Physiological Site of Ethylene Effects on Carbon Dioxide Assimilation in Glycine max L. Merr

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Abstract
The physiological site of ethylene action on CO2 assimilation was investigated in intact plants of Glycine max L., using a whole-plant, open exposure system equipped with a remotely operated single-leaf cuvette. The objective of the study was met by investigating in control and ethylene-treated plants the (a) synchrony in response of CO2 assimilation, stomatal conductance to water vapor, and substomatal CO2 partial pressure; (b) response of CO2 assimilation as a function of a range of substomatal CO2 partial pressures; and (c) response of CO2 assimilation as a function of a range of photon flux densities. After exposure to 410 micromoles per cubic meter of ethylene for 2.0 hours, CO2 assimilation and stomatal conductance declined in synchrony, while substomatal CO2 partial pressure remained unchanged until exposure times equaled and exceeded 3.0 hours. Because incipient changes in CO2 assimilation occurred without a change in the CO2 partial pressure in the leaf interior, it is concluded that both stomatal physiology and the chloroplast''s CO2 assimilatory capacity were initial sites of ethylene action. After 3.5 hours the effect of ethylene on stomatal conductance and CO2 assimilation exhibited saturation kinetics, and the effect was substantially more pronounced for stomatal conductance than for CO2 assimilation. Based on the response of CO2 assimilation to a range of substomatal CO2 partial pressures, ethylene did not affect either the CO2 compensation point or carboxylation efficiency at subsaturating CO2 partial pressures. Above-ambient supplies of CO2 did not alleviate the diminished rates of CO2 assimilation. In partitioning the limitations imposed on CO2 assimilation in control and ethylene-treated plants, the stomatal component accounted for only 16 and 4%, respectively. The response of CO2 assimilation to a range of photon flux densities suggests that ethylene reduced apparent quantum yield by nearly 50%. Thus, the pronounced decline in net photosynthetic CO2 assimilation in the presence of ethylene was due more to a loss in the mesophyll tissue''s intrinsic capacity to assimilate CO2 than to a reduction in stomatal conductance.