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.