The Influence of Water Stress on the Photosynthesis of a Simulated Sward of Perennial Ryegrass

Abstract

The rate of canopy photosynthesis, single leaf photosynthesis, leaf resistance to gaseous exchange, and leaf water potential of simulated swards of perennial ryegrass ( Lolium perenne cv. S24) in a controlled environment, were determined during a period of increasing water stress and recovery from that stress. Canopy photosynthesis did not decline immediately water was withheld but continued at an undiminished rate for several days; thereafter it fell rapidly, particularly at first. As water stress increased successive relationships between canopy photosynthesis and irradiance became more curved, indicating that the effect of water stress increased with increasing irradiance. After the swards were re-watered canopy photosynthesis rose, most rapidly during the first 24 h. In general, the pattern of change of leaf water potential was similar to that of canopy photosynthesis, although a more detailed examination of this relationship showed it to be hysteresial ; in particular, the fall in leaf water potential preceded that of canopy photosynthesis. Single leaf photosynthesis appeared to be the main agent through which water stress influenced canopy photosynthesis although in the more severely stressed swards (leaf water potentials of about -15 bars) some leaf tissue died and so limited the recovery of canopy photosynthesis. The leaf resistance to gaseous diffusion increased with increasing water stress, as did the C02 compensation point, thereby influencing single-leaf photosynthesis and through it canopy photosynthesis.