INFLUENCE OF INCREASING CARBON-DIOXIDE CONCENTRATION ON THE PHOTOSYNTHETIC AND GROWTH-STIMULATION OF SELECTED C-4 CROPS AND WEEDS

Plants of six weedy species (Amaranthus retroflexus, Echinochloa crus- galli, Panicum dichotomiflorum, Setaria faberi, Setaria viridis, Sorgh um halapense) and 4 crop species (Amaranthus hypochondriacus, Saccharu m officinarum, Sorghum bicolor and Zea mays) possessing the C-4 type o f photosynthesis were,at-own at ambient (38 Pa) and elevated (69 Pa) c arbon dioxide during early development (i.e. up to 60 days after sowin g) to determine: (a) if plants possessing the C-4 photosynthetic pathw ay could respond photosynthetically or in biomass production to future increases in global carbon dioxide and (b) whether differences exist between weeds and crops in the degree of response. Based on observatio ns in the response of photosynthesis (measured as A, CO2 assimilation rate) to the growth CO2 condition as well as to a range of internal CO 2 (C-i) concentrations, eight of ten C-4 species showed a significant increase in photosynthesis. The largest and smallest increases observe d were for A. retroflexus (+30%) and Z. mays (+5%), respectively. Weed species (+19%) showed approximately twice the degree of photosyntheti c stimulation as that of crop species (+10%) at the higher CO2 concent ration. Elevated carbon dioxide also resulted in significant increases in whole plant biomass for four C-4 weeds (A. retroflexus, E. crus-ga lli, P. dichotomiflorum, S viridis) relative to the ambient CO2 condit ion. Leaf water potentials for three selected species (A. retroflexus, A. hypochondriacus, Z. mays) indicated that differences in photosynth etic stimulation were not due solely to improved leaf water status. Da ta from this study indicate that C-4 plants may respond directly to in creasing CO2 concentration, and in the case of some C-4 weeds (e.g. A. retroflexus) may show photosynthetic increases similar to those publi shed for C-3 species.