Swards of perennial ryegrass (Lolium perenne) were grown in the field and in boxes with low water-holding capacity (simulated swards). Water stress was allowed to build up in half the swards by using rain covers and withholding irrigation. Stress developed slowly in the field swards (over a period of 4 wk) but rapidly in the simulated swards (over a period of 5 days). Comparisons of leaf water potential, osmotic potential and stomatal resistance were made between these stressed swards and field and simulated swards whose soil water content were kept within 20 mm of field capacity. Measurements were made on the youngest fully expanded leaves. In the field swards the leaves on which measurements were made had grown during the development of stress, whereas in the simulated swards they were fully developed before the imposition of stress.
Daily minimum values of leaf water potential fell to —12 bars (-1.2 M Fa) in both field and simulated irrigated swards, to —16 bars in the stressed field swards and to —20 bars in the stressed simulated swards. Osmotic potential declined in the stressed leaves; in the field swards this ensured that turgor was maintained under stress but in the rapidly stressed simulated swards turgor fell to zero for a significant part of the day.
In all swards, leaf resistance measurements indicated that stomata began to close at leaf water potentials below —13 bars. There was no clear difference between slowly and rapidly stressed swards in the sensitivity of stomata to the decrease in leaf water potential.
Leaves which grew under slowly developing stress were morphologically different from leaves from non-stressed swards. The leaves were smaller, had a higher stomatal density, smaller epidermal cells and showed deeper ridging on their adaxial surface.