Submitted to: New Phytologist
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/5/1997
Publication Date: N/A
Citation: Interpretive Summary: There are large land areas, particularly in the western U.S., where crop production has been impacted by the accumulation of high levels of salinity in the soil. Increasing soil salinity has resulted from irrigation and fertilization practices, and has reduced the growth and economic yield of many important crops. A considerable research effort has recently been focused on mechanisms of salt toxicity in crop plants, and also directed towards understanding how some plant species can tolerate toxic levels of salt in the soil. By gaining a better understanding of mechanisms of salt toxicity and tolerance in plants, it may be possible to develop agricultural practices as well as plant species and cultivars that will enable farmers to cultivate these saline, marginal soils. It has previously been shown that salt toxicity involves, in part, inhibition of the transport of calcium (an essential mineral nutrient in plants) to plant shoots. In this study, we investigated the interaction between toxic level of salt (as sodium chloride or sulfate) and the transport pathway for calcium from the root to the shoot. Both radioactive forms of calcium and calcium-selective electrodes were used to quantify calcium transport throughout the plant. It was found that salt stress had a much bigger effect on cell-to-cell calcium transport through the root than it did on calcium movement through root cell walls (the two primary pathways for radial mineral and water movement across the root). These findings indicate that future research in this area of salt toxicity should focus on specific cell types within the root (endodermal cells) that are involved in the regulation of calcium movement from the root to the plant shoot.
Technical Abstract: Salinity stress inhibits Ca translocation to the shoot, leading to Ca deficiency. The objective of this study was to determine whether salt stress inhibits Ca translocation through effects on younger root regions, where radial Ca transport is largely apoplastic, or through effects on mature regions of the roots, where radial Ca transport is largely symplastic. Roots were excised from 4-day old dark-grown barley seedlings. Calcium translocation was studied by application of 45Ca 3 or 6 cm from the root tip and measurement of its appearance in the root exudate. Calcium uptake along the axis of excised roots was studied with a vibrating Ca+2 microelectrode. Salt stress (60 mM NaCl) inhibited Ca translocation more from the 6 cm region than from the 3 cm region. Imposition of osmotic and ionic stress (with 110 mM mannitol and 60 mM KC1) affected Ca translocation nsimilarly to salt stress. The salt and mannitol treatments were more dele- terious for Ca translocation in the 6 cm region than in the 3 cm region, and mannitol and KC1 inhibited Ca translocation more than the sodium treat- ments. Supplemental Ca could overcome the inhibition of Ca translocation by NaC1 stress. Calcium uptake into the root at the 3 and 6 cm regions was not reduced by NaCl, but was inhibited at the root apex. We conclude that sym- plastic Ca transport is inhibited more significantly than apoplastic trans- port in salinized roots, and that the osmotic component of salt stress causes the majority of the inhibition. Since symplastic trans-port is inhibited, the possibility exists that research into the effects of salinity on the transport functions of endodermal cells will yield informa- tion that can be used to improve Ca translocation in salt-affected plants.