ACCUMULATION AND CONSUMPTION OF SOLUTES IN SWARDS OF LOLIUM-PERENNE DURING DROUGHT AND AFTER REWATERING

Simulated swards of Lolium perenne were grown in containers 0.9 m deep in the glasshouse and subjected to drought for 0 (control) 2, 4, 6 or 8 wk. Leaf growth, water relations and accumulation of solutes (sugars, amino acids and minerals) were measured in mature leaf laminae and in young meristematic leaf bases. The swards were then rewatered and fertilized, and growth and solute changes measured over the next 8 d. After 8 wk of drought leaf extension had stopped, leaf laminae were scorched at the tips, cell membranes were damaged, and osmotic potential (psi-s) had fallen to -2.6 and -2.8 MPa in laminae and bases respectively. Hydration (ratio of tissue water to dry matter) declined steadily to less than half control values, but relative water content declined significantly only after more than 6 wk drought. Nearly all osmotic adjustment was the result of these changes in water content, since molarity of total measured solutes on a dry weight basis increased by only 11% in laminae and fell by 6% in bases. During drought, water soluble carbohydrate contents increased, mostly due to accumulation of large fructan molecules having little effect on psi-s. In bases smaller fructans (having degrees of polymerization, DP, of 3-5), glucose and fructose decreased, whereas in laminae fructan of DP3 and sucrose increased. Relatively large amounts of neo-ketose accumulated in laminae after 8 wk drought. Proline contents of bases increased steadily during drought, whilst in laminae proline contents were generally very low, and increased only after severe drought. Minerals made the major contribution to psi-s. Total mineral contents of laminae rose slightly due mostly to accumulation of Ca2+ and Mg2+, whereas in bases mineral contents tended to decline. Phosphate content, however, declined in both bases and laminae. Following rewatering, tissues rehydrated within 2 d. Water potential rose more rapidly than psi-s, so that after 2 d turgor temporarily exceeded control values. Leaf extension rates changed almost in parallel with turgor. Carbohydrates in leaf bases changed rapidly: fructans of DP3-5 and fructose initially increased and then declined, whereas sucrose and glucose contents fell by more than 50% and remained low. Proline contents fell to control values within 1-2 d, whereas phosphate contents (already low) continued to decline. The changes were generally more rapid in the bases than in the laminae. The roles of solutes in osmotic adjustment and as substrates for regrowth are discussed. The differences between slow, relatively natural drought and rapid stressing of pot-grown plants are emphasized.