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Articles

Hypobaric Control of Ethylene-Induced Leaf Senescence in Intact Plants of Phaseolus vulgaris L. ¹

Department of Horticulture, Iowa State University, Ames, Iowa 50011

A controlled atmospheric-environment system (CAES) designed to sustain normal or hypobaric ambient growing conditions was developed, described, and evaluated for its effectiveness as a research tool capable of controlling ethylene-induced leaf senescence in intact plants of Phaseolus vulgaris L.

Senescence was prematurely-induced in primary leaves by treatment with 30 parts per million ethephon. Ethephon-derived endogenous ethylene reached peak levels within 6 hours at 26°C. Total endogenous ethylene levels then temporarily stabilized at approximately 1.75 microliters per liter from 6 to 24 hours. Thereafter, a progressive rise in ethylene resulted from leaf tissue metabolism and release. Throughout the study, the endogenous ethylene content of ethephon-treated leaves was greater than that of nontreated leaves.

Subjecting ethephon-treated leaves to atmospheres of 200 millibars, with O₂ and CO₂ compositions set to approximate normal atmospheric partial pressures, prevented chlorophyll loss. Alternately, subjecting ethephon-treated plants to 200 millibars of air only partially prevented chlorophyll loss. Hypobaric conditions (200 millibars), with O₂ and CO₂ at normal atmospheric availability, could be delayed until 48 hours after ethephon treatment and still prevent most leaf senescence. In conclusion, hypobaric conditions established and maintained within the CAES prevented ethylene-induced senescence (chlorosis) in intact plants, provided O₂ and CO₂ partial pressures were maintained at levels approximating normal ambient availability.

An unexpected increase in endogenous ethylene was detected within nontreated control leaves 48 hours subsequent to relocation from winter greenhouse conditions (latitude, 42°00'' N) to the CAES operating at normal ambient pressure. The longer photoperiod and/or higher temperature utilized within the CAES are hypothesized to influence ethylene metabolism directly and growth-promotive processes (e.g. response thresholds) indirectly.

¹ Supported by projects 1893, 2001, 2038, and 2308 of the Agriculture and Home Economics Experiment Station, Ames, IA 50011.