Stomatal control and water transport in the xylem

[Extract] The movement of plants from aquatic to terrestrial habitats in the Ordovician placed new and conflicting demands on the structure and function of photosynthetic organs. Protection against desiccation led to the formation of an epidermis with low permeability to water, while the requirement to facilitate entry of gaseous CO2 close to photosynthetic tissue favored an increase in epidermal porosity. This dilemma was solved early during the evolution of land plants with the advent of stomata, which, through their ability to regulate epidermal porosity, provided a means by which to optimize the tradeoff between carbon gain and water loss. However, without a reliable water supply to the sites of photosynthesis, stomata remain closed and cells starve. Since their evolution, the stomata and xylem have been inextricably connected in the transpiration pathway. For a given environment and leaf photosynthetic biochemistry, the rates of carbon uptake and transpirational water loss are controlled by stomatal conductance. Thus, for the same environment, higher photosynthetic rates result in higher transpiration rates, and xylem investment represents a major part of the cost of maintaining a transpirational flux while keeping the photosynthetic tissue hydrated. This chapter examines coordination between xylem and stomata, focusing on how the evolution and function of these water-conducting tissues in plants has led to a linkage between living and ostensibly nonliving tissue