Lee Seong Hee, Chung Gap Chae, Steudle Ernst
Lehrstuhl Pflanzenökologie, Universität Bayreuth, D-95440 Bayreuth, Germany.
J Exp Bot. 2005 Mar;56(413):985-95. doi: 10.1093/jxb/eri092. Epub 2005 Feb 25.
Effects of low temperature (8 degrees C) on the hydraulic conductivity of young roots of a chilling-sensitive (cucumber, Cucumis sativus L.) and a chilling-resistant (figleaf gourd, Cucurbita ficifolia Bouche) crop have been measured at the levels of whole root systems (root hydraulic conductivity, Lp(r)) and of individual cortical cells (cell hydraulic conductivity, Lp). Exposure of roots to low temperature (LRT) for up to 6 d caused a stronger suberization of the endodermis in cucumber compared with figleaf gourd, but no development of exodermal Casparian bands in either species. Changes in anatomy after 6 d of LRT treatment corresponded with a reduction in hydrostatic root Lp(r) of cucumber roots by a factor of 24, and by a factor of 2 in figleaf gourd. In figleaf gourd, there was a reduction only in hydrostatic Lp(r) but not in osmotic Lp(r) suggesting that the activity of water channels was not much affected by LRT treatment in this species. Changes in cell Lp in response to chilling and recovery were similar to the root levels, although they were more intense at the root level. Activation energies (E(a)) and Q10 of water flow as measured at the cell level were high in cucumber (E(a)=109+/-13 kJ mol(-1); Q(10)=4.8+/-0.7; n=6-10 cells), but small in figleaf gourd (E(a)=11+/-2 kJ mol(-1); Q10=1.2+/-0.1; n=6-10 cells). Roots of figleaf gourd recovered better from LRT treatment than those of cucumber. In figleaf gourd, recovery (at both the root and cell level) often resulted in Lp and Lp(r) values which were even bigger than the original, i.e. there was an overshoot in hydraulic conductivity. These effects were larger for osmotic (representing the cell-to-cell passage of water) than for hydrostatic Lp(r). After a short-term (1 d) exposure to 8 degrees C followed by 1 d at 20 degrees C, hydrostatic Lp(r) of cucumber nearly recovered and that of figleaf gourd still remained higher due to the overshoot. By contrast, osmotic Lp(r) and cell Lp in both species remained high by a factor of 3 compared with the control, possibly due to an increased activity of water channels. After preconditioning of roots at LRT, increased hydraulic conductivity was completely inhibited by HgCl2 at both the root and cell levels. Different from figleaf gourd, recovery from chilling was not complete in cucumber after longer exposure to LRT. It is concluded that at LRT, both changes in the activity of aquaporins (AQPs) and alterations of root anatomy determine the water uptake in both species. The high temperature dependence of cell Lp in cucumber suggests conformational changes of AQPs during LRT treatment which result in channel closure and in a strong gating of AQP activity by low temperature. This mechanism is thought to be different from that in figleaf gourd where AQPs reacted in the conventional way, i.e. low temperature affected the mobility of water molecules in AQPs rather than their open/closed state, and Q(10) was low.
在整个根系水平（根水力导度，Lp(r)）和单个皮层细胞水平（细胞水力导度，Lp）上，测定了低温（8摄氏度）对冷敏感作物（黄瓜，Cucumis sativus L.）和抗冷作物（黑籽南瓜，Cucurbita ficifolia Bouche）幼根水力导度的影响。将根系暴露于低温（LRT）长达6天，与黑籽南瓜相比，黄瓜内皮层的栓质化作用更强，但两种植物的外皮层凯氏带均未发育。LRT处理6天后的解剖结构变化与黄瓜根静水压Lp(r)降低24倍、黑籽南瓜降低2倍相对应。在黑籽南瓜中，仅静水压Lp(r)降低，而渗透Lp(r)未降低，这表明该物种中水分通道的活性受LRT处理影响不大。响应低温及恢复过程中细胞Lp的变化与根系水平相似，尽管在根系水平上变化更为强烈。在细胞水平上测定的水流活化能（E(a)）和Q10在黄瓜中较高（E(a)=109±13 kJ mol(-1)；Q(10)=4.8±0.7；n = 6 - 10个细胞），而在黑籽南瓜中较小（E(a)=11±2 kJ mol(-1)；Q10=1.2±0.1；n = 6 - 10个细胞）。黑籽南瓜的根系比黄瓜的根系从LRT处理中恢复得更好。在黑籽南瓜中，恢复（在根系和细胞水平）通常导致Lp和Lp(r)值甚至比原来更大，即水力导度出现超调。这些效应在渗透（代表细胞间水分传递）方面比静水压Lp(r)更大。在短期（1天）暴露于8摄氏度随后在20摄氏度下放置1天后，黄瓜的静水压Lp(r)几乎恢复，而黑籽南瓜由于超调其静水压Lp(r)仍然较高。相比之下，两种植物的渗透Lp(r)和细胞Lp与对照相比仍高3倍，这可能是由于水分通道活性增加所致。在将根系在LRT下预处理后，根系和细胞水平的水力导度增加均被HgCl2完全抑制。与黑籽南瓜不同，黄瓜在长时间暴露于LRT后，低温恢复并不完全。得出的结论是，在LRT下，水通道蛋白（AQP）活性的变化和根系解剖结构的改变共同决定了两种植物的水分吸收。黄瓜中细胞Lp对温度的高度依赖性表明，LRT处理期间AQP发生构象变化，导致通道关闭以及低温对AQP活性的强烈门控作用。这种机制被认为与黑籽南瓜不同，在黑籽南瓜中AQP以传统方式反应，即低温影响AQP中水分子的流动性而非其开放/关闭状态，且Q(10)较低。
