Water stress in field‐grown perennial ryegrass.

SUMMARY Two sets of irrigated and water‐stressed swards of perennial ryegrass (Lolium perenne) growing in the field were compared. One set of swards was grown normally (field swards) and the other was established in boxes of low water‐holding capacity (simulated swards). Rain covers were used to exclude rain from half of both sets of swards: the others were kept within 20 mm of field capacity. Daily minimum values of leaf water potential fell to —12 bars (—1.2 MPa) in the irrigated swards, —16 bars in the stressed field swards and —20 bars in the stressed simulated swards. Dry‐matter production was reduced in the stressed field swards and leaf extension declined markedly in both the field and simulated swards. Canopy photosynthesis at saturating light intensity was reduced by about half in the stressed field swards and by more than 80% in the stressed simulated swards. In the former case a proportion of this reduction was due to the lower leaf area but the mean rate of leaf photosynthesis at saturating light intensity (Pmax/LAI) was reduced by about 40% and this was attributable to increased stomatal resistance calculated from canopy transpiration rates or measured with a porometer. The more marked decline in photosynthesis in the stressed simulated swards was not only the consequence of more complete stomatal closure but also a decrease in the quantum efficiency of photosynthesis. When water stress occurs naturally in the field it appears that changes in the leaves take place as stress is developing and these changes result in a less severe effect of water stress on leaf photosynthesis. When stress is imposed with unnatural rapidity, as in the simulated swards, there is no opportunity for these changes in the leaf to occur and stress leads to a rapid and severe decline in leaf photosynthesis. The importance of these findings for the grass sward under water stress is discussed.