Effects of water stress on leaf growth in tall fescue, Italian ryegrass and their hybrid: rheological properties of expansion zones of leaves, measured on growing and killed tissue

In Expt 1, plants of tall fescue (Festuca arundinacea Schreb.), Italian ryegrass (Lolium multiflorum Lam.) and their F1 hybrid were grown in soil-based compost in a controlled environment, and subjected to full or partial irrigation for 20 d. In Expt 2, plants of the parent species were grown in nutrient solution in the same environment and subjected to osmotic stress (0.76 MPa) for 2 d. In both experiments, distribution of growth in the leaf growing zone (at the base of the growing leaf) was determined, and elastic and plastic compliances were measured on methanol-killed samples of growing zone and of mature lamina using an extensiometer. In Expt 2 plastic compliance, coefficient of extension, extensibility, and hydraulic conductance were calculated from changes in leaf extension rate occasioned by imposing linear stress.

Plastic and elastic compliances of growing zones were 10–20 times greater than those of mature laminae. In both species, drought reduced (a) leaf extension rate, (b) the length of the growing zone, (c) the height of maximum growth, (d) the plastic compliance of whole bases (Expt 1), and (e) hydraulic conductance. The elastic compliance of whole leaf bases was unaffected by drought, but when expressed per unit length of growing zone was increased by drought. Killing with methanol reduced the plastic compliance of leaf bases in control plants, but not in droughted plants.

F. arundinacea differed from L. multiflorum in having (a) a lower leaf extension rate (although drought reduced extension by the same proportion in both species), (b) a longer growing zone in droughted plants in Expt 2, (c) a lower elastic and plastic compliance of whole killed leaf bases and laminae, (d) slightly higher plastic compliance in attached growing leaves, and (e) lower plastic compliance per unit length of growing zone in attached leaves. The hybrid was generally intermediate between the parents. The results are discussed in relation to methodology and to crop improvement.