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Harvard Forest Symposium Abstract 2011
- Title: The timing of leaf senescence in the temperate trees Quercus rubra and Acer saccharum is not regulated by xylem-transported cytokinins
- Primary Author: Juan Pablo Giraldo (Harvard University)
- Additional Authors: Noel Michele Holbrook (Harvard Forest)
- Abstract:
Through its downregulating effect on photosynthesis leaf senescence influences tree growth, competition between trees with diverse phenologies and net ecosystem carbon uptake in temperate forests. While temperature and photoperiod have been commonly identified as environmental cues of leaf senescence in the temperate region, the mechanisms regulating this developmental process lack general experimental evidence. Furthermore, the role of endogenous signals in regulating leaf senescence of temperate trees is poorly understood. Cytokinins have been traditionally associated with delayed leaf senescence and even re-greening of senesced leaves in crop plants. Recent studies provide evidence that the liquid phase conductance of water through the leaves (Kleaf) controls the timing of leaf senescence in crop plants via cytokinin delivery. To test this hypothesis in Quercus rubra and Acer saccharum trees at Harvard Forest, MA, we (1) monitored temporal patterns of Kleaf, chlorophyll, cytokinins, as a function of leaf age in the lower canopy, (2) assessed the impact of leaf aging on gas exchange regulation, (3) manipulated under field conditions both transpiration rates and photosynthesis using humidity and shade treatments to examine the role of hydraulic limitations to xylem transport and CO2 uptake in controlling leaf senescence. Kleaf, stomatal conductance, and assimilation remained relatively stable before the onset of leaf senescence in both species studied. However, loss of gas exchange control with ageing in A. saccharum leaves was observed in stomatal response assays to variations in vapor pressure deficit. Leaf cytokinin concentrations could not be explained by hydraulic regulation. Furthermore, reducing the flow of xylem-transported compounds and photosynthesis in high humidity and shade treatments did not have an impact on leaf chlorophyll content. Thus, we propose that leaf cytokinin delivery does not control leaf senescence in temperate deciduous species. Instead, leaf senescence might be linked to sink pool activation for nitrogen storage during the dormant period.
- Research Category: Physiological Ecology, Population Dynamics, and Species Interactions
