© 2007 Heron Publishing—Victoria, Canada
Leaf exchange in a Mediterranean shrub: water, nutrient, non-structural carbohydrate and osmolyte dynamics
R. Milla (1, 2), S. Palacio (3), M. Maestro-Martínez (3) and G. Montserrat-Martí (3)
1. Department of Forest Resources, University of Minnesota, St. Paul, MN 55108, USA / 2. Corresponding author (ruben.milla@gmail.com) / 3. Instituto Pirenaico de Ecología (C.S.I.C.), P.O. Box 202, 50080 Zaragoza, Spain / Received June 18, 2006; accepted October 23, 2006; published online April 2, 2007
Summary
Leaf exchange is an abrupt phenological event that drastically modifies the morphology and physiology of the aerial portion
of the plant. We examined if water and osmolyte differences between old leaves and new organs trigger leaf exchange, and whether
the differences are closely linked to the resource resorption process in senescing leaves. We monitored concentrations of
osmolyte, water, non-structural carbohydrate, nitrogen and potassium in senescing leaves and in emerging new leaves and inflorescences
of a Mediterranean leaf exchanger (Cistus laurifolius L.) growing in NE Spain. Old leaves rehydrated markedly during most of the senescence process, which co-occurred with the
extension of new shoots, suggesting the lack of a clear-cut switch in water supply from old to new organs. The accumulation
of osmolytes in the early stage of leaf senescence might account for this rehydration. Osmolyte dynamics in old leaves depended
largely on the progression of resource resorption from senescing organs but were mostly unrelated to water content during
late senescence. We conclude that dehydration of old leaves is not a prerequisite for the triggering of leaf exchange. The
finding that most nutrients and carbohydrates accumulated in new organs before senescing leaves massively exported resources,
and the absence of relevant differences between the dynamics of old leaves at the base of inflorescences and those at the
base of vegetative shoots, indicate that the nutrient and carbohydrate demands of new organs do not trigger leaf exchange.
Keywords:
Cistus laurifolius, leaf-exchangers, nitrogen, osmotic pressure, resource dynamics, sink-source, sugar.
