Stimulation of chlorophyll synthesis by benzyladenine and potassium in excised and intact cucumber cotyledons

Both benzyladenine (BA) and potassium (K) stimulated chlorophyll synthesis in cucumber ( Cucumus sativus L. cv. National Pickling) cotyledons. However, differences existed between the effects of BA and K. Stimulation of chlorophyll synthesis by BA (1 mg l^?1, 4.4 μ M ) was observed in excised cotyledons after 4 and 8 h of illumination but not after 24 h, whereas the stimulation caused by K (40 m M ) continued. In contrast to BA, K was unable to eliminate the lag phase of chlorophyll production, and it also required light for its stimulation of cotyledon expansion. Both BA and K were required to maximize cotyledon expansion and chlorophyll production. In intact plants, K was not limiting for chlorophyll synthesis since foliar or soil pretreatments with K did not markedly stimulate greening. Foliar pretreatment with BA stimulated chlorophyll levels in intact plants, whereas soil pretreatment with BA inhibited chlorophyll production, probably because BA was not readily transported from the roots to the shoot and created a "sink" effect. Inhibitor studies showed that stimulation by K of greening did not depend on RNA or chloroplastic protein synthesis to the extent that has been reported for BA. Thus it appears that BA and K stimulate chlorophyll synthesis via different mechanisms, although both cytokinins and K are essential for maximum rates of greening.