Diurnal variations in gas exchange and chlorophyll fluorescence in rice leaves: the cause for midday depression in CO2 photosynthetic rate

Abstract

Gas exchange and chlorophyll fluorescence analysis were carried out to investigate the diurnal variations in photosynthesis in leaves of rice (Oryza sativa L.). Leaf CO2 photosynthetic rate (Pn) showed a bimodal diurnal pattern and midday depression in Pn was observed at 13:00 h. Depression in Pn at midday was mostly attributed to stomatal limitation since the reduction in Pn was followed by the significant reduction in stomatal conductance (Gs). Midday depression in Pn was found to be associated with reversible inactivation of Photo-system II (PS II) reaction centers and increase of photo-inhibition in response to high intensity as evidenced by the maximum efficiency of PS II (Fv/Fm) decreased with increase of light intensity from 6:00 h to 16:00 h of a day. The minimal fluorescence (Fo) gradually increased with increasing light intensity and reached its highest value at 13:00 h and on contrary the maximal fluorescence (Fm) decreased and reached its lowest value at 13:00 h. Quantification of several chlorophyll fluorescence parameters (JIP-test) like area above the fluorescence curve between Fo and Fm, phenomenological energy fluxes like electron transport per cross section (ETo/CS), active PS II reaction center per exited cross-section (RC/CSo) and performance index (Pi) were low in early morning, increasing with time and reaching a maximum at 9:00 h subsequently decreasing and reaching a minimum value at 13.00 h. On contrary the dissipation per cross-section (Dio/CS) gradually increased with increasing light intensity and reached its highest value at 13:00 h. It is likely that PS II down-regulation and heat dissipation co-operated together to prevent the chloroplast from photo damage.