Characterization of the Na+ delivery from roots to shoots in rice under saline stress: Excessive salt enhances apoplastic transport in rice plants

Sodium transport to the shoots of rice (Oryza sativa L.) plants grown under salt stress conditions was characterized. The rate of Na⁺ transport to shoots increased exponentially depending on the increase in the NaCl concentration of the rooting solution, however, the rates were independent of the plant transpiration. Excessive NaCl in the rooting solution was found to enhance apoplastic transport in rice plants by using the apoplastic dye, 3-hydroxy-5,8,10-pyrene trisulfonic acid (PTS) and Fluostain I (also known as Calcoflour White M2R New). The results suggest that excessive Na⁺ in the rooting solution enlarges the apoplastic pathways, which facilitates Na⁺ intrusion into the xylem vessels, resulting in an excessive accumulation of Na⁺ in rice shoots.     

Seawater‐irrigation effects on growth,ion concentration,and photosynthesis of transgenic poplar overexpressing the Na+/H+ antiporter AtNHX1

Tonoplast Na⁺/H⁺ antiporters increase the salt resistance of various plant species, but very little is known about the role of these antiporters in the salt resistance of trees. Understanding the physiological responses of plants to salinity stress is of paramount importance in examining the salt resistance of transgenic plants. In this study, the wild‐type poplar (WT; Populus × euramericana var. Neva) and its transgenic varieties (TR) that overexpress the AtNHX1 gene were exposed to various seawater concentrations (0%, 10%, 20%, and 30%) for 30 d to determine the effects of seawater on seedling growth, ion content, and photosynthetic productivity. Results show that TR plants grew much better than WT under saline conditions. Differences between WT and TR in most parameters were significant after 30 d exposure to 20% and 30% seawater concentrations. The dry weight of TR was higher than that of WT for each seawater treatment. Transgenic variety was able to maintain higher photosynthetic ability than WT upon exposure to salinity and maintained higher K⁺ concentrations and K⁺ : Na⁺ ratio but had less Cl^– compared with WT. This suggests that AtNHX1 has a critical role in the regulation of K⁺ homeostasis, which in turn affects plant K⁺ nutrition and salt resistance.     

Comparison of the response of ion distribution in the tissues and cells of the succulent plants Aloe vera and Salicornia europaea to saline stress

There exists a great variability among plant species regarding their sensitivity and resistance to high salinity in soil, and most often this variability is related with the ability of a particular plant species to regulate ion homeostasis and transport. In this study, we have investigated the effects of NaCl on growth rate, water status, and ion distribution in different cells and tissues of two succulent plants, Aloe vera and Salicornia europaea. Our results showed that the growth of A. vera seedlings was significantly decreased in response to salinity. However, the growth of S. europaea seedlings was greatly stimulated by high concentrations of NaCl. Under saline conditions, S. europaea seedlings maintained K⁺ and Ca²⁺ uptake in roots and showed a higher root‐to‐shoot flux of Na⁺ and Cl^– as compared to A. vera. Despite great accumulation of Na⁺ and Cl^– in photosynthetically active shoot cells in S. europaea, its growth was enhanced, indicating S. europaea is capable of compartmentalizing salt ions in the vacuoles of shoot cells. Aloe vera seedlings, however, showed a low transport rate of Na⁺ and Cl^– to leaves and suppressed uptake of K⁺ and Ca²⁺ in roots during NaCl treatment. Our results also implicate that A. vera may be able to accumulate Na⁺ and Cl^– in the metabolically inactive aqueous cells in leaves and, as a result, the plant can survive and can maintain growth under saline conditions.     

Negative correlation between the ratio of K+ to Na+ and proline accumulation in tobacco suspension cells

Abstract

The concentrations of K⁺, Na⁺, and proline and the ratio of K⁺ to Na⁺ (K⁺ / Na⁺) were analyzed in NaCl-unadapted and NaCl-adapted tobacco (Nicotiana tabacum) cells in suspension culture. At 3 to 5 d after inoculation, the NaCl-unadapted cells cultured in 100 mmol L^?1 NaCl saline culture medium (Na100 medium) accumulated 28.7 mmol L^?1 proline with a low ratio of K⁺ to Na⁺ (= 2.8) and the NaCl-adapted cells cultured in the Na100 medium contained 6.28 mmol L^?1 proline with a high K⁺ / Na⁺ ratio (≧ 7.5). The contents of amino acids for the NaCl-adapted cells in the Na100 medium were similar to those for the NaCl-unadapted cells in a modified LS medium (standard medium). At 14 d after inoculation, the NaCl-unadapted cells in the Na100 medium contained 4.77 mmol L^?1 proline and restored the K⁺ / Na⁺ ratio from 2.8 to 6.2. These results indicate the presence of a negative correlation between the K⁺ / Na⁺ ratio and proline accumulation and suggest that a balance between the K⁺ / Na⁺ ratio and proline accumulation may be the factor involved in determining the salt tolerance of plant cells.     

Factors related to iron uptake by dicotyledonous and monocotyledonous plants. II. The reduction of Fe3+ as influenced by roots and inhibitors

In a companion paper (10), varieties of four plant species [two monocotyledons (oats and corn) and two dicotyledons (soybeans and tomato)] were shown to differ widely in their ability to respond to Fe‐stress. The ability of the more Fe‐efficient varieties was manifested by a lowering of the pH of the ambient medium of the root and/or by loss of reductants from the root. Both effects can enhance uptake of Fe by the roots, since Fe is taken up primarily, if not entirely, as Fe²⁺ ions. Thus, a given stressed plant has a means, under some degree of metabolic control, for modifying the root environment and, thereby, alleviating its chlorotic condition.

The present investigation deals with environmental factors, particularly chemical inhibitors, modifying the effectiveness of the stress response. Without inhibitors, excised root samples of the four species exhibited a wide range of abilities to reduce Fe³⁺ to Fe²⁺. Roots of the dicotyledonous species reduced about twice as much Fe³⁺ as did equal weights of the monocotyledonous species. Iron‐efficient tomato, soybean, and oat roots reduced more Fe³⁺ than did roots of the Fe‐inefficient varieties. The two corn varieties were about equal in their effectiveness.

Comparable samples of roots were also exposed to chemicals that induce or aggravate Fe chlorosis. Those found to be very effective inhibitors of Fe³⁺ reduction by the roots included: hydroxide, orthophosphate, pyrophosphate, Cu²⁺ and Ni²⁺. Other ions (includ ing Mn²⁺, Zn²⁺ and molybdate) and ethyl ammonium phosphate also inhibited Fe³⁺ reduction but to a lesser degree. Citrate, however, enhanced Fe³⁺ reduction. The degree of inhibition or enhancement differed for each of the varieties. In general, the Fe‐efficient plants were best able to reduce Fe³⁺ in spite of the inhibitory influence of the imposed treatments. Thus, our findings indicated that inhibition of the Fe³⁺ ‐reduction process at, or near, the periphery of the root is an apparent cause of Fe chlorosis.