Influence of Different Cytokinins on the Transpiration and Senescence of Excised Oat Leaves

In order to investigate the possibility that cytokinins control transpiration indirectly through affecting leaf senescence, a direct comparison was made of the effect of different cytokinins on transpiration and senescence of oat leaves (Avena sativa L. cv. Forward). Senescence was assessed by measuring chlorophyll loss. The synthetic cytokinins N⁶ benzyladenine (BA) and kinetin delayed senescence and increased transpiration of oat leaves to a greater extent than did the naturally occurring compounds zeatin, N^b-Δ² isopentenyladenine (i⁶ Ade) and 6-ø-hydroxybenzyladenosine (hyd-BA riboside). During the early stages of the transpiration experiment zeatin showed similar or greater activity than BA. This period was longest when freshly excised leaves were used, was reduced when leaves were used after incubation in distilled water in the dark for 20 h and was eliminated by incubation in cytokinin solution in the dark. After this period the activity of zeatin declined relative to BA. The effect of cytokinins in increasing transpiration occurred only in the light; no effect was observed in the dark. BA showed higher activity than zeatin in senescence tests but both cytokinins were less effective as the tests progressed, this decrease in activity being more rapid when older leaves were used. The results are discussed in relation to the mechanisms by which endogenous cytokinins might control sensecence and transpiration in oat leaves and to the value of the oat leaf senscence and transpiration bioassays as tests for cytokinin activity of plant extracts.