The interactive effects of light, temperature and CO2/O2 ratios on photosynthesis of Coix lachryma-jobi L

A portable infra-red gas analyser was used to investigate the interactive effects of light, temperature, and CO 2 /O 2 ratios under controlled environmental conditions in an attempt to model gas exchange characteristics of Coix lachryma - jobi L. Plotting light response curves as a function of temperature (20, 25, 30 and 35°C) revealed no sign of light saturation even at a photosynthetic photon flux density (PPFD) close to 2000 μmol m -2 s -1 . High net assimilation rates ( A ) of approximately 24 μmol CO 2 m -2 s -1 were realized at 30 – 35°C. Assimilation ( A ) versus internal CO 2 partial pressure ( C i ) curves showed a steep rise in A with increase in C i but saturated at approximately 150 μl l -1 and the effect was similar in either the absence or presence of O 2 , under all temperature regimes. C . lachryma - jobi exhibited low CO 2 compensation points ( Γ *) between 0 and 10 μl l -1 at either 0 or 21% O 2 . The slopes of double reciprocal plots of 1/ A versus 1/ C i , were nearly identical and crossed the y-intercept at almost identical points under all O 2 concentrations. This data indicated that there was no apparent O 2 inhibition and that the apparent inhibitor constant ( K ) for O 2 at the site of carboxylation did not change with an increase in [O 2 ] from 0 to 21%. These observations were further confirmed by results obtained from the analysis of apparent carboxylation efficiency where no inhibition of A with increase of [O 2 ] occurred. These characteristics are thus consistent with typical features of C 4 photosynthesis.