Watson R, Pritchard J, Malone M
The School of Biosciences, Division of Agriculture and Horticulture, The University of Nottingham, Sutton Bonington Campus, Loughborough, Leicestershire LE12 5RD, UK.
J Exp Bot. 2001 Sep;52(362):1873-81. doi: 10.1093/jexbot/52.362.1873.
The xylem-feeding insect Philaenus spumarius was used to analyse sodium and potassium fluxes in the xylem of intact, transpiring wheat plants. Two cultivars were compared: the salt-excluding (Chinese Spring) and the non-excluding (Langdon). Chinese Spring accumulated much less sodium in its leaves than the salt-sensitive Langdon. After 7 d in 150 mol m(-3) NaCl, the sodium concentration in the leaf sap of Langdon reached over 600 mol m(-3). This was some three-fold greater than that in Chinese Spring. Similar findings have previously been reported from these cultivars. The reduced ion accumulation was specific to sodium; accumulation of K(+) was unaffected by NaCl in Chinese Spring, such that it developed a much lower leaf Na(+)/K(+) ratio than Langdon. The spittlebug, P. spumarius was used to sample xylem sap from both cultivars. This approach showed that the leaf xylem sap of Chinese Spring had much lower levels of sodium than that of Langdon. In the 150 mol m(-3) NaCl treatment, sodium levels in the leaf xylem reached only 2-3 mol m(-3) in Chinese Spring, compared with 8-10 mol m(-3) in Langdon. Transpiration rates were found to be similar in the two varieties. The lower leaf xylem content alone was thus sufficient to account for the reduced accumulation of sodium in leaves of Chinese Spring. The mechanisms by which xylem sodium might be lowered are discussed and it is concluded that sodium is probably excluded from the xylem in the root of Chinese Spring.
以取食木质部的昆虫沫蝉为实验对象，分析完整的、处于蒸腾状态的小麦植株木质部中的钠钾通量。对两个品种进行了比较：耐盐品种（中国春小麦）和非耐盐品种（兰登）。中国春小麦叶片中积累的钠远少于对盐敏感的兰登。在150 mol m(-3) NaCl环境中处理7天后，兰登叶片汁液中的钠浓度超过600 mol m(-3)。这大约是中国春小麦的三倍。此前对这些品种已有类似研究发现。离子积累的减少是钠特异性的；中国春小麦中钾离子的积累不受NaCl影响，因此其叶片中的钠钾比远低于兰登。利用沫蝉从两个品种中采集木质部汁液。该方法表明，中国春小麦叶片木质部汁液中的钠含量远低于兰登。在150 mol m(-3) NaCl处理下，中国春小麦叶片木质部中的钠含量仅达到2 - 3 mol m(-3)，而兰登为8 - 10 mol m(-3)。发现两个品种的蒸腾速率相似。因此，仅较低的叶片木质部钠含量就足以解释中国春小麦叶片中钠积累减少的现象。文中讨论了降低木质部钠含量的机制，并得出结论，中国春小麦根部可能将钠排除在木质部之外。
