Photosynthetic rate and canopy development in the drought-deciduous shrub Anthyllis cytisoides L.

Seasonal changes in phenology, shoot demography and physiology of the summer-deciduous shrubAnthyllis cytisoides L. were monitored at a field site in semi-arid south-eastern Spain. Vegetative growth took place during the cool season and the shrubs were leafless for c. 3 months during the dry season. In shrubs which had been supplied with water since late spring, leaf and shoot growth continued in the summer. Photosynthetic rate was limited by low plant water status for most of the observation period. Maximum photosynthetic rates of 14 μmol CO2m−2s−1were measured in early April but then declined progressively to practically zero in early July. Shrubs supplied with water maintained the same maximum photosynthetic rate in June. Superimposed on the environmentally controlled seasonal variation in the photosynthetic rate was a change in the leaf to stem area ratio as a result of leaf development, senescence (decrease in leaf nitrogen) and abscission. Anthyllis cytisoides is a drought avoider with a predictable, but temporally variable resting period. At various times of the seasonal cycle, the summer-deciduous habit allows only limited phenological and physiological responses to temporally favourable environmental conditions, which constrains carbon gain and productivity