Siwninary. Homogenates of leaflets of desiccated tomato plants show increased ribonuclease activity compared to homogenates of turgid controls. Much of this increase is iindependent of changes in translocation to and from the leaflet. Interruption of translocation through living cel'ls by detachment of leaflets or steam damage to the petiolules prodluces increased ribonuclease activity, but this activity is increased further when excised leaflets are allowed to wilt. Increases in rihonuclease often parallel or precede increases in the soluble nitrogen content. Further increases in activity occur when excised leaves become yellow. Exposure of leaflets to CO,-,free air has little effect on activity at low-light intensitv (120 ft-c). These results suggest that water stress directly affected ribontuclease activity at the cellular level.Increased ribonuclease activity results from various physiological disturbances inl leaves. Kessler and co-workers found greater activity in homogenates of leaves stressed by zinc deficiency (6) and dehydration (4) thani in homogenates of unstressed controls. Ribonuclea;e activity of apple leaves increases with leaf age (5). However, barley leaf discs show less ribonuclease activity when they are floated on water ( 11). These data aDparently confl,ict wvith the w idelv-held belief that detachment of leaflets causes an acceleration of normal patterns of senescence (14).TranSlocation between leaves and the remainder of the pDlant body is often redutced during drought (110, 13) and the assimilation of CO, is also reduced (1). Changes 'in ribonuclease activity during drought may tlherefore be a dlirect response to dehydration of leaf cells or may result from changes in the immediate cellular environment brought about by factors such1 as stomatal closure and reduced translocation.

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.

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Root systems from plants grown in nutrient solution were exposed to air and either transferred to fresh nutrient solution containing (32)P-labeled phosphate or placed in a psychrometer to determine their water potential. The amount of (32)P absorbed by maize and soybean roots in the hour following their exposure to air was proportional to their water potential at the time they were transferred. Some cells, probably located in the stele, were more resistant to moisture stress than others. Absorption of (32)P by all cells was severely inhibited by water potentials below-12 to-15 bars. Nearly normal amounts of the radioisotope and total phosphate were absorbed within 72 hr following root exposure of 4 of 5 species of detopped plants; some phosphorus was lost to the nutrient solution. Uptake of (32)P by passive processes was increased slightly by exposure of roots of intact maize plants to air, but the increase did not compensate for the substantial reduction in actively-absorbed (32)P.

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.