Control of the Acclimation of Photosynthesis to Light and Temperature in Relation to Partitioning of Photosynthate in Developing Soybean Leaves

Abstract

Photosynthetic acclimation was examined by exposing third trifoliolate leaves of soybeans to air temperatures of 20 to 30°C and photosynthetic photon flux densities (PPFD) of 150 to 950μmol photons m−2 s−1 for the last 3 d before they reached maximum area. In some cases the environment of the third leaf was controlled separately from that of the rest of the plant. Photosynthesis, respiration and dry mass accumulation were determined under the treatment conditions, and photosynthetic capacity, and dry mass and protein content were determined at full expansion. Photosynthetic capacity, the light-saturated rate of net carbon dioxide exchange at 25°C and 34 Pa external partial pressure of carbon dioxide, could be modified between 21 and 35 μmol CO2 m−2 s−1 by environmental changes after leaves had become exporters of photosynthate. Protein per unit leaf mass did not differ between treatments, and photosynthetic capacity increased with leaf mass per unit area. Photosynthetic capacity of third leaves was affected by the PPFD incident on those leaves, but not by the PPFD on other leaves on the plant. Photosynthetic capacity of third leaves was affected by the temperature of the rest of the plant, but not by the temperature of the third leaves. Photosynthetic capacity was linearly related to carbon dioxide exchange rate in the growth regimes, but not to daytime PPFD. At high PPFD, and at 25 and 30°C, mass accumulation was about 28% of the mass of photosynthate produced. At lower PPFD, and at 20°C, larger percentages of the photosynthate produced accumulated as dry mass. The results suggest that photosynthate supply is an important factor controlling leaf structural growth and, consequently, photosynthetic acclimation to light and temperature.