Carbohydrate and amino acid composition in phloem sap of Lolium perenne L. before and after defoliation

Abstract

Carbohydrate and amino acid composition of phloem sap was studied in the grass Lolium perenne L., before and after defoliation. Leaf exudate was collected in a 5 mmol·L–1 EDTA solution from cut leaf blades or stubble, and phloem sap was obtained through excised aphid (Rhopalosiphum padi L.) stylets. Results indicate that leaf exudates obtained from leaves devoid of petiole might not be relevant predictors of carbohydrate content of pure phloem sap. Sucrose was the dominating carbohydrate, accounting for 93% of the total soluble sugars in the phloem sap. Myo-inositol, glucose, and fructose were present in trace amounts, while fructans, raffinose, and loliose have never been detected. Predominant amino acid in the phloem sap was glutamine followed by glutamate, aspartate, and serine. Phloem sap component concentration declined during the first hours following defoliation. Sucrose was the main sugar transported in the phloem sap of Lolium perenne, despite the fact that the product of fructan degradation was fructose and not sucrose. The results are discussed in relation with the physiological mechanisms that contribute to plant recovery after defoliation.Key words: fructan, sucrose, loliose, defoliation, phloem sap, amino acids.