The Effects of Shade on the Gas Exchange of Seedlings of Four Tropical Trees from Mexico

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Abstract
The effects of two light regimes, "sun" and "shade", on the gas exchange of four tropical tree species [Brosimum alicastram, Swietenia macrophylla, Cedrella odorata and Cordia alliodora] from Mexico were evaluated. Photosynthetic and stomatal responses to light and humidity were studied using a controlled environment cabinet and an infra-red gas analyzer. A non-linear model was fitted to the photosynthetic responses and the physiological parameters found were used to compare treatments and species. Species from the secondary forest displayed higher maximal values of photosynthesis than those species normally found in primary forests. These species were also more sensitive to shade, i.e. maximal values of photosynthesis were grown in "shade". Plants grown under "sun" conditions displayed enhanced dark respiration, higher values of photosynthesis and their rate of photosynthesis continued to increase above a photon flux density of 400 .mu.mol m-2 s-1. When plants were raised in "sun" conditions, their leaves contained more nitrogen (on a leaf area basis) and this was associated with an increased mesophyll conductance. The species differed from each other much more in their growth than in their rates of photosynthesis and it is concluded that assimilate distribution may be more diagnostic of the ecological status of a species than its rate of assimilate production per area of leaf.

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