Analysis of respiratory metabolism correlates well with the response of Eucalyptus camaldulensis seedlings to NaCl and high pH

Abstract

Growth of sand-cultured Eucalyptus camaldulensis Dehnh. (river red gum) seedlings from six wide-ranging provenances was reduced in the presence of 150 mM NaCl, a high pH of 9.5, and combined NaCl and high pH, compared with no applied NaCl and neutral pH. Effects of these stress conditions on respiration rates and substrate carbon conversion efficiencies of rapidly-expanding leaf tissue were measured with calorespirometric techniques. Growth rates were calculated from respiration parameters. Respiration rate, measured as metabolic heat production rate (q), showed no consistent trend with either NaCl or high pH, whereas the rate measured as CO₂ production rate (R _CO2) was generally lower with both treatments. The ratio of heat lost per mole of CO₂ produced [q/(R _CO2)] was consistently increased by both stresses. Stress causes a larger fraction of metabolic energy produced by aerobic metabolism to be lost as heat, relative to non-stressed controls. Consequently, a larger fraction of photosynthetic product in stressed seedlings must be metabolized to CO₂ per mole of C incorporated into biomass. Our results indicate that 0.42 mol substrate C is converted to CO₂ per mole C incorporated into biomass for control plants, compared with 0.96 mol for plants treated with combined NaCl and high pH. Respiratory responses to treatment varied with provenance. Specific growth rates, calculated from repiratory parameters (q and R_CO2) of stressed E. camaldulensis seedlings, generally paralleled experimentally-determined reduced growth (dry weight) of these seedlings. Thus, measurements of leaf respiration allow calculation of growth inhibition caused by NaCl and high pH stress. However, we could not discriminate among provenances in this experiment with only one level of NaCl and pH.