Effects of water deficit and competition on net photosynthesis of spring wheat (Triticum aestivum L.) and two annual weeds (Phalaris minor Retz. and Chenopodium album L.)

Abstract

Two pot experiments were performed to investigate the effects of water deficit and competition on net photosynthesis (Pn) of spring wheat cv. Alexandria and two C3 annual weed species (Phalaris minor Retz., Chenopodium album L.). The treatments tested included pure stands of wheat and weed species and a mixture containing them in equal proportions combined factorially with two water deficit treatments (well watered, watering withheld to maintain soil moisture at 50% of field capacity during short periods following the expansion of leaf 5 and the flag leaf of wheat). Water deficit significantly decreased Pn of all species, the observed decreases being slightly greater in wheat and P. minor than in C. album. The decreases in Pn were mainly due to decreases in stomatal conductance (gs), but changes in the concentration of sub-stomatal CO2 (Ci) indicated that non stomatal factors were also responsible, particularly in P. minor. In wheat and C. album, but not in P. minor, Pn recovered following re-watering. In the experiment with C. album the effect of interspecific competition was to increase Pn of wheat and decrease Pn of the weed. This was observed in well-watered plants and those subjected to water deficit. In the experiment with P. minor the effects of interspecific competition on Pn varied with growth stage and were modified by water deficit. An early deficit increased the relative competitive ability of the weed, and a late deficit increased the relative competitive ability of the wheat.