Nitrogen Deficiency in Small Closed Communities of S24 Ryegrass. I. Photosynthesis, Respiration, Dry Matter Production and Partition

Abstract

Small communities of S24 ryegrass were grown under supplementary lights in a glasshouse at 20°C, and abundantly supplied with a complete nutrient solution containing 300 p.p.m. of nitrogen, until they had a leaf area index of 5 and fully intercepted the light. Half were then given a solution containing only 3 p.p.m. of nitrogen (LN) while the rest were kept at 300 p.p.m. (HN). The LN plants had a rate of single leaf photosynthesis lower than that of the HN plants at all but the lowest light intensities (33 per cent lower at the saturating irradiance of 170 W m ⁻² ). Similarly, the LN communities had rates of canopy gross photosynthesis (P _sc ) markedly lower than those of the HN communities. A comparison of the observed rates of P _sc with those predicted by a mathematical model of canopy photosynthesis indicated that it was the effect of nitrogen on single leaf photosynthesis, rather than differences between the communities in leaf area, which led to the observed differences in P _sc . The superiority of the HN communities in terms of P _sc was partly offset by a higher rate of respiration so that they only exceeded the LN communities in terms of canopy net photosynthesis at irradiances in excess of 180 W m ₋₂ , and produced only 15 per cent more total dry matter. Nevertheless, the HN plants directed less of that dry matter into root and more into tops so that they came to possess twice the weight of live laminae, and the HN communities twice the leaf area, of their nitrogen deficient counterparts.