Improvement of the enhanced phloem exudation technique to estimate phloem concentration and turgor pressure in tomato

A critical appraisal of the EDTA-enhanced exudation technique was performed and improvements for this technique were proposed. Two buffer solutions were used (HEPES and EDTA) at a concentration of 20 mM. HEPES is a buffering agent widely used in cell culture. Exudation from cut-ends of petioles and peduncles of tomato was allowed to occur over a period of 7 h. Variations of the buffer solution volume, carbohydrates and potassium concentrations, as well as osmotic pressure, were measured in the exudates. The use of HEPES buffer solution alone is proof of the large uptake of the buffer solution by the plant through the xylem. The effect of this uptake on the classical estimation of the carbohydrate flow and concentration of phloem is presented. An improvement of the technique is proposed in order to correctly estimate: (i) the phloem flow; (ii) its composition (carbohydrates and others solutes); and (iii) the turgor pressure of the phloem. The improved EDTA-enhanced exudation technique was applied to investigate the effect of water stress on the driving variables involved in the carbohydrate allocation within the plant. Phloem composition varied with the date and was affected by water stress as well as turgor pressure, which decreased. Furthermore, differences arose in the turgor pressure at the petiole and peduncle sites. Results showed that the exudation technique could be a useful tool to estimate local phloem composition and turgor pressure and to infer resource allocation within the plant.