Limitations on photosynthesis under environment-simulating culturein vitro

Abstract

Limitations on photosynthesis, characterized by leaf CO2 exchange, chlorophyll fluorescence, and thylakoid structure, were studied under environmental conditions simulating culturein vitro. These were simulated by growingPhaseolus vulgaris plants in nutrient solution under high relative humidity of air (>90%), and CO2 concentrations (ca) that decreased with the development of photosynthetic activities during plant ontogeny (1200 to 300 mg m−3). The ontogeny of such model plants was more rapid, primary leaves reached photosynthetic maturity 2 to 3 d earlier and their life span was 7 to 14 d shorter than in control plants. Their photosynthetic activityin situ was limited, after reaching “photosynthetic maturity”, similarly to plants grownin vitro. When measured under optimal conditions, however, 50 to 70% higher net photosynthetic rates (PN) were found in leaves of different ages as compared with plants grown under ca of 700 mg m−3 and a lower air humidity (30–35%). This increase in PN was associated with a high conductance for CO2 transfer by adaxial and abaxial epidermes. In model plants, the dark respiration rate (RD) was almost twice that in the control, while the photorespiration rates were similar to controls; CO2 compensation concentration was about 50% of that in controls. The ratios PN/RD were similar in control and in model plants. Chlorophylla+b content in leaves of the model plants was lower than that in the control plants. Grana extent increased with plant age in the model plants while it decreased in the control ones. In both the stromal and granal membranes of the chloroplasts in model plants, a marked accumulation of carotenoids occurred independent of age. The ratio of variable to maximal fluorescence, Fv/Fm, did not differ in the model and the control plants. In the control plants, photochemical quenching (qP) slightly increased with plant age and was not affected by CO2 concentration present during measurement. In the model plants, qP increased with elevated CO2 concentration in young plants and decreased in saturating CO2 concentrations in older plants. Nonphotochemical quenching (qNP) was lower in the model plants and increased under CO2 saturating conditions. Vitality index, Rfd, was markedly lower in the model plants than in the control ones and a decline was found in saturating CO2 concentration.