| Leaf senescence signals the end of leaf's photosynthetic activity, thus influencing competition between plant phenological types, seasonal rhythms of plant-herbivore interactions, and annual ecosystem carbon uptake and energy fluxes in seasonal forests. Despite its importance in plant biology it is not well understood how leaves control the timing of senescence. The marked changes in expression of senescence associated genes (SAGs) during the onset of senescence indicate that rather than a degenerative process this is a highly regulated developmental transition.;I tested the hypothesis that the leaf vascular system controls the onset of leaf senescence via regulating the flow of cytokinins, key xylem-transported hormones known to delay leaf senescence and even cause re-greening of senescing leaves. As leaves age in tomato (Solanum lycopersicum) a decline in leaf hydraulic conductance (Kleaf) precedes the onset of leaf senescence, limiting the delivery of isopentenyladenosine-type cytokinins (iPA). This in turn weakens cytokinin downregulation of SAG expression, making leaves vulnerable to senescence. In the tropical evergreen tree Anacardium excelsum, iPA cytokinins also influence the timing of leaf senescence, but despite Kleaf reduction with age, leaf cytokinin content is not limited by hydraulic supply. In contrast, temperate deciduous trees Acer saccharum and Quercus rubra show neither age-related changes in leaf hydraulic capacity nor xylem-transported cytokinins impact their timing of leaf senescence. The underlying causes of Kleaf reduction preceding leaf senescence are poorly understood. My results indicate that in tomato expansion of leaf conductive area without proportional increase in xylem hydraulic supply and lamina tissue permeability impairs leaf vascular transport with age.;Through its regulation of xylem iPA cytokinin delivery the leaf vascular system is well positioned to act as an age-dependent signaling system controlling leaf senescence. The adaptive advantage of this mechanism is that allows plants to recognize leaf developmental age for nutrient resorption during new vegetative or reproductive growth. Incorporating the physiological mechanisms underlying leaf senescence into ecosystem dynamic models should allow greater phenology predictability in future climate scenarios. Identifying the mechanisms controlling leaf longevity will contribute to the development of drought resistant crops that have delayed leaf senescence, thus preventing leaves from being prematurely shed. |
