Contrasting CO2 and temperature effects on leaf growth of perennial ryegrass in spring and summer

The effects of increased atmospheric carbon dioxide (CO2) of 700 mu mol mol(-1) and increased air temperature of +4 degrees C were examined in Lolium perenne L. cv. Vigor, growing in semi-controlled greenhouses, Leaf growth, segmental elongation rates (SER), water relations, cell wall (tensiometric) extensibility (%P) and epidermal cell lengths (ECL) were measured in expanding leaves in spring and summer. In elevated CO2, shoot dry weight (SDW) increased in mid-summer, In both seasons, SDW decreased in elevated air temperatures with this reduction being greater in summer as compared to spring, Specific leaf area (SLA) decreased in elevated CO2 and in CO2 x temperature in both seasons, In spring, increased leaf extension and SER in elevated CO2 were linked with increased ECL, %P and final leaf size whilst in summer all were reduced, In high temperature, leaf extension, SER, %P and final leaf size were reduced in both seasons. In elevated CO2 x temperature, leaf extension, SER, %P, and ECL increased in spring, but final leaf size remained unaltered, whilst in summer all decreased. Mid-morning water potential did not differ with CO2 or temperature treatments. Leaf turgor pressure increased in elevated CO2 in spring and remained similar to the control in summer whilst solute potential decreased in spring and increased in summer, Contrasting seasonal growth responses of L. perenne in response to elevated CO2 and temperature suggests pasture management may change in the future, The grazing season may be prolonged, but whole season productivity may become more variable than today.