After 45 d of limited water supply, cassava (Manihot esculenta Crantz) exhibited pronounced reduction in shoot growth, high leaf fall, and decreased stomatal conductance. However, the water status of the remaining leaves was unaffected. This was combined with an amplified heliotropic response and drooping which minimises radiant energy interception at mid-day, suggesting that leaves are sensitive to high irradiance (I). In well-irrigated plants, CO2-saturated oxygen evolution and net photosynthetic rate (PN) in air were markedly higher (5-fold) in young (expanding) leaves than in mature leaves. Water limitation did not strongly modify CO2-saturated oxygen evolution but it altered PN in air for both types of leaves, although differently. The mature leaves of drought-adapted plants displayed residual rate of P N and deteriorated photosystem 2 (PS2) photochemistry estimated from chlorophyll (Chl) a fluorescence measurements. In young leaves at moderate I, PN was depressed by only 66 % in stressed plants. Moreover, the photochemical quenching of Chl a fluorescence and the quantum efficiency of PS2 photochemistry in young leaves were comparable in both control and stressed plants. In contrast at high I, PN was almost null and marked decreases in the two fluorescence parameters were apparent. Hence the strong heliotropic response and drooping displayed by young leaves under water limitation is an important strategy for avoiding inactivation of PN by high I and therefore for cassava tolerance to drought. and P.-A. Calatayud ... [et al.].
The objectives of the study were to characterize photosynthesis of temperate fallow C3 herbaceous species and examine the performance of a simple photosynthesis model (based on the Farquhar's equations) to simulate carbon fluxes at the leaf and canopy levels. The maximum rate of carboxylation at 25°C (V m0) was estimated for sunlit leaves using in situ gas exchange data under saturating irradiance. Throughout the seasons, leaf measurements indicate that values of V m0 were similar for the four major species of the fallow. The rate declined from March (100 µmol m-2 s-1) to July (50 µmol m-2 s-1) and remained almost constant until November. The maximum quantum yield estimated for Potentilla reptans L. (dominant species) was 0.082 mol(CO2) mol-1(photon absorbed), similar to values already published for C3 species. Leaf area index (LAI) increased from winter (less than 0.2 m2 m-2) to spring (up to 4 m2 m-2). Rates of canopy photosynthesis (measured with a canopy chamber) strongly depended on LAI and temperature, in addition to irradiance. They reached a maximum of 25 µmol m-2 s-1 and were intermediate between those published for C4 grassland or cultivated species, and on woody species. At leaf level, simulations gave realistic predictions. At canopy level, the model had the ability to reproduce the effects of environmental and seasonal conditions. However, simulations underestimated the photosynthetic activity of the fallow canopy. and M. Gouasmi ... [et al.].