Age‐governed Responses of Detached Tomato Leaves to Phytokinins

Abstract

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.

Plant Nucleases

“…Changes in RNase levels in plants have been associated with injury (2, 7), infection (7, 12, 13), growth or growth inhibition (1,14), RNA synthesis (9), senescence (11,15), and endosperm maturation (8,5 (1) found that a light-mediated increase in RNase of lupine hypocotyls was associated with polysomes and ribosomes. This RNase appeared to be either RNase I or II.…”

“…3.1.4.x), which has a pH optimum near 6, produces 5'-nucleotides from both RNA and DNA, and is associated with large cell particles (20, 21). Pure RNase I has been obtained from corn endosperm (22), while partially purified RNase II and Nuclease I have been obtained from corn roots (20).Changes in RNase levels in plants have been associated with injury (2, 7), infection (7, 12, 13), growth or growth inhibition (1,14), RNA synthesis (9), senescence (11,15), and endosperm maturation (8,5 (1) found that a light-mediated increase in RNase of lupine hypocotyls was associated with polysomes and ribosomes. This RNase appeared to be either RNase I or II.…”

Changes in Ribonuclease Activity of Wheat Plants during Water Stress

“…actinomycin D before injury completely prevented the rise in RNase activity (Bagi and Farkas, 1967). The increase in RNase activity was greater in light than that in darkness (McHale and Dove, .1969;and Sodek and Wright, 1969) during senescence. Fragments of Avena leaves were incubated in Petri dishes in the presence of cycloheximide (50 ~g/ml), and the increase in RNase activity following excision of leaf tissues was completely inhibited by cycloheximide (Udvardy, et al, 1969).…”

“…The production of reducing sugar promoted by gibberellic acid was inhibited by treatments with abscisic acid and pancreatic RNase during germination of barley seeds (Leshem, 1971). Cytokinin inhibited an increase in RNase activity in detached tomato leaves (McHale and Dove, 1969).…”

Ribonuclease Activity in Tomato Leaves as Related to Development and Senescence

“…Differences in responses of fifth and eighth leaves during senescence might be expected to be reflected in responses to phytokinin-induced delay of senescence. These differences will be discussed in a subsequent paper (McHale and Dove, 1968).…”