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SUMMARYRibonuclease (RNase) activity, greenness (reflectance) and chlorophyll content were determined for different ages of attached and detached leaves from 6 to 8 week old tomato plants. RNase activity of homogenates was correlated with a general indicator of senescence, chlorophyll loss. Young leaves were different from old leaves in their response to detachment. RNase activity increased almost linearly in older leaves, but younger leaves increased in activity to an 'RNase plateau' which was maintained for 2 days before a second increase in activity occurred. High light intensity, which is known to delay senescence, apparently delayed this second increase in RNase activity. Rooting of detached young leaves delayed loss of greenness and eventually returned leaves to the dark green condition and low level of RNase activity associated with attached leaves.RNase activity increases during senescence were not attributed to dechne of inhibitors of the enzyme, or to rupture of hypothetical lysosomes. Suppression of RNase activity increases by actinomycin D suggested instead that enzyme synthesis was at least partially responsible for additional activity during senescence.

Summary Young tomato leaves differed from older leaves in their response to phytokinin treatment after leaf detachment. N,6‐benzyladenine kept old leaves at a ribonuclease (RNase) activity level similar to that of freshly harvested material while controls increased in RNase activity. RNase activity of younger leaves was not suppressed until after a longer period of detachment but the initial post‐harvest treatment induced this effect. Young leaves treated with kinetin at first had a higher RNase activity level than controls but RNase activity in controls had increased by 8 days after detachment while activity in kinetin‐treated tissue remained near the level observed 4 days after detachment. Early response in young leaves to phytokinins was interpreted as slightly toxic, reminiscent of the more toxic response of attached leaves to phytokinins. Loss of greenness was delayed by phytokinin in both young and old leaves but RNase activity was affected first. Young leaves detached from water‐stressed plants showed an early response to N,6‐benzyl‐adenine which was similar to that obtained from older leaves after detachment. Water‐stress altered an age‐governed response to phytokinin. Phytokinins probably did not directly inhibit RNase activity, increase destruction of the enzyme, or preserve membrane‐bound bodies containing the enzyme. It is suggested that phytokinins affect RNase synthesis directly and that this phase of leaf senescence is more fundamental to the life of the cells than mobilization phenomena.

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