Influence of increasing carbon dioxide concentration on the photosynthetic and growth stimulation of selected C4crops and weeds

Plants of six weedy species (Amaranthus retroflexus, Echinochloa crus-galli, Panicum dichotomiflorum, Setaria faberi, Setaria viridis, Sorghum halapense) and 4 crop species (Amaranthus hypochondriacus, Saccharum officinarum, Sorghum bicolor and Zea mays) possessing the C4type of photosynthesis were grown at ambient (38 Pa) and elevated (69 Pa) carbon dioxide during early development (i.e. up to 60 days after sowing) to determine: (a) if plants possessing the C4photosynthetic pathway 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 observations in the response of photosynthesis (measured as A, CO2assimilation rate) to the growth CO2condition as well as to a range of internal CO2(Ci) concentrations, eight of ten C4species showed a significant increase in photosynthesis. The largest and smallest increases observed were for A. retroflexus (+30%) and Z. mays (+5%), respectively. Weed species (+19%) showed approximately twice the degree of photosynthetic stimulation as that of crop species (+10%) at the higher CO2concentration. Elevated carbon dioxide also resulted in significant increases in whole plant biomass for four C4weeds (A. retroflexus, E. crus-galli, P. dichotomiflorum, S. viridis) relative to the ambient CO2condition. Leaf water potentials for three selected species (A. retroflexus, A. hypochondriacus, Z. mays) indicated that differences in photosynthetic stimulation were not due solely to improved leaf water status. Data from this study indicate that C4plants may respond directly to increasing CO2concentration, and in the case of some C4weeds (e.g. A. retroflexus) may show photosynthetic increases similar to those published for C3species.

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