Water deficit and induction of summer dormancy in perennial Mediterranean grasses.

BACKGROUND AND AIMS: Summer dormancy is a trait conferring superior drought survival in Mediterranean perennial grasses. As the respective roles of environmental factors and water deficit on induction of summer dormancy are unclear, the effect of intense drought were tested under contrasting day lengths in a range of forage and native grasses. METHODS: Plants of Poa bulbosa, Dactylis glomerata 'Kasbah' and Lolium arundinaceum 'Flecha' were grown in pots (a) from winter to summer in a glasshouse and subjected to either an early or a late-spring drought period followed by a summer water deficit and (b) in controlled conditions, with long days (LD, 16 h) or short days (SD, 9 h) and either full irrigation or water deficit followed by rehydration. Leaf elongation, senescence of aerial tissues and dehydration of basal tissues were measured to assess dormancy. Endogenous abscisic acid (ABA) in basal tissues was determined by monoclonal immunoassay analysis. KEY RESULTS: Even under irrigation, cessation of leaf elongation, senescence of lamina and relative dehydration of basal tissues were triggered only by a day length longer than 13 h 30 min (late spring and LD) in plants of Poa bulbosa and Dactylis glomerata 'Kasbah' which exhibit complete dormancy. Plants of Lolium arundinaceum 'Flecha' maintained leaf growth under irrigation irrespective of the day length since its dormancy is incomplete. ABA concentrations were not higher during late-spring drought than early, and could not be associated with spring dormancy induction. In summer, ABA concentration in bulbs of the desiccation-tolerant Poa were greater than in basal tissues of other species. CONCLUSIONS: The results of both experiments tend to invalidate the hypothesis that water deficit has a role in early summer-dormancy induction in the range of tested grasses. However, a late-spring drought tends to increase plant senescence and ABA accumulation in basal tissues of forage grasses which could enhance summer drought survival.

Survival and recovery of perennial forage grasses under prolonged Mediterranean drought: I. Growth, death, water relations and solute content in herbage and stubble.

Swards of Dactylis glomerata cultivars (cvs) KM2 and Lutetia and of Lolium perenne cvs Aurora and Vigor were grown under full irrigation or prolonged summer drought (80 d) in a field experiment in the South of France. After irrigation was withheld, leaf extension rates of all cvs fell by 90% within 9-12 d, and rapid scorching of laminae followed. Tiller mortality at the end of the drought was very different in the cocksfoot cvs (4% for KM2 and 76% for Lutetia) and intermediate (41%) for both ryegrass cvs. Following re-watering, rates of herbage regrowth were closely correlated with tiller survival. Measured minerals contributed c. 0·52 MPa to osmotic potential in all treatments, whereas water-soluble carbohydrates (WSC) contributed 0·25 MPa under irrigation and 0·46 MPa during drought. There was no systematic difference between the two species for summer survival under severe drought, but large differences between the cocksfoot cvs. The traits most strongly associated with superior survival were: (a) a deep root system and greater water uptake at depth; (b) low water and osmotic potentials in surviving laminae, i.e. better tolerance to dehydration; (c) large pool-size of WSC reserves (fructans having degree of polymerization >4) in entire tiller bases (stubble); (d) low accumulation of proline in stubble; (e) rapid nitrogen uptake after rewatering.

Survival and recovery of perennial forage grasses under prolonged Mediterranean drought: II. Water status, solute accumulation, abscisic acid concentration and accumulation of dehydrin transcripts in bases of immature leaves.

Swards of cocksfoot (cvs KM2, Lutetia) and perennial ryegrass (cvs Aurora, Vigor) were grown under full irrigation or severe (80 d) drought in a field experiment in the South of France. Responses of the bases of immature leaves plus enclosed tissues were made during the drought period and after rewatering. By the end of the drought, water content had fallen from 3·0 to 0·8 gwater g-1 dm , and osmotic potential from -1·0 to -4·5 MPa in all cvs. Measured minerals and water-soluble carbohydrates contributed, respectively, c 19 and 44% to osmotic potential in droughted leaf bases. The drought-sensitive cocksfoot cv. Lutetia was characterized by a large proportion of fructans having a low degree of polymerization (DP=3, 4). As drought progressed, accumulation of dehydrin transcripts and ABA were higher in leaf bases of the sensitive cv. Lutetia than in the resistant cv. KM2. After rewatering, the water status of immature leaf bases returned to control levels in 1-2 d, and then increased further as leaves began to grow and new tissue was produced. High-DP-fructans remained unchanged in leaf bases of 'Lutetia' but were depleted by over 55%, and therefore remobilized, in leaf bases of other cvs after 8 d. It is concluded that enclosed immature leaf bases survive drought by tolerating a low water status and that changes conventionally associated with desiccation tolerance are expressed most strongly in susceptible plants least able to maintain their water supply.