Phloem translocation of weak acids in ricinus <i>communis</i>

Abstract

Abstract Uptake of weak acids by roots of Ricinus communis and their translocation to shoots increased as the pH of the nutrient solution decreased, the behaviour being closely similar to that observed for the same compounds in barley. Phenoxyacetic acids (p K a ˜3) with log K ow , for the undissociated acid of between 1 and 3 were readily taken up and translocated in phloem to the apical leaf and the upper stem of R. communis following injection into the petioles of mature leaves; strong retention of these chemicals in the stem rapidly depleted their concentration in phloem down the stem so that very little was found in roots. A more polar and a more lipophilic phenoxyacetic acid were poorly translocated from the petioles, and flamprop (pK a 3.7, log K ow , 2.9) was predominantly xylem‐mobile. Maleic hydrazide (pK a 5.65, log K ow , −0.6) was taken up from the petioles rather slowly but was readily phloem‐translocated from the mature leaves, and little was retained by the stem, permitting appreciable amounts to reach the roots and nutrient solution. Concentrations of these weak acids in leaf phloem were up to ten times higher than in the adjacent leaf cells, and could be predicted reasonably well assuming that accumulation in phloem was purely a physico‐chemical process occurring due to the ion‐trap effect, taking values obtained from barley roots for the ratio of the permeabilities of membranes to the undissociated and dissociated forms of the acids.