Abstract

Limitations to photosynthesis were examined for Encelia farinosa Toney et A.Gray, a common C₃ sub-shrub in arid regions of south-westem 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 (Ψ₁) 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 Ψ₁ 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.