Drought effects on the dynamics of leaf production and senescence in field-grown Medicago arborea and Medicago citrina

Forage shrub production in the Mediterranean region is frequently limited by soil water availability. To study plant responses to water deficit under such conditions is important for improving crop management and for selecting better yielding forage shrub species. Pre-dawn leaf water potential (Psi(pd)), plant leaf area (PLA), leaf area per stem (LA(s)), leaf appearance rate (LAR(l)), leaf senescence rate (LSR), individual leaf area (LA) and maximal leaf elongation rate (LER) were studied throughout the year for Medicago arborea (MA) and Medicago citrina (MC) under irrigated (control) and low rainfall field conditions, at the experimental field site of the University of the Balearic Islands in Spain. With irrigation, the highest LA and LER were observed in autumn and spring and the lowest in winter and summer. LAR(l) was similar for both species in autumn and winter. Throughout the spring, LAR(l) was higher for MC compared to MA. PLA was similar for both species during the autumn, winter and spring seasons; however, during the summer PLA of MA was significantly reduced by 53%. This decline was attributed to higher leaf senescence during seed maturity. As a consequence, MC maintained higher leaf area (approximate to 5 m(2) plant(-1)) than MA (3 m(2) plant(-1)). Under natural field conditions, soil water deficit increased from February to late August. The main effect of water stress was a marked reduction in LAR(s), LA and LER reflected in lower LA(s) and PLA. Leaf area was severely reduced for both species during the summer, but much more intensively in MA, which developed full leaf senescence. Thus, MC maintained higher PLA than MA (0.5 m(2) compared to 0.0 m(2)). Throughout the year, but especially in the driest months, MC was superior to MA in leaf growth parameters and PLA.