An in situ approach for characterizing photosynthetic gas exchange of rice panicle

Abstract Background: Photosynthesis of reproductive organs in C 3 cereals is generally regarded as important to crop yield. Whereas, photosynthetic characteristics of reproductive organs are much less understood as compared to leaf photosynthesis, mainly due to methodological limitations. To date, many indirect methods were developed to study photosynthesis of reproductive organs and its contribution to grain yield, such as shading, application of herbicides or photosynthetic measurement of excised organs or tissues, which might be intrusive and cause biases. Thus, a robust and in situ approach needs to be developed. Results: Here we report the development of a custom-built panicle chamber (P-chamber), which can be connected to a standard infrared gas analyzer to study photosynthetic/respiratory rate of a rice panicle. With this P-chamber, we measured photosynthetic characteristics of panicles for seven high-yielding elite japonica , japonica-indica hybrid and indica rice cultivars. Results show that, 1) rice panicle is photosynthetically active during grain filling, and there are substantial inter-cultivar variations in panicle photosynthetic and respiratory rates, both for a whole panicle and on a single spikelet basis; 2) among the seven testing cultivars, light saturated whole-panicle gross photosynthetic rates at early grain filling stage are 17 – 54 nmol s -1 , which are estimated to be 20-38% of the corresponding flag leaves; 3) rice panicle photosynthesis has higher apparent CO 2 compensation point, light compensation point and apparent CO 2 saturation point, as compared to that of a typical leaf; 4) there is a strong and significant positive correlation between gross photosynthetic rate at early grain filling stage on a single spikelet basis and grain setting rate at harvest (Pearson correlation coefficient r = 0.93, p -value < 0.0001). Conclusions: Overall, the P-Chamber can be used as a tool to study in situ photosynthetic characteristics of irregular non-foliar plant organs, which is a relatively less explored area in current cereal breeding community.