productivity in a wide variety of tropical species, and (2) increases in productivity and photosynthesis may be related to physiological adaptation(s) to increased CO.

Sudden but transient changes in the fraction or illuminated foliage area in a well‐watered 7‐year‐old Pinus radiata D. Don tree were imposed by completely covering either the upper 22% or the lower 78% of the foliage for periods of up to 36 h. Measurements of transpiration flux density (E), tree conductance (gt), stomatal conductance (gs) and net photosynthesis (A) were made to test the hypothesis that compensatory responses would occur in the remaining illuminated foliage when the cover was installed. When the lower foliage was covered there was an immediate decrease in gt. However, when tree conductance was normalized with respect to the illuminated leaf area (gt'), it increased between 50 and 75%, depending on the value of air saturation deficit (D). The effect was also apparent from concurrent measurements of increases in gs and A up to 59 and 24%, respectively, for needles in the top third of (he crown. When the cover was removed these effects were reversed. The changes in the lower foliage when the upper foliage was covered were much smaller. Changes in bulk needle water potential were small. It is suggested that the observed responses occurred because of a perturbation to the hydraulic pathway in the xylem that could have triggered the action of a chemical signal to regulate stomatal conductance and photosynthesis.

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Plants that resprout after fires often have higher rates of photosynthesis than before a fire. To elucidate the mechanism of this response, we studied gas exchange and chlorophyll fluorescence in Quercus ilex L. plants growing on control (unburned) sites and on sites that had been burned the preceding summer. In early July, photosynthetic rates and stomatal conductance were similar in plants on unburned and burned plots, and in young and old foliage within unburned plots. At this time, photochemical efficiency of photosystem II (PSII), nonphotochemical quenching of chlorophyll fluorescence (NPQ), and the de-epoxidation of violaxanthin to zeaxanthin were also similar among leaves of different ages and treatments. In late July, photosynthetic rates and stomatal conductances were much greater in resprouts on the burned areas than in unburned plants. From early to late July, unburned plants showed an increase in NPQ and the de-epoxidation of violaxanthin to zeaxanthin, indicating increased photoprotection as a result of enhanced nonradiative dissipation of excess light energy. Plants on the burned plots did not show these changes. Leaves of all ages and treatments showed no substantial reduction in potential quantum yield of PSII (F(v)/F(m)) at midday or predawn, indicating that there was little or no photoinhibition. Leaf nitrogen and soluble protein contents varied with leaf age during July, but did not vary between treatments. We conclude that the primary effect of burning is an increase in water availability to resprouting plants that eliminates the need for photoprotection, at least in the short term. The decrease in photosynthetic rates of unburned leaves in late July was the result of reduced stomatal conductance. We suggest that lowered stomatal conductance is the primary limiting factor in Q. ilex leaves, governing the regulation of carboxylation activity and energy dissipation processes.

Abstract. Very little attention has been directed at the responses of tropical plants to increases in global atmospheric CO2 concentrations and the potential climatic changes. The available data, from greenhouse and laboratory studies, indicate that the photosynthesis, growth and water use efficiency of tropical plants can increase at higher CO2 concentrations. However, under field conditions abiotic (light, water or nutrients) or biotic (competition or herbivory) factors might limit these responses. In general, elevated atmospheric CO2 concentrations seem to increase plant tolerance to stress, including low water availability, high or low temperature, and photoinhibition. Thus, some species may be able to extend their ranges into physically less favourable sites, and biological interactions may become relatively more important in determining the distribution and abundance of species. Tropical plants may be more narrowly adapted to prevailing temperature regimes than are temperate plants, so expected changes in temperature might be relatively more important in the tropics. Reduced transpiration due to decreased stomatal conductance could modify the effects of water stress as a cue for vegetative or reproductive phenology of plants of seasonal tropical areas. The available information suggests that changes in atmospheric CO2 concentrations could affect processes as varied as plant/herbivore interactions, decomposition and nutrient cycling, local and geographic distributions of species and community types, and ecosystem productivity. However, data on tropical plants are few, and there seem to be no published tropical studies carried out in the field. Immediate steps should be undertaken to reduce our ignorance of this critical area.

Figure 1. Sketch of the canopy crane operating in a forest showing the tower (a), jib (b), counterjib and counterweight (c), the operator's cab (d), and the gondola (e) attached to the hook. The radius and height of the jib are 82 and 52 m, respectively.

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