Physiological Studies on Pea Tendrils. V. Membrane Changes and Water Movement Associated with Contact Coiling

Abstract

The coiling of excised pea tendrils in response to mechanical stimulation is accompanied by an increased efflux from their cut bases of electrolytes and label from previously absorbed 14C-acetate and 14C-sucrose. The major excreted cation is H+; H+ loss is potentiated by pretreatment with benzoic acid, which also leaves the tendrils during coiling. Label from previously absorbed tritiated water is excreted during coiling, mainly from the ventral side of the tendril, which contracts in the initial phase of coiling. Such label does not pass from the ventral to the dorsal side. Similarities between this and other rapidly moving systems in plants are surveyed and a hypothesis to explain turgor movements is advanced. The rapid movements of plant parts have often been aittributed to turgor changes resuliting from alteration in cell membrane permeability (17). This view has been based largely on changes in size of motor celils before and after movement (10, 17) and effects on motor cells of reagents such as diethyl ether and ethyl alcohol, known to affect memlbranes (5, 9,15, 16). Recently we were able to implicate membrane changes in ithe phytochrome-controlled rapid nycti-nastic movements of excised pinnae of Albizzia julibrissin (8). We reasoned that if contact coiling of tendri-ls involves the rapid efflux of water from cellis of the contracting ventral side (5), the efflux of solutes and of water from such coiling ten,drils might be detectable by sensit'ive techniques. This paper reports evidence for such movement. Materials and Methods Un'branched tendri4ls from the fifth node of 10 to 13 day old Alaska pea plants, grown as previousily descriibed i(5), were used in 2 ways. In one procedure , previously described (5), 10 excised tendrils were shaken in 10 ml of solution in a petri dish contatining 0.1 or 0.01 % Tween-20 as a wetting agent. Where test addenda were inciltuded, 30 mM phosphate buffer (pH 6.4) was added, but all con-ductance measurements were made on effluents subsequen(tly collected in distilled water. In a second procedure, tendril's were excised under water containing 0.01 % Tween-20, usually with about 1 cm of subjacent petidle attached, and placed tnpright in 2 ml of solution in a plasitic viail. The occasionally flaccid tendrillis were allowed to 1 Present address: Department of Botany, Ohio University , Athens, Ohio 45701. regain turgor by an overnight incubation in distilled waiter or in 100 AM benzoic acid. Three ten-dril's pe.r vial were used for the electrolyte efflux experiments and 1 per vial for the experiments with tritiated water. The efflux of label from previously absoTbed 14C-Glabeled sucrose and sodium acetate was measured as follows. Tendriils were floated in solutions of the labeled material for 2 hours, rinsed, and then either shaken or allowed to remain at rest foir 30 minuites in fresh buffer. The radioactivity of the buffer was then determined by mixing an aliquot with scintillation fluid and counlting in an Ansitron liquid scintillation spectrometer. Since both acetate and sucrose were partially metabolized, the resulits were expressed as cipm per mg final fresh weight of the 10 tendrils rather than in molar terms. Ellec.trolyte efflux was determined on tendrils incu-bated in distilled water containing 0.01 % Tween-20 for 2 hours. Conductance measuremenits of the ba.thing soltultion were mmade art the start and end of the incubation period with a previously described apparatus (3). The efflux of electrolytes from the cut base of the tendril was determined as follows. Leaf No. 5 was severed from the plant at the base of the petiole and the par.ts below the tendiriil immersed in water containing 0.01 % Tween-20. The leaflets and alil but 1 cm of petiole base were then excised under the water leaving the tendrill attached to the pejtiolar stump, which remained immersed. All subsequent handling was genitle, and limi(ted to the petiolar stump. The system was allowed to recover overnight, usually in the presence of 100 uM benzoic acid, after which the solution was removed, and after some rinsing replaced by distilled water whose conductance was measured at time zero and again after 30 minultes. Some of the tendrils were then stimulated bv stroking their ventral surfaces with 537