The nature of the perennial response in Mediterranean grasses. II. Senescence, summer dormancy, and survival in Phalaris

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Abstract
Adaptation to summer drought has been one of the major factors influencing the life cycle of the Mediterranean grasses. The nature of this adaptation has been studied by comparing the behaviour of a typical perennial, Phalaris tuberosa, with a closely related annual species P. minor, under simulated summer drought conditions. One of the most important factors controlling the plant survival of these two species is the difference in the rate and pattern of senescence after flowering under the influence of moisture stress. In the annual, senescence is rapid and complete, but in the perennial it is retarded, and function is retained in certain organs, including the lower stem and root system, throughout the summer stress. This permits survival of dormant buds which contain the meristems vital for regrowth in the autumn. Another feature which may be important in the perennial is the limited carbon assimilation by the dormant culms during the summer which contributes a source of energy for the maintenance of root activity, and for the regeneration of dormant buds. The evidence suggests that these factors may be of general significance, and could well form the basis of the perennial response in other Mediterranean grass species.