Water Relations in Pulvini from Samanea saman: I. Intact Pulvini

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 approximate 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.