Control of Na+ and K+ transport in Spergularia manrina. II. Effects of plant size, tissue ion contents and root-shoot ratio at moderate salinity

In this paper we continue our analysis of Na⁺ and K⁺ uptake in vegetative Spergularia marina (L.) Griseb. plants growing on 0.2x sea water in solution culture. We consider the relationships between isotope uptake and plant size, root: shoot ratio (RSR) and total ion contents, both individually (with the linear effects of time removed) and in combinations through stepwise multiple linear regression. The results differ from those of other studies, representing in our case inherent variability in a homogeneous population under steady-state growth conditions. The results were broadly similar for ²²Na⁺ and ⁴²K⁺. Total uptake was significantly and negatively correlated with RSR and root weight (W_r), and positively with root K⁺ content (K_r). These 3 variables were mutually correlated, however, and this was reflected in the multiple analyses as a reduction or loss of significance of one or more of the measures. Transport to the shoot was very highly correlated with total uptake (r² > 0.99 for both isotopes), resulting in nearly identical regression results. In multiple regression analyses of root data alone, accumulation was related only to root contents, but in a manner inconsistent with the allosteric regulation hypothesis, the most significant correlation being positive with K_r. The results were nearly identical for the two isotopes. The results were not consistent with a single factor regulatory system involving only initial root plamalemma ion influx. The observed Na⁺-K⁺ and root-shoot balances seem to require at least involvement of symplast-to-medium and symplast-to-xylem transport steps. Though the biochemical and biophysical signalling and transduction steps are not known, a physiological working hypothesis is presented, in which the positive correlation of uptake with root contents is balanced by a negative feedback signal deriving from plant size and by the diluting effects of growth. Considered over the vegetative period, these would produce the observed stability of plant contents during growth. The negative interaction with RSR is postulated to manifest the integrating system required to deliver ions to the shoot at the required rates.