van Ieperen W, van Meeteren U, van Gelder H
Department of Plant Sciences, Wageningen University, The Netherlands.
J Exp Bot. 2000 Apr;51(345):769-76. doi: 10.1093/jexbot/51.345.769.
The direct effect of fluid composition on xylem hydraulic conductance is investigated in excised stem segments of chrysanthemum (Dendranthema x grandiflorum Tzvelev cv. Cassa) plants. Dynamic changes in hydraulic conductance are accurately measured at 30 s intervals before and after modifications of the composition of the standard fluid (deionized water). It is investigated whether osmotic properties of the flowing solution influence overall hydraulic conductance by affecting the hydraulic conductance of vessel-to-vessel pit membranes, as has previously been suggested. Various iso-osmotic salt solutions (20 mOsm kg-1) of different composition raised the hydraulic conductance of 20 cm long stem segments approximately 5-8% compared to deionized water. In contrast, carbohydrate solutions with similar osmotic strength and pH did not cause any change in hydraulic conductance. KCl solutions that greatly differed in osmotic strength all increased hydraulic conductance, but the response was not correlated with the osmotic strength of the solution. Increasing the number of vessels that were open from one cut end to the other by shortening the stem segments greatly increased the hydraulic conductance response. Changing from deionized water to a salt solution caused an immediate increase in hydraulic conductance, while a shift back to deionized water resulted in a slow decline. This decline lasted longer when the salt solution contained divalent cations compared to monovalent cations. It is concluded that the presence of cations and not the osmotic strength in the flowing solution influenced the hydraulic conductance. The phenomenon is not caused by the vessel-to-vessel pit membranes, which in fact suppressed the effect, due to their large contribution to the overall resistance to water flow.
在菊花（Dendranthema x grandiflorum Tzvelev cv. Cassa）植株的离体茎段中研究了流体成分对木质部水力导度的直接影响。在标准流体（去离子水）成分改变之前和之后，每隔30秒精确测量水力导度的动态变化。研究了流动溶液的渗透特性是否如先前所提出的那样，通过影响导管间纹孔膜的水力导度来影响整体水力导度。与去离子水相比，不同成分的各种等渗盐溶液（20 mOsm kg-1）使20厘米长的茎段的水力导度提高了约5 - 8%。相比之下，具有相似渗透强度和pH值的碳水化合物溶液并未引起水力导度的任何变化。渗透强度差异很大的KCl溶液均增加了水力导度，但这种响应与溶液的渗透强度无关。通过缩短茎段来增加从一个切割端到另一个切割端开放的导管数量，极大地增加了水力导度响应。从去离子水转变为盐溶液会导致水力导度立即增加，而转回去离子水则导致缓慢下降。与单价阳离子相比，当盐溶液含有二价阳离子时，这种下降持续的时间更长。得出的结论是，流动溶液中阳离子的存在而非渗透强度影响了水力导度。这种现象不是由导管间纹孔膜引起的，实际上由于它们对整体水流阻力的巨大贡献，抑制了这种影响。
