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 interact ive effects of light, temperature, and CO2/O2 ratios under controlled environmental conditions in an attempt to model gas exchange character istics of Coix lachryma-jobi L. Plotting light response curves as a fu nction of temperature (20, 25, 30 and 35-degrees-C) revealed no sign o f light saturation even at a photosynthetic photon flux density (PPFD) close to 2000 mumol m-2 s-1. High net assimilation rates (A) of appro ximately 24 mumol CO2 m-2s-1 were realized at 30 - 35-degrees-C. Assim ilation (A) versus internal CO2 partial pressure (C(i)) curves showed a steep rise in A with increase in C(i) but saturated at approximately 150 mul l-1 and the effect was similar in either the absence or prese nce of O2, under all temperature regimes. C. lachryma-jobi exhibited l ow CO2 compensation points (GAMMA) between 0 and 10 mul l-1 at either 0 or 21% O2. The slopes of double reciprocal plots of 1/A versus 1/C( i), were nearly identical and crossed the y-intercept at almost identi cal points under all O2 concentrations. This data indicated that there was no apparent O2 inhibition and that the apparent inhibitor constan t (K(i)) for O2 at the site of carboxylation did not change with an in crease in [O2] from 0 to 21%. These observations were further confirme d by results obtained from the analysis of apparent carboxylation effi ciency where no inhibition of A with increase of [O2] occurred. These characteristics are thus consistent with typical features of C4 photos ynthesis.