RESPONSES OF SELECTED SPECIES OF WOODY DICOTYLEDONS TO SUN AND SHADE

Ten species of native trees, ranging from obligate sun plants to facultative sun/shade plants were grown for several seasons in full sun and under an artificial shade passing about 17% of the incident photosynthetically-active radiation. Both whole plant and leaf responses were studied. The data suggest that the capacity to store food reserves is a very important factor contributing to shade tolerance. Every species experienced a reduction in stem-growth in the shade; almost every species showed greater reduction in the shade the second season than the first. Greater growth reduction the second season was probably due to the smaller carry-over of stored food reserves to support the spring flush of growth in the shade. Less growth reduction the second year in sugar maple probably resulted from the capacity to store a greater portion of photosynthates than are utilized in the current season\u27s growth. Death of cottonwood evidently was due to lack of stored reserves. Sun leaves were slanted 10-75(DEGREES) (mean 33(DEGREES)) and shade leaves 8-33(DEGREES) (mean 13.5(DEGREES)) from horizontal. No preferential orientation for compass direction was observed. The sun position will reduce ineffective light absorption and so enhance photosynthetic capacity and efficiency, on a whole plant basis, and promote individual leaf cooling during high solar radiation. The shade position will maximize the individual leaf photosynthetic input, and so maximize photosynthetic efficiency at low light intensity. Measurements of metabolic factors show different sun/shade responses, depending on the basis of comparison: unit area, unit fresh weight, unit protein or unit chlorophyll. On a unit area basis, sun leaves had a higher maximum photosynthetic rate, dark respiration rate, and light compensation point than shade leaves. Initial slopes of the light response curves were the same. This was also true per unit chlorophyll, for maximum photosynthetic rate and respiration. Per unit fresh weight, maximum photosynthetic rate and respiration were either higher in shade leaves or nearly the same as sun leaves of the same species. The same relationship was found using unit protein as for unit fresh weight. The initial slope of the light response curves was greater for shade leaves on a fresh weight basis. All of these findings are interpreted as indicating that on a per cell basis, shade leaves have more chlorophyll but the same total (or limiting) enzymic complement as sun leaves. Expressing these data in a different manner supports this interpretation. Density thickness (fr wt/area) measurements show that sun leaves were about twice as thick as shade leaves. Sun and shade leaves had about the same amount of chlorophyll per unit area, but shade leaves had double the chlorophyll per unit fresh weight. Total protein was the same in sun and shade leaves per unit fresh weight, but sun leaves had twice as much per unit area. Direct measurements of enzyme content, while less reliable, tended to follow the protein distribution. Furthermore, sun/shade ratios of enzyme activity followed those of maximum photosynthetic rate and respiration; ratios on an area basis were equal to those on a chlorophyll basis while ratios on a fresh weight basis were equal to those on a protein basis. Thus, the nature of the physiological sun/shade response in the leaves studied seems to be such that cell content of enzymes remains constant, but leaf thickness and pigment content per cell vary inversely. Combined with the response of leaf attitude, these responses appear designed to maintain an optimum relationship between the actual light energy input and the enzymatic capacity to use that energy. It is suggested that the change in cell content is the result of change in the structure of the chloroplast membranes