Phloem sap exudation in Ricinus communis: elastic responses and anatomical implications

Abstract

Abstract. Ricinus communis plants have an unusually high capacity to exude considerable quantities of phloem sap from bark incisions. We have used Ricinus as an experimental system to study different aspects of sap exudation. Dimensional changes in the bark, monitored by a displacement transducer, showed that pressure release in the sieve tubes was accompanied by elastic shrinkage. The rate of exudation was also controlled by the degree of pressurization and elastic properties of the sieve tubes. A displacement transducer was used to measure the elastic modulus (ɛ) of phloem samples by immersing them in a range of different osmotica. The cells had a low elastic modulus (ɛ= 1.62 ± 0.41 MPa at full turgor). ɛ of phloem tissue in massage pretreated bark, from which exudation was enhanced, was not significantly different from that of unmassaged bark in contrast with the suggestion of Lee (1981). However, anatomical studies showed that massage pretreatment has a stimulating influence on the cambial cell division, which increased the phloem tissue cross‐section up to 160%. The newly‐formed sieve tubes were ‘spliced’ into existing ones in the unmassaged zone to re‐establish vascular continuity. Plants with a greater capacity to exude phloem sap from a given stem location had a greater cross‐sectional area of sieve tubes in the vicinity.The significance of observations with respect to other sap exudation phenomenon is discussed. The importance of the present work in understanding the technique of palm tapping, on which the palm sugar industry depends, is also considered.