Diurnal regulation of photosynthesis in understory saplings

Photosynthetic rates of plants grown in natural systems exhibit diurnal pat terns often characterized by an afternoon decline, even when measured under constant light and temperature conditions. Since we thought changes in the carbohydrate status could cause this pattern through feedback from starch and sucrose synthesis, we studied the natural fluctuations in photosynthesi s rates of plants grown at 36 and 56 Pa CO2 at a FACE (free-air-CO2-enrichm ent) research site. Light-saturated photosynthesis varied by 40% during the day and was independent of the light-limited quantum yield of photosynthes is, which varied little through the day. Photosynthesis did not correspond with xylem water potential or leaf carbohydrate build-up, but rather with d iurnal changes in air vapor-pressure deficit and light. The afternoon decli ne in photosynthesis also corresponded with decreased stomatal conductance and decreased Rubisco carboxylation efficiency which in turn allowed leaf-a irspace CO2 partial pressure to remain constant. Growth at elevated CO2 did not affect the afternoon decline in photosynthesis, but did stimulate earl y-morning photosynthesis rates relative to the rest of the day. Plants grow n at 56 Pa CO2 had higher light-limited quantum yields than those at 36 Pa CO2 but, there was no growth-CO2 effect on quantum yield when measured at 2 kPa O-2. Therefore, understory plants have a high light-limited quantum yi eld that does not vary through the day. Thus, the major diurnal changes in photosynthesis occur under light-saturated conditions which may help unders tory saplings maximize their sunfleck-use-efficiency.