Size-Dependent Changes in Biophysical Control of Tree Growth: The Role of Turgor

Abstract

AbstractXylem pressure at the tops of trees decreases with increasing tree height as a result of gravitational force and friction associated with water movement through the xylem. Unless altered by osmotic adjustment, the turgor of cells in the apical and vascular meristems will decline proportionally with xylem pressure. In the tallest coniferous species on earth osmotic adjustment is insufficient to compensate for the negative impact of vertical gradients of xylem pressure on turgor of developing and mature tissue. This chapter synthesizes the research addressing turgor-related constraints on growth at the cellular, tissue, and whole tree levels. We examine the role of turgor in plant growth and foliar and stem structural and functional characteristics as they are influenced by height-related trends in turgor. We also evaluate height-related trends in turgor as a cause of observed patterns of tree height and tree biomass growth with an emphasis on how reductions in turgor can influence sink/source relationships for photosynthate in tall trees. Height-related reductions in cell turgor generate a cascade of direct and indirect effects on shoot extension and expansion. The multiple consequences of turgor-limited growth may act in a synergistic manner to further constrain shoot extension as trees approach species- and site-specific height limits. Turgor-limited tissue expansion has permanent anatomical and morphological consequences that govern height-dependent changes in an array of growth-related physiological processes. These include the impact of reduced xylem conduit dimensions on efficiency of water supply to growing tissues at the terminal portions of the water transport pathway, reduced growth efficiency associated with increased leaf mass per area, and increased resistance to CO2 diffusion through the mesophyll to the sites of carboxylation.KeywordsSpecific Leaf AreaOsmotic AdjustmentTurgor PressurePhloem TransportXylem PressureThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.