Developing Phloem δ13C and Sugar Composition as Indicators of Water Deficit in Lupinus angustifolius

Abstract

The use of naturally occurring carbon isotope ratios (expressed as δ13C) obtained from phloem sap offers considerable promise for evaluations of short-term physiological status of plants. We investigate the suitability of δ13C among carbon pools isolated from both leaves and phloem sap as surrogate measures of plant gas exchange in Lupinus angustifolius L. We also investigate the use of phloem osmotic potential (ψS), and metabolite concentrations as surrogate measures of δ13C. Phloem sap carbon bled from the distal tip of the fruit offers considerably improved correlations between δ13C and plant gas exchange compared with δ13C of carbon pools obtained from extractions of excised phloem tissue and leaf soluble carbon. The concentration of phloem sugars and sugar alcohols correlates strongly with measures of phloem δ13C but not with phloem ψS. We discuss these results in the context of developing fast, reliable, in-field tools for the assessment of plant physiological status.