Water Relations in Pulvini from Samanea saman: II. Effects of Excision of Motor Tissues.

Pulvinar motor tissues of Samanea saman are often excised for in vitro studies of ion transport. Because ion transport may be regulated in part by hydrostatic pressure (P), this study explores how P and water potential (Psi) change when motor tissues are excised. Water potential (Psi) of excised extensor and flexor tissues was measured by the Chardakov method and compared with Psi measurements made on extensor and flexor tissues of intact pulvini (HL Gorton 1987 Plant Physiol 83: 945-950). Psi values for excised extensor and flexor tissues were always substantially more negative than for the same tissues in intact pulvini. Extensor tissues excised from open pulvini had slightly more negative Psi than excised flexor tissues, and the opposite was true for closed pulvini. Extensor and flexor tissues elongate immediately when excised from open or closed pulvini, suggesting that in intact pulvini they are constrained from elongating by the nonextensible vascular core. In addition, both tissues in both open and closed pulvini are under compression imposed by oppositely positioned motor tissue. Excision relieves constraint and compression, decreasing P, and thus decreasing Psi. This finding may explain, at least in part, the difference between Psi measurements on intact and excised motor tissues. Implications of these data for the planning and interpretation of in vitro experiments requiring excised strips of extensor and flexor tissues are discussed.