Maintenance respiration and carbon balance of plants at low levels of sodium chloride salinity

The daily course of carbon influx and efflux was measured in young plants of Phaseolus vulgaris, Xanthium strumarium, Zea mays, and Atriplex halimus, exposed to low levels of salinity (NaCl) and varying daytime light intensities. Maintenance respiration (R_M) was calculated. In Phaseolus, Xanthium, and Atriplex, R_M rose with increasing salinity, approximately up to those levels of salinity above which apparent signs of toxicity appear. At higher levels of salinity R_M declined. There was no response of R_M to salinity in Zea. At the levels of salinity tested, salinity did not affect the ratio of growth respiration to photosynthesis.At -5 × 10⁵ Pa of NaCl salinity, the increase in R_M in Xanthium was calculated to account for 24% of the growth reduction caused by salt. The remainder could be ascribed to reduced photosynthesis. The increase of R_M is considered to be indicative of an adaptive mechanism, not present in the very salt-sensitive Zea.