PHOTOSYNTHETIC CHARACTERISTICS OF SUN VERSUS SHADE PLANTS OF ENCELIA-FARINOSA AS AFFECTED BY PHOTOSYNTHETIC PHOTON FLUX-DENSITY, INTERCELLULAR CO2 CONCENTRATION, LEAF WATER POTENTIAL, AND LEAF TEMPERATURE

Limitations to photosynthesis were examined for Encelia farinosa Torrey et A.Gray, a common C-3 sub-shrub in arid regions of south-western United States, for plants grown in full sunlight and those shaded to 40% of full sunlight. The initial slopes of CO2 assimilation (A) versus intercellular CO2 concentration curves were similar for sun and shade plants at low photosynthetic photon flux density (PPFD) but higher for sun plants as the PPFD increased, indicating a greater limitation by carboxylation capacity in shade plants. Sun plants had higher electron transport rates but a lower ratio of electron transport capacity to carboxylation capacity (V-max); the ratio was inversely proportional to mesophyll conductance for both sun and shade plants. Dark respiration decreased with decreasing leaf water potential (psi(1)) in sun plants but remained unchanged in shade plants; day respiration was little affected by PPFD for both sun and shade plants. Stomatal conductance (g(s)) was similar for sun and shade plants under high soil-moisture conditions but higher in sun plants as psi(1) decreased; for all data considered together, changes in the leaf-air vapour pressure difference accounted for 71% of the variation in g(s). The lower A for shade plants of E. farinosa apparently resulted from a lower V-max, as well as a lower g(s) when plants were under water stress.