Annals of Botany

Dying for a drink

I have just returned from a spring visit to the Mediterranean coast of Spain. Many plants were in full bloom and much of the open ground was a profusion of colour against a green background. But, by high summer, much of this will be brown. The spring annuals will have completed their life cycles and many other herbaceous plants will be dormant. It is this phenomenon of summer dormancy that has been the subject of a thorough study by a joint French–Australian team (Volaire et al., pp. 981–990). They compared genotypes of cocksfoot grass, Dactylis glomerata, that differ in their summer dormancy. Interestingly, drought avoidance (dormancy) and increased tolerance of soil water deficit occur in the same variety: ‘Kasbah’ (summer dormant) was much better able to survive low soil water content than ‘Oasis’ (non-dormant). However, at the tissue level, there was no difference; leaf bases did not survive at water contents below 0.45 g H₂O g^–1 d. wt (in well-watered plants water content was maintained at 5.0 g H₂O g^–1 d. wt). In relation to dormancy behaviour, ‘Kasbah’ maintained a lower tissue water content than ‘Oasis’ in spring and summer, and exhibited a higher level of above-ground tissue senescence in the absence of addition of new biomass. This behaviour started as early as April in droughted plants but also occurred in well-watered plants (again showing that avoidance and tolerance are independent characters). There were also differences in carbohydrate metabolism: ‘Oasis’ accumulated less fructan but more sucrose than ‘Kasbah’. In the latter variety there was also a very marked decline in monosaccharide content in the spring, and the resulting very low level was maintained through the summer period, even in well-watered plants. The patterns of accumulation of the putative desiccation-protection proteins, dehydrins, were not, however, correlated with drought tolerance or avoidance: in both varieties the presence of dehydrins was related to tissue water status.