Journal of Stress Physiology & Biochemistry, Vol. 7, No. 4,
2011, pp. 175- 186. ISSN 1997-0838
Original Text Copyright (cc) 2011 by Panda
Diurnal variations in gas exchange and chlorophyll fluorescence in rice leaves: the cause for midday depression in CO2 photosynthetic rate
Debabrata Panda
Division of Biochemistry, Plant
Physiology and Environmental Sciences, Central Rice Research Institute,
Cuttack-753 006, Orissa, India.
Present address: Rubber Research Institute of India, Tura, Meghalaya-794 001, India.
Tel: 91-3651-232413; Fax: 91-3651-232413
E-mail: dpanda80@gmail.com
Received September 13, 2011
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.
Key words: Chlorophyll fluorescence; Photosynthesis; Photo-system II; Photo-inhibition
