Irradiance frequently fluctuates under natural conditions and in greenhouses. When light intensity suddenly increases, leaf photosynthesis progressively increases rather than instantaneously increasing to the steady-state level (a process termed photosynthetic induction). The main limiting processes for photosynthetic induction are typically described as RuBP (ribulose-1,5-bisphosphate) regeneration, Rubisco (ribulose-1,5-bisphosphate carboxylase oxygenase) activation and stomatal opening. At high [CO2], photosynthesis induction tends to be faster mainly due to faster Rubisco activation rate and a reduction in stomatal limitation. Interestingly, we found that when CO2 level was very high (e.g. 1000 ppm, Fig. 1), leaf photosynthesis was quickly induced during the first 50-60 seconds, followed by a “dip” in photosynthesis rate for 1-2 minutes, after which photosynthesis rate stabilised. This “dip” is most likely caused by the insufficient activation of sucrose phosphate synthase (SPS), an enzyme in the triose utilization pathway following CO2 fixation by Rubisco. This slow activation of SPS leads to a transient phosphate limitation in the chloroplast, which negatively feeds back on assimilation rate. Nevertheless, the “dip” is not well described and characterized in literature. The aim of this project is to (1) explore at which [CO2] the “dip” happens and (2) to quantify to what extent the “dip” affects leaf net carbon gain during photosynthetic induction.
Fig. 1. Photosynthetic induction curve measured at 1000 ppm [CO2]. The leaf was firstly adapted to a light intensity of 50 μmol m-2 s-1, then at time point 0s, light intensity increased to 1000 μmol m-2 s-1
- Gas exchange measurements
- Data analysis
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