The interactive effects of light, temperature and CO[subscript 2]/O[subscript 2] ratios on photosynthesis of Coix lachryma-jobi L.

Abstract

A portable infra-red gas analyzer was used to investigate the interactive effects of light, temperature, and CO[subscript 2]/O[subscript 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 degrees C) revealed no sign of light saturation even at a photosynthetic photon flux density (PPFD) close to 2000 µmol m[superscript -2]s[superscript -1]. High net assimilation rates (A) of approximately 24 µmol CO[subscript 2] m[superscript -2]s[superscript -1] were realized at 30-35 degrees C. Assimilation (A) versus internal CO[subscript 2] partial pressure Ci curves showed a steep rise in A with increase in C[subscript i] but saturated at approximately 150 µl[superscript -1] and the effect was similar in either the absence or presence of O[subscript 2], under all temperature regimes. C. lachryma-jobi exhibited low CO[subscript 2] compensation points (Γ*) between 0 and 10 µl[superscript -1] at either 0 or 21% O[subscript 2]. The slopes of double reciprocal plots of 1/A versus 1/C[subscript i], were nearly identical and crossed the y-intercept at almost identical points under all O[subscript 2] concentrations. This data indicated that there was no apparent O[subscript 2] inhibition and that the apparent inhibitor constant (K[subscript i]) for O[subscript 2] at the site of carboxylation did not change with an increase in [O[subscript 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[subscript 2]] occurred. These characteristics are thus consistent with typical features of C[subscript 4] photosynthesis.