Limits to growth of Salicornia bigelovii Torr. at suboptimal salinity

Abstract

In spite of the fact that it is one of the most widely researched subjects in plant biology, the physiology of salt tolerance is still not well understood. This research was undertaken to investigate salt tolerance in the extreme halophyte, Salicornia bigelovii Torr. Halophytes, plants that are naturally salt tolerant, are well suited to research on salt tolerance because millions of years of evolution have fine tuned their adaptation to high salinity to the extent that their growth is inhibited when they are grown in reduced salinity. S. bigelovii was grown in different concentrations of NaCl and growth responses were compared. My emphasis was on salinity effects on cell wall extensibility, wall yielding threshold (minimum turgor required for growth), and water relations. When S. bigelovii was grown in low salinity, relative growth rate was slower, fresh and dry weight, relative water content and succulence were reduced, and both epidermal and cortex parenchyma cells were smaller. The plants also accumulated less Na⁺ and more K⁺, Ca²⁺ and Mg²⁺. These results prompted two specific questions. Was the excess Ca²⁺ accumulated by the plants grown in low salinity bound to cell walls and did it cause reduced cell wall extensibility and increased yield threshold? Was growth inhibition a consequence of low turgor due disturbances in water relations? Even though there was three times more Ca²⁺ in the walls of the plants grown in I salinity, cell wall extensibility was not significantly different between salinity treatments. However, the wall yielding threshold of the plants grown in low salinity was significantly lower. Turgor was also significantly lower in these plants. But, since the minimum turgor required for growth was even lower, reduced turgor was not responsible for growth inhibition. Based upon the results of this research, I have concluded that growth inhibition in S. bigelovii in reduced salinity was not due to disturbances in water relations nor was it the result of detrimental changes in cell wall properties. It is becoming increasingly clear that specific ion effects play an important role in limiting the growth of S. bigelovii in low salinity. This research has led to the identification of several new directions for future investigation into the salt tolerance mechanisms of this unique plant.