Guard Cell Photosynthesis
Tracy Lawson and James I. L. Morison, Department of Biological Sciences, University of Essex, Wivenhoe Park, Colchester, UK
As the leaf cuticle is almost impermeable to CO2 and H2O, stomata regulate gas exchange between the inside of the leaf and the external environment. Stomatal function is critical for controlling CO2 uptake for photosynthesis and water loss through transpiration and therefore plant water use efficiency (WUE; amount of carbon gained per unit water lost, see Morison et al. 2008). Changing climatic conditions and the increasing global population is forcing plant breeders and researchers to find plants with improved yields and greater WUE. As stomata ultimately determine these two characteristics, they potentially provide a manipulation route to produce crop plants with improved yield, using less water. Before such a route could be considered viable the mechanism(s) that links mesophyll assimilation rate with stomatal conductance must first be established and the signalling and sensory mechanisms that allow stomatal guard cells to respond to the continually fluctuating environment must be understood. The fact that guard cells contain functional chloroplasts suggests this could be the location for sensory or regulatory mechanisms, but this is controversial. This essay briefly reviews the characteristics of guard cell chloroplasts and discusses their possible functions. In particular, we will examine the evidence for and against guard cell photosynthetic CO2 fixation in relation to function, as well as illustrate some recent developments using transgenic plants. We will also illustrate how chlorophyll fluorescence has been used to assess photosynthetic function within the intact leaf (Lawson et al. 2002; 2003; von Caemmerer et al. 2004).
Guard Cell Chloroplasts
Chloroplasts are a common feature of guard cells in the majority of species examined, yet the function of these organelles...