Ribonuclease (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.