Control of Na+ and K+ transport in Spergularia marina. III. Relationship between ion uptake and growth at moderate salinity

Abstract

In this paper, we continue our analysis of Na+ and K+ uptake by mid‐vegetative Spergularia marina (L.) Griseb. plants growing on 0.2x sea water medium, with attention to the relationship of ion uptake and growth. In the first part of the paper, growth analysis techniques are used to compare relative growth rates (RGR) and relative accumulation rates (RAR) for Na+ and K+. Under constant growth conditions, a high correlation between RGR and RAR indicated that growth and accumulation of both ions were well balanced, resulting in Na+ and K+ concentrations within the plants which were stable after adjustement to the saline medium. The analysis confirmed the existence of a Na+ ‐related growth stimulation in S. marina and an associated increase in the efficiency of K+ utilization for growth. When plants were subjected to more rapid salinization and step changes in the light intensity of the growth chamber, RGR and RAR were again similar, even through the discontinuities in growth conditions, suggesting that growth and ion accumulation were co‐regulated rather than simply correlated.The growth analysis data were then transformed to give net uptake rates for Na+ and K+ and the results were compared to those of isotope studies under similar growth conditions. In roots, the rates estimated by the two techniques differed substantially; net uptake rates reflected primarily growth, while isotope studies indicated a substantial ion exchange rate between mature cells and the growth medium. The rates of transport of either Na+ or K+ to the shoot were very similar using the two estimation techniques. As the rates measured with isotopes were taken from studies lasting at most a few hours, this suggested a very rapid turnover of the upwardly mobile Na+ and K+ pools in the roots.