Leaf exchange in a Mediterranean shrub: water, nutrient, non-structural carbohydrate and osmolyte dynamics.

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.

Costs of reproduction as related to the timing of phenological phases in the dioecious Shrub Pistacia lentiscus L.

Females of woody dioecious species usually devote more resources to reproduction than males. This may lead to a decrease in female survival and growth. The costs of reproduction, however, can be lightened through a number of mechanisms, as for example avoiding the temporal coincidence of reproduction and vegetative growth. The aim of this study was to evaluate whether males and females of P. lentiscus differ in the timing of their vegetative growth, and to assess whether the sequencing of vegetative growth and reproduction reduces reproductive costs. We monitored phenology in males and females. We also compared male and female allocation of nutrients and biomass in the branch, and the developmental stability of the growing shoots. We did this both prior to and at the end of the fruiting period. Males and females showed similar vegetative and flowering phenologies. Males invested more biomass in flowering, but the sexes showed equal vegetative biomass and nutrient content prior to the fruiting period. In female branches, no trade-off was found between fruit load and current-year vegetative growth. In P. lentiscus, avoiding the overlap of flowering, vegetative growth and fruiting probably contributes to reduce the immediate costs of reproductive efforts, both in males and females.

Environmental constraints on phenology and internal nutrient cycling in the Mediterranean winter-deciduous shrub Amelanchier ovalis Medicus.

The functional adjustments of winter-deciduous perennials to Mediterranean conditions have received little attention. The objectives of this study were: (i) to determine whether Amelanchier ovalis, a winter-deciduous shrub of Mediterranean and sub-Mediterranean regions, has nutritional and phenological traits in common with temperate zone deciduous trees and shrubs and (ii) to determine the constraints of Mediterranean environmental conditions on these traits. Over two years, phenology and nitrogen, and phosphorus concentrations were monitored monthly in the crown of A. ovalis. Leaf longevity, survival and nutrient resorption from senescing leaves were used to infer nutrient use efficiency and retention times of nutrients within the crown. In A. ovalis, bud burst was much earlier than in temperate deciduous trees and shrubs. Most vegetative and reproductive growth occurred in spring. Limited phenological development took place during the summer drought period. Unexpectedly, leaf shedding was very gradual, which might be related to water shortages in summer. Leaf longevity, nutrient resorption from senescing leaves, and maximum leaf nutrient concentrations indicated that nutrient retention times were short and nutrient use efficiency was low compared to that found in temperate deciduous plants and co-occurring Mediterranean evergreens. A. ovalis exhibited phenological development appropriate for a Mediterranean climate, although its limited ability to retain nutrients likely restricts the types of sites that it can occupy.

Relationships between phenology and the remobilization of nitrogen, phosphorus and potassium in branches of eight Mediterranean evergreens.

Few studies have examined the effects of plant growth on nutrient remobilization in phenologically contrasting species. Here we evaluated the consequences of above-ground seasonality of growth and leaf shedding on the remobilization of nutrients from branches in eight evergreen Mediterranean phanaerophytes that differ widely in phenology. Vegetative growth, flower bud formation, flowering, fruiting, leaf shedding, and the variations in nitrogen (N), phosphorus (P) and potassium (K) pools in branches throughout the year were monitored in each species. Nitrogen and P remobilization occurred in summer, after vegetative growth and synchronously with leaf shedding. Despite the time-lag between growth and remobilization, the branches that invested more nutrients in vegetative growth also remobilized more nutrients from their old organs. Potassium remobilization peaked in the climatically harshest periods, and appears to be related to osmotic requirements. We conclude that N and P remobilization occurs mainly associated with leaf senescence, which might be triggered by factors such as the replenishment of nutrient reserves in woody organs, the hormonal relations between new and old leaves, or the constraints that summer drought poses on the amount of leaf area per branch in summer.

Seasonal branch nutrient dynamics in two Mediterranean woody shrubs with contrasted phenology.

BACKGROUND AND AIMS: Mediterranean woody plants have a wide variety of phenological strategies. Some authors have classified the Mediterranean phanaerophytes into two broad phenological categories: phenophase-overlappers (that overlap resource-demanding activities in a short period of the year) and phenophase-sequencers (that protract resource-demanding activities throughout the year). In this work the impact of both phenological strategies on leaf nutrient accumulation and retranslocation dynamics at the level of leaves and branches was evaluated. Phenophase-overlappers were expected to accumulate nutrients in leaves throughout most of the year and withdraw them efficiently in a short period. Phenophase-sequencers were expected to withdraw nutrients progressively throughout the year, without long accumulation periods. METHODS: To test this hypothesis, variations in phenology and leaf NPK in the crown of a phenophase-overlapper Cistus laurifolius and a phenophase-sequencer Bupleurum fruticosum were monitored monthly during 2 years. KEY RESULTS: Changes in nutrient concentration at the leaf level were not clearly related with the different phenologies. Nitrogen and phosphorous resorption efficiencies were lower in the phenophase-overlapper, and accumulation-retranslocation seasonality was similar in both species. Changes in the branch nutrient pool agreed with the hypothesis that the phenophase-overlapper accumulated nutrients from summer until the bud burst of the following spring, recovering a large nutrient pool during massive leaf shedding. The phenophase-sequencer did not accumulate nutrients from autumn until early spring, achieving lower nutrient recovery during spring leaf shedding. CONCLUSIONS: It is concluded that phenological demands influence branch nutrient cycling. This effect is easier to detect by assessing changes in the branch nutrient pool rather than changes in the leaf nutrient concentration.