Rubisco activity in Mediterranean species is regulated by the chloroplastic CO2 concentration under water stress

Water stress decreases the availability of the gaseous substrate for ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) by decreasing leaf conductance to CO2. In spite of limiting photosynthetic carbon assimilation, especially in those environments where drought is the predominant factor affecting plant growth and yield, the effects of water deprivation on the mechanisms that control Rubisco activity are unclear. In the present study, 11 Mediterranean species, representing different growth forms, were subject to increasing levels of drought stress, the most severe one followed by rewatering. The results confirmed species-specific patterns in the decrease in the initial activity and activation state of Rubisco as drought stress and leaf dehydration intensified. Nevertheless, all species followed roughly the same trend when Rubisco activity was related to stomatal conductance (gs) and chloroplastic CO2 concentration (Cc), suggesting that deactivation of Rubisco sites could be induced by low Cc, as a result of water stress. The threshold level of Cc that triggered Rubisco deactivation was dependent on leaf characteristics and was related to the maximum attained for each species under non-stressing conditions. Those species adapted to low Cc were more capable of maintaining active Rubisco as drought stress intensified

Analysis of gas exchange in seedlings of Acer saccharum : integration of field and laboratory studies

In the field, photosynthesis of Acer saccharum seedlings was rarely light saturated, even though light saturation occurs at about 100 μmol quanta m -2 s -1 photosynthetic photon flux density (PPFD). PPFD during more than 75% of the daylight period was 50 μmol m -2 s -1 or less. At these low PPFD's there is a marked interaction of PPFD with the initial slope (CE) of the CO 2 response. At PPFD-saturation CE was 0.018 μmol m -2 s -1 /(μl/l). The apparent quantum efficiency (incident PPFD) at saturating CO 2 was 0.05–0.08 mol/mol. and PPFD-saturated CO 2 exchange was 6–8 μmol m -2 s -1 . The ratio of internal CO 2 concentration to external ( C i / C a ) was 0.7 to 0.8 except during sunflecks when it decreased to 0.5. The decrease in C i / C a during sunflecks was the result of the slow response of stomates to increased PPFD compared to the response of net photosynthesis. An empirical model, which included the above parameters was used to simulate the measured CO 2 exchange rate for portions of two days. Parameter values for the model were determined in experiments separate from the daily time courses being sumulated. Analysis of the field data, partly through the use of simulations, indicate that the elimination of sunflecks would reduce net carbon gain by 5–10%.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47755/1/442_2004_Article_BF00378907.pd

Interactive effects of light, leaf temperature, CO 2 and O 2 on photosynthesis in soybean

A biochemical model of C 3 photosynthesis has been developed by G.D. Farquhar et al. (1980, Planta 149, 78–90) based on Michaelis-Menten kinetics of ribulose-1,5-bisphosphate (RuBP) carboxylase-oxygenase, with a potential RuBP limitation imposed via the Calvin cycle and rates of electron transport. The model presented here is slightly modified so that parameters may be estimated from whole-leaf gas-exchange measurements. Carbon-dioxide response curves of net photosynthesis obtained using soybean plants ( Glycine max (L.) Merr.) at four partial pressures of oxygen and five leaf temperatures are presented, and a method for estimating the kinetic parameters of RuBP carboxylase-oxygenase, as manifested in vivo, is discussed. The kinetic parameters so obtained compare well with kinetic parameters obtained in vitro, and the model fits to the measured data give r 2 values ranging from 0.87 to 0.98. In addition, equations developed by J.D. Tenhunen et al. (1976, Oecologia 26, 89–100, 101–109) to describe the light and temperature responses of measured CO 2 -saturated photosynthetic rates are applied to data collected on soybean. Combining these equations with those describing the kinetics of RuBP carboxylase-oxygenase allows one to model successfully the interactive effects of incident irradiance, leaf temperature, CO 2 and O 2 on whole-leaf photosynthesis. This analytical model may become a useful tool for plant ecologists interested in comparing photosynthetic responses of different C 3 plants or of a single species grown in contrasting environments.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47469/1/425_2004_Article_BF00395048.pd