Rapid Changes in Transpiration in Banana Leaves.

Rapid changes in the transpiration rate from a leaf or leafy shoot as a result of excision of that leaf or shoot have been observed in many plants bv many investigators (1, 2, 5, 7, 10, 11, 12, 13, 16). In banana plants, both photosynthesis and transpiration have been found to change rapidly following excision of the leaf (7). The mechanism underlying such rapid changes in transpiration has been the subject of a recent exchange of opinion between Heath (13), and Meidner and Heath (15) on the one hand and Rufelt (16, 17, 18) on the other hand. Rufelt (16) invokes the interpretation, originally formulated by Iwanoff (14), that excision causes a sudden release of tension in the water supply to the leaf. The increased water supply to the leaf is presumably responsible for increased stomatal opening and increased transpiration. Other investigators have adopted this same interpretation (2, 3, 4, 5, 7) and Allerup (2, 3, 4) who has demonstrated increased stomatal opening as a result of excision of young barley leaves considers this as supporting evidence for Rufelt's hypothesis. Heath (13), and Meidner and Heath (1I5) oln the other hand, hold that these rapid changes in transpiration are occasioned by an increased stomatal opening which they ascribe, as did Darwin and Pertz (9), to a wilting of the mesophyll cells withdrawing water from the epidermal and subsidiary cells thus permitting the guard cells to bulge outward, opening the stomatal pores. Excising a leaf in air can only, according to their argument. result in a decreased water supply to the leaf, and it is this decreased water supply which causes the mesophyll cells to wilt. The present work constitutes an effort to shed some light on this problem by studying in a controlled environment the rates of photosynthesis and transpiration, the average stomatal opening (by direct observation), and the relative turgidity (by leaf thickness measurements) of banana leaves before, (tlrinig anid after excision from the plant.