Shoots of the tropical latex-producing tree Hevea brasiliensis (rubber tree) grow according to a periodic pattern, producing four to five whorls of leaves per year. All leaves in the same whorl were considered to be in the same leaf-age class, in order to assess the evolution of photosynthesis with leaf age in three clones of rubber trees, in a plantation in eastern Thailand. Light-saturated CO2 assimilation rate (Amax) decreased more with leaf age than did photosynthetic capacity (maximal rate of carboxylation, Vcmax , and maximum rate of electron transport, Jmax), which was estimated by fitting a biochemical photosynthesis model to the CO2-response curves. Nitrogen-use efficiency (Amax/Na, Na is nitrogen content per leaf area) decreased also with leaf age, whereas Jmax and
Vcmax did not correlate with Na. Although measurements were performed during the rainy season, the leaf gas exchange parameter that showed the best correlation with Amax was stomatal conductance (gs). An asymptotic function was fitted to the Amax-gs relationship, with R2 = 0.85. Amax, Vcmax, Jmax and gs varied more among different whorls in the same clone than among different clones in the same whorl. We concluded that leaf whorl was an appropriate parameter to characterize leaves for the purpose of modelling canopy photosynthesis in field-grown rubber trees, and that stomatal conductance was the most important variable explaining changes in Amax with leaf age in rubber trees. and B. Kositsup ... [et al.].
The crop sensitivity to ozone (O3) is affected by the timing of the O3 exposure, by the O3 concentration, and by the crop age. To determine the physiological response to the acute ozone stress, tomato plants were exposed to O3 at two growth stages. In Experiment I (Exp. I), O3 (500 μg m-3) was applied to 30-d-old plants (PL30). In Experiment II (Exp. II), three O3 concentrations (200, 350, and 500 μg m-3) were applied to 51-d-old plants (PL51). The time of the treatment was 4 h (7:30-11:30 h). Photosynthesis and chlorophyll fluorescence measurements were done 4 times (before the exposure; 20 min, 20 h, and 2-3 weeks after the end of the treatment) using a LI-COR 6400 photosynthesis meter. The stomatal pore area and stomatal conductance were reduced as the O3 concentration increased. Ozone induced the decrease in the photosynthetic parameters of tomato regardless of the plant age. Both the photosystem (PS) II operating efficiency and the maximum quantum efficiency of PSII photochemistry declined under the ozone stress suggesting that the PSII activity was inhibited by O3. The impaired PSII contributed to the reduced photosynthetic rate. The greater decline of photosynthetic parameters was found in the PL30 compared with the PL51. It proved the age-dependent ozone sensitivity of tomato, where the younger plants were more vulnerable. Ozone caused the degradation of photosynthetic apparatus, which affected the photosynthesis of tomato plants depending on the growth stage and the O3 concentration., A. A. Thwe, G. Vercambre, H. Gautier, F. Gay, J. Phattaralerphong, P. Kasemsap., and Obsahuje bibliografii