Is xylem sap calcium responsible for reducing stomatal conductance after soil

18 Understanding the regulation of calcium uptake, xylem transport and its impacts on growth and leaf 19 gas exchange is a subject that has received insufficient recent attention. Calcium (Ca) is unique 20 within the group of key elements required for plant growth in that it also has a role in cellular 21 signalling via regulation of changes in its cytoplasmic concentration. Its mobility, within the plant, is 22 however somewhat constricted by its chemistry and cellular signalling role, and its adsorptive 23 capacity within the aopoplast and the xylem. Supply and demand for Ca is achieved by a homeostatic 24 balance which if perturbed can cause a number of distinctive physiological conditions, often related 25 to Ca deficiency. In this issue Rothwell and Dodd present experiments with bean (Phaseolus vulgaris) 26 and pea (Pisum sativum) plants grown in a field soil exposed to the processes of soil liming 27 (application of Ca carbonate (CaCO3). Given that there is evidence of free Ca in the xylem sap 28 altering stomatal conductance it is reasonable to ask the question does liming elevate Ca in the 29 transpiration stream which may explain the observed reduced growth which they hypothesise is due 30 to Ca-induced stomatal closure. They show that liming doubled soil exchangeable Ca, reduced 31 stomatal conductance and shoot biomass in both species compared with unlimed controls. 32 However, xylem sap Ca concentration increased only in bean. Interestingly, the same was not true 33 for the pea where the root xylem sap concentration remained unchanged despite an increase in soil 34