干旱胁迫对油松和侧柏水分运输安全性和有效性的影响

通过采用改良的冲洗法测定5年生苗木油松(Pinus tabulaeformis)和侧柏(Platycladus orientalis)在不同干旱胁迫时期的水力结构参数,结果表明随着干旱胁迫增加,不同分枝级和茎段所在区域的导水率损失(PLC)增加,比导率(Ks)减少。油松和侧柏在0级,1级和2级分枝级的发生栓塞的水势阈值分别为-0.55、-0.49、-0.43和-0.90、-0.78、-0.74MPa。随着相对分枝级的增加,油松和侧柏的水势阈值增大,栓塞脆弱性变大。非限速区PLC大而Ks小,限速区PLC小而Ks大。油松和侧柏相对分枝级和茎段所在区域的栓塞脆弱性大小为2级1级0级,限速区非限速区,且油松大于侧柏。油松和侧柏在不同干旱胁迫,不同相对分枝级,不同茎段所在区域采取不同的方式来适应由水势降低而引起的栓塞变化。其采取的生态策略包括:保持较高的水分安全性;减轻安全性而对有效性的折衷;同时降低有效性和安全性但不终止任何生产力或树高的组织生长所需水的限制。

Influences of drought stress on hydraulic safety and efficiency in the saplings of Pinus tabulaeformis and Platycladus orientalis

Hydraulic architecture parameters of 5-year-old saplings of Pinus tabulaeformis and Platycladus orientalis under drought stress were measured using the improved flushing method. The hydraulic architecture of trees is based on differences in the hydraulic properties of the xylem and is characterized by two important functional aspects, hydraulic safety and hydraulic efficiency. The hydraulic efficiency is proportional to the specific conductivity (Ks) of the xylem, while the hydraulic safety describes the resistance of the xylem against embolism formation, which quantized through the percent loss of conductivity (PLC). The aims were to study the compromises between hydraulic safety and hydraulic efficiency and adaptations by determining the hydraulic architecture parameters changed with drought stress of P. tabulaeformis and P. orientalis, which have different leaf types and ecological strategies, and to search the mechanism influenced the trade-offs. The results showed that PLC increased and Ks decreased in all the ramification and stem segments with increasing drought stress. The water potential thresholds were -0.55, -0.49, -0.43 MPa for P. tabulaeformis and -0.90, -0.78, -0.74 MPa for P. orientalis respectively, in the three categories of branches 0, 1 and 2. As the water thresholds increased, embolisms vulnerability was highest in branch 2, followed by branch 1 and branch 0 in both species, and higher in P. tabulaeformis than that in P. orientalis. This may relate to the number of water transportation tissues in different branches or the ability of resistance to the embolism. PLC increased and Ks decreased in non-restrict area stem segment, visa versa in the restrict area. As the PLC reached the maximum, the water potential were -5.08, -4.62, -4.07 and -6.79, -6.40, -6.21 MPa respectively in the three different branches. Therefore, the ecological strategy of P. tabulaeformis and P. orientalis were as much as possible to serve the trunk or lower ramification branch in this phase. The vulnerability was higher in non-restrict area stem segment than in restrict area stem segment. P. tabulaeformis showed higher vulnerability than P. orientalis, may be relate to the number or diameter of the tracheid and less possibility to cavitation in restrict area stem segment. Under drought stress, P. tabulaeformis and P. orientalis showed different strategies to adapt to the embolism caused by the decreased water potential with relative ramification in different area stem segment. The strategies included: (1) keeping higher hydraulic security, such as the lowest PLC reduction of P. tabulaeformis and P. orientalis under light drought mean higher hydraulic safety; (2) decreasing hydraulic security to tradeoff the hydraulic efficiency, i.e. branch 2 of P. orientalis had highest Ks reduction and lowest PLC reduction under heavy drought, which represented higher hydraulic and lower hydraulic efficiency than other branches; and (3) decreasing hydraulic safety and hydraulic efficiency to level at which tree growth is not restricted. That occurred in the branch 0 of P. tabulaeformis under serious drought, showed that both hydraulic safety and hydraulic efficiency were decreased.