Ribonuclease Activity of Stressed Tomato Leaflets

L. D. Dove
1967 Plant Physiology  
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
more » ... s 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. Direct effects of water stress may be sel)arated fronm tranllocation effects by comlparinig the ribonuclease activity of homnogenates fronm (a) wilted attached leaflets, (b) Nvilted detached leaflets, (c) unwilted attached leaflets, and (d) unwilted detachled leaflets. Exposure of turgid leaflets to air low in CO, should show if ribonuclease activity responds to changes in amount of CO2 available unider the light intensity used in these experiments. TI'his paper describes the results of these comparisons. Changes in soluble nitrogen content and leaflet co!or provide additional criteria to aid in assessment of damage wvrith eacch stress. Materials and Methods AMar-lobe variety tomato planits were grow-n in a greenhou2Se in sand with applications of nutrient solution (8) unitil thev were 6 weeks old and tlhen trans-ferred to a laboratory witlh cool-white fluorescent lights which gave constant ilIumination of 120 ft-c and a temperature of approximuately 210. Lower leaves of each plant were removed untill 5 leaves over 5 cm long remained. Leaflets of the lowermost leaf were sampled when they lhad reached full size. The experiments are summarized in table 1. Two ml of distilled water maintained a varying number of leaflets from each p!ant in a turgid condition in centrifuge tubes while a varying number of leaflets were al1lowed to wilt. Wilted leaflets and unwilted leaflets wN-ere harvested from time to time after detachment. The petiolules of other detached leaflets were immersed in water and the leaflets were enclosed for 5 days in centrifuge tubes containing vials of 10 % (W/V) NaOH solution. Changes in these leaflets Aere compare(d to changes in detached leaflets exposed to normal air. In another experiment, the sand was brought to f'eld capac.tv wvith distil'ed water and the plants were allowed to lose water until half of them were wilted. Water deficit, soluble nitrogen content and ribonuclease activity of leaflets attached to wilted plants wNere sampled and compared to data from unwilted controls. Petiolules were treated with a thin jet of steam in another experiment and the leaflets were sampled later. Unsteamed leaflets of the same leaf served as controls. All leaflets remained unwilted. Effects of CO., deficiency on ribonuclease activity were also studied. Leaflets attached to unwilted plants were enclosecl for 5 days in glass flasks above a 10 % (W/V) NaOH solution to reduce the content of CO, in the air. Unenclosed leaflets served as controls. Combined etlhanol and water extracts were digested tusing H2 SO4 and 11O. (2), followed by treatmlent with N\'essler's reagent (3). Nitrogen content of these extracts was estimated using a Bausch and Lonmb 1176 www.plantphysiol.org on August 17, 2017 -Published by Downloaded from
doi:10.1104/pp.42.9.1176 pmid:16656636 fatcat:ksy6bna5jzhcnid727xkcoe2fq