|Authors: ||D.H. Greer, J.N. Wünsche|
|Keywords: ||apple, carboxylation, photosynthesis, temperature response, seasonal response, RuBP regeneration|
Field measurements of gas exchange in leaves of apple (Malus domestica) trees growing at the Universität Hohenheim orchard were undertaken from mid stage of development until after harvest to enhance our understanding of seasonal limitations to photosynthesis.
Photosynthetic responses to CO2 (A/ci) at different leaf temperature (10-35°C) were measured on leaves of selected bourse shoots each subtending a fruit and also when the fruit were harvested.
A/ci responses were analysed to determine rates of carboxylation (Vcmax) of Ribulose 1, 5 Bisphosphate (RuBP) and rates of RuBP regeneration (Jmax) at each temperature.
Results showed that CO2 and light-saturated photosynthesis as well as Jmax were optimal at 25°C, but Vcmax was maximal at 30°C. It was also evident that at temperatures below about 25°C, photosynthesis was limited by regeneration of RuBP but above this, carboxylation of RuBP limited photosynthesis, in line with other species.
From mid to late season, the maximum photosynthetic capacity remained about constant, at 31 µmol m-2 s-1 as did Jmax but Vcmax declined about 3-fold.
After harvest, net photosynthesis was again reasonably constant but both Jmax and Vcmax increased.
However, photosynthesis was primarily limited by RuBP regeneration because of the temperatures declining during this period.
The conclusion of this study was that changes in sink capacity at harvest had no effect on the processes limiting photosynthesis.
However, because of the temperature-dependent transition from RuBP regeneration to RuBP carboxylation, photosynthesis was most likely carboxylation-limited in the early season when temperatures were high whereas regeneration of RuBP limited photosynthesis at the cooler time of harvest.
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