Control of Na+ and K+ transport in Spergularia marina I. Transpiration effects

In this paper we begin our study of factors controlling Na⁺ and K⁺ uptake in the halophyte Spergularia marina (L.) Griseb., with emphasis on plants growing at moderate salinity (0.2x sea water). The involvement of transpiration was considered first because of its potential to account for much or all of the transport of ions, and particularly of Na⁺, to the shoot under these growth conditions. Transpiration was constant with time through most of the light period, quickly dropping to 6% of the day time rate at night. ²²Na⁺ uptake, on the other hand, showed much less day/night variation, and relative transport to the shoot was constant. After establishing that transpiration was linearly related to leaf weight, possible transpiration effects were further considered as correlations between leaf weight and transport to the shoot. Under constant, day-time conditions, with linear effects of time and plant size removed, total transport of ²²Na⁺ to the shoot (per plant) was not correlated to leaf weight. A similar result was found when transport was expressed per gram of root, and when partitioning of total label to the shoot was considered. Finally, the correlation was considered between leaf weight and a Na⁺/K⁺ enrichment factor defined as the Na⁺/K⁺ ratio in the leaves divided by that in the roots. This correlation was also insignificant. The results indicate that analysis of control of Na⁺ and K⁺ uptake and transport in this experimental system need not consider effects of transpiration.