The developmental responses of papaya leaves to simulated canopy shade

The developmental responses of plants to shade underneath foliage are influenced by reductions in irradiance and shifts in spectral quality (characterized by reductions in the quantum ratio of red to far-red wavelengths, R:FR). Previous research on the influence of shadelight on leaf development has neglected the reductions in R:FR characteristic of foliage shade, and these studies have almost certainly underestimated the extent and array of developmental responses to foliage shade. We have studied the effects of reduced irradiance and R:FR on the leaf development of papaya (Carica papaya L., Caricaceae). Using experimental shadehouses, replicates of plants grown in high light conditions (0.20 of sunlight and R:FR = 0.90) were compared to low light conditions (0.02 of sunlight) with either the spectral quality of sunlight (R:FR = 0.99) or of foliage shade (F:FR = 0.26). Although many characteristics, such as leaf thickness, specific leaf weight, stomatal density, palisade parenchyma cell shape, and the ratio of mesophyll air surface/leaf surface were affected by reductions in irradiance, reduced R:FR contributed to further changes. Some characters, such as reduced chlorophyll a/b ratios, reduced lobing, and greater internode length, were affected primarily by low R:FR. The reduced R:FR of foliage shade, presumably affecting phytochrome equilibrium, strongly influences the morphology and anatomy of papaya leaves.

Correlates of leaf optical properties in tropical forest sun and extreme-shade plants

Thirteens hade-adaptedr ain forest species were comparedw ith twelve sun-adaptedt ropical forest species for correlates to leaf optical properties (described previously in Amer. J. Bot. 73: 1100-1108). The two samples were similar in absorptance of quanta for photosynthesis, but the shade-adaptedt axa: 1) had significantlyl ower specificl eaf weights,i ndicatinga more metabolically efficient production of surface for quantum capture; 2) synthesized less chlorophyll per unit area; and 3) used less chlorophyll for capturing the same quanta for photosynthesis. The anatomical features that best correlate with this increased efficiency are palisade cell shape and chloroplast distribution. Palisade cells with more equal dimensions have more chloroplasts on their abaxial surfaces. This dense layer of chloroplasts maximizes the light capture efficiency limited by sieve effects. The more columnar palisade cells of sun-adapted taxa allow light to pass through the central vacuoles and spaces between cells, making chloroplasts less efficient in energy capture, but allowing light to reach chloroplasts in the spongy mesophyll. Pioneer species may be an exception to these two groups of species. Three pioneer taxa included in this study have columnar palisade cells that are extremely narrow and packed closely together. This layer allows little penetration of light, but exposure of the leaf undersurface may provide illumination of spongy mesophyll chloroplasts in these plants.