Collection of the phloem sap, pros and cons

Phloem tissue is essential for the translocation of nutrients, water, energy, and signals in plants. In order to study the chemical composition of phloem sap, several methods have been used for its collection including the dipotassium ethylenediamine tetraacetic acid (K₂-EDTA) exudation, incision, and the stylectomy by aphids. Each method has advantages and disadvantages. Unfortunately, there is no ideal method that can be used for all plants or to collect ultrapure phloem sap with no cellular contamination. However, K₂-EDTA exudation is the most used method because it is easy, fast, and results in a high quantity of phloem sap. In woody plants, it is easy to separate the bark. Using the bark which contains the phloem tissue would avoid the contamination with xylem sap when phloem sap is collected. Lately, we developed a simple and a quick method for the collection of the citrus phloem sap depending on the centrifugation of the detached bark tissue. Here, I report the advantages of the centrifugation method over the K₂-EDTA exudation in collecting phloem sap from citrus. To emphasize the purity of collected saps, phloem sap (from bark tissue) and xylem sap (from inner part of stem) were collected using both methods. Using gas chromatography mass spectrometry, the centrifugation method showed less artifact peaks than the K₂-EDTA exudation indicating more pure saps were collected. For instant, less hexoses were detected in phloem sap and the absence of sucrose in xylem sap in centrifugation method than in K₂-EDTA exudation. More importantly, centrifugation method allowed accurate estimation of the concentrations of metabolites. This method could be successfully used for the collection of saps of citrus and other trees until the invention of a more specific method to collect ultra-pure phloem sap.