Aspects of light acclimation

A field study recognises three ways in which a forest canopy modifies the incoming solar radiation to give the light climate on the forest floor. First, it attenuates the intensity of light. Second , it alters the spectral quality of that light. Third , it reduces the effective daylength. The ability of photosynthetic species to respond to each of these was thus examined. Atherosperma moschatum and Elaeocarpus holopetalus are both shown to alter their development and carbon exchange characteristics in response to different light intensities. The carbon exchange characteristics indicate that A. moschatum is the more shade-tolerant of the two species, a conclusion supported by a field study of a forest where the two species co-dominate. A. moschatum is shown to be sensitive to nightbreaks of red/near infrared irradiation. In the experiment only stem elongation was significantly affected. Microcystis aeruginosa, a cyanobacterium, changes its carbon exchange and carbohydrate partitioning characteristics in response to different photoperiods and light intensities. The changes seem to relate to light and carbon use efficiencies under light limited conditions. A. mosqhaium is shown to respond in a similar way when conditioned to different daylengths, nightlengths and light intensities. Ananas comosus, pineapple, undergoing crassulacean acid metabolism is shown to acclimaJe to nightlength in a similar way to A. moschatum. The acclimation involves changes to both C3 and C4 photosynthetic metabolism. Within a single sapling of A. moschatum leaves are shown to acclimate to different light intensities in a manner similar to that shown earlier for comparisons between plants. The laboratory results are brought to bear on the developmental variation shown by saplings of A. moschatum growing in the same forest from which the light measurements which began this study were taken. The dimensions of leaves and internodes indicate light to be a major determinant of form within this forest.