Water Relations in Pulvini from Samanea saman

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Abstract
The movement of Samanea leaflets depends upon changes in the curvature of the pulvinus at the base of each leaflet. Pulvinar bending and straightening, in turn, are driven by the movement of water between opposing (extensor and flexor) sides of the pulvinus. Although water movement depends on water potential (.psi.) and thus on osmotic potential (.pi.) and hydrostatic pressure (P), none of these parameters have been measured in Samanea. In this investigation, .psi. and .pi. were measured and P was calculated for extensor and flexor tissues of excised, whole pulvini that were open in the light and closed in the dark. In fully open pulvini, .pi. in the extensor was generally between 800 and 1000 milliosmol per kilogram and exceeded .pi. in the flexor by 300 to 450 milliosmol per kilogram. In fully closed pulvini the reverse was true, with .pi. in the flexor between 800 and 1000 milliosmol per kilogram, exceeding .pi. in the extensor by 300 to 450 milliosmol per kilogram. To obtain approximately values of .psi. of pulvinar tissues, shallow cuts in extensor and flexor sides of oil-covered pulvini were filled with droplets of polyethylene glycol solutions of known .psi.. Droplets maintaining constant size were assumed to have the same .psi. as the tissue. Extensor and flexor halves of open pulvini had very different .psi. (extensor, about -1.4 MPa; flexor, about -0.3 MPa), but both sides of closed pulvini had similar .psi. (about -0.3 MPa). Measurements of .psi. and .pi. and calculations of P indicate: (a) In open pulvini, P is about the same in extensor and flexor. The large .psi. gradient is caused by a large osmotic gradient. (b) In closed pulvini, P is approximately 50% higher in the flexor than in the extensor. This difference in P compensates for differences in .pi. such that the .psi. gradient is small. (c) Pulvini close as P increases in the flexor and reopen as flexor P decreases; extensor P values are similar in open and closed pulvini.