Chloroplasts, isolated from the primary leaves of 7-day-old seedlings, were incubated in vitro at 256C with 2-chloroethylphosphonic acid (ethephon) under light (0.16 milliwatts per square centimeter) and dark conditions. Ethephon at 1 micromolar (0.1445 ppm), 0.1 and 1 millimolar, or 5 microliters ethylene promoted the deterioration of chloroplasts, increased proteolysis, and reduced the chlorophyll content and PSI and PSII during 72 hours under,both light and dark conditions. The decline in PSI and PSII occurred prior to a measurable loss of chlorophyll. The loss of photosynthetic activity affected by ethephon was initiated prior to 12 hours of incubation. After 24 hours in light, 0.1 millimolar (1.445 ppm) epthephon significantly reduced PSI and PSII and promoted the total free amino acid liberation in isolated chlorophsts. In darkness the rate of loss of PSI activity was about 50%O of that in light. After 24 hours, in light at 1 millimolar epthephon, PSII activity was 55% of the control, yet nearly 90% of the chlorophyll remained, which indicates that the loss of thylakoid integrity was promoted by ethephon. Ethylene injected in the chloroplast medium at 5 microliters (0.22 micromolar per milliliter) reduced PSI by nearly 50% of the initial in 12 hours. In leaf sections floated in 5 microliters per milliliter suspension medium, a 36% loss of chlorophyll of the control in 36 hours was observed. Cycloheximide at 0.5 millimolar masked the effect of 1 millimolar ethephon and maintained the initial chlorophyll content during the 72 hour period.To promote maturity or color of apples, blackberries, blueberries, cranberries, pineapple, and tomatoes, foliar sprays of an ethephon solution ranging from 200 to 1000 actual ppm have been the recent field practice (24). Ethylene evolved from ethephon has been known to hasten maturity and the senescence phenomena in plant tissues. Ethylene released from ethephon promotes the loss of Chl (4, 18, 1-9), hydrolysis of polymers (12,14), climacteric rise in respiration-(1, 14), and early maturation or accelerated senescence (1,2,13 MATERIALS AND METHODSChloroplasts were isolated from oat (Avena sativa cv. 'Garry Spring') seedlings which were grown for 7 d in vermiculite at 25°C. Cool white fluorescent tubes provided light for 14 h/d at approximately 0.81 mW cm-2 at the plant level. A bundle of 10 g of apical 5-cm leaves were chilled, cut, surface sterilized, finely minced with a razor blade, and homogenized in a freshly prepared chilled isolation medium A (5). This medium A contained 30 mm Tes, 0.33 M sorbitol, 1 mM EDTA, 1 mM MgCl2, 5 mM sodium ascorbate, and 0.1% BSA fraction V and was adjusted to pH 7.4 with 1 N NaOH. Chloroplasts were aseptically isolated at 0 to 4°C from the homogenate as previously described (6)