Ethylene reduces gas exchange and growth of lettuce plants under hypobaric and normal atmospheric conditions.

Elevated levels of ethylene occur in controlled environment agriculture and in spaceflight environments, leading to adverse plant growth and sterility. The objectives of this research were to characterize the influence of ethylene on carbon dioxide (CO 2 )assimilation (C A ), dark period respiration (DPR) and growth of lettuce (Lactuca sativa L. cv. Buttercrunch) under ambient and low total pressure conditions. Lettuce plants were grown under variable total gas pressures of 25 kPa (hypobaric) and 101 kPa (ambient) pressure. Endogenously produced ethylene accumulated and reduced C A , DPR and plant growth of ambient and hypobaric plants. There was a negative linear correlation between increasing ethylene concentrations [from 0 to around 1000 nmol mol -1 (ppb)] on C A , DPR and growth of ambient and hypobaric plants. Declines in C A and DPR occurred with both exogenous and endogenous ethylene treatments. C A was more sensitive to increasing ethylene concentration than DPR. There was a direct, negative effect of increasing ethylene concentration reducing gas exchange as well as an indirect ethylene effect on leaf epinasty, which reduced light capture and C A . While the C A was comparable, there was a lower DPR in hypobaric than ambient pressure plants - independent of ethylene and under non-limiting CO 2 levels (100 Pa pCO 2 , nearly three-fold that in normal air). This research shows that lettuce can be grown under hypobaria (≅25% of normal earth ambient total pressure); however, hypobaria caused no significant reduction of endogenous ethylene production.