Effects of Ethylene on Photosynthesis and Partitioning in Tomato,Lycopersicon esculentumMill

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Abstract
The extended period of ethylene release from ethephon (2-chloroethylphosphonic acid) after application to intact tomato plants has provided a model system in which the effects of ethylene on photosynthetic metabolism and carbon partitioning has been studied. Ethylene release from leaf tissue after ethephon treatment was 10 times greater than that from untreated control leaves. The specific activity of 14C2H4 released from [14C] ethephon remained constant over several days demonstrating that the ethylene was derived from the applied ethephon. The ethephon-treated plants exhibited extreme epinasty of the leaves and 24 h after application the flower buds in the first visible cluster had abscised, leaf expansion at the apex had ceased and developing adventitious roots were visible on the lower stem. Rates of steady-state photosynthesis, respiration, photorespiration and transpiration were the same in treated and control leaves 24 h after ethephon application. Both treated and control leaves partitioned similar proportions of newly-fixed 14C from 14CO2 into neutral (46.4%), acidic (14.0%), basic (5.0%) and insoluble (34.0%) leaf fractions under steady-state conditions. The speed of 11C-assimilate movement in the stems of control plants (3.62±0.42 cm min-1 towards the apex and 4.03±0.15 cm min-1 towards the roots) was more rapid than in the ethephon-treated plants (2.90±0.31 cm min-1 upwards and 2.59±0.22 cm min-1 downwards). Furthermore, in the control plants 20.0±5.4% of the 14C exported to the plant from the source leaf was transported towards the developing flower cluster and young leaves. Twenty-four hours after ethephon application only 6.5 ±1.7% of the exported 14C was translocated towards the shoot. Contrary to some reports ethylene did not affect steady-state gas exchange processes while carbon partitioning was significantly altered indicating that ethylene effects on photosynthetic carbon metabolism are indirect and not due to direct effects on photosynthetic processes per se.