Domec J C, Scholz F G, Bucci S J, Meinzer F C, Goldstein G, Villalobos-Vega R
Department of Wood Science and Engineering, Oregon State University, Corvallis, OR 97331, USA.
Plant Cell Environ. 2006 Jan;29(1):26-35. doi: 10.1111/j.1365-3040.2005.01397.x.
Vulnerability to water-stress-induced embolism and variation in the degree of native embolism were measured in lateral roots of four co-occurring neotropical savanna tree species. Root embolism varied diurnally and seasonally. Late in the dry season, loss of root xylem conductivity reached 80% in the afternoon when root water potential (psi root) was about -2.6 MPa, and recovered to 25-40% loss of conductivity in the morning when psi root was about -1.0 MPa. Daily variation in psi root decreased, and root xylem vulnerability and capacitance increased with rooting depth. However, all species experienced seasonal minimum psi root close to complete hydraulic failure independent of their rooting depth or resistance to embolism. Predawn psi root was lower than psi soil when psi soil was relatively high (> -0.7 MPa) but became less negative than psi soil, later in the dry season, consistent with a transition from a disequilibrium between plant and soil psi induced by nocturnal transpiration to one induced by hydraulic redistribution of water from deeper soil layers. Shallow longitudinal root incisions external to the xylem prevented reversal of embolism overnight, suggesting that root mechanical integrity was necessary for recovery, consistent with the hypothesis that if embolism is a function of tension, refilling may be a function of internal pressure imbalances. All species shared a common relationship in which maximum daily stomatal conductance declined linearly with increasing afternoon loss of root conductivity over the course of the dry season. Daily embolism and refilling in roots is a common occurrence and thus may be an inherent component of a hydraulic signaling mechanism enabling stomata to maintain the integrity of the hydraulic pipeline in long-lived structures such as stems.
对四种共生的新热带稀树草原树种侧根中水分胁迫诱导栓塞的脆弱性以及天然栓塞程度的变化进行了测量。根栓塞随昼夜和季节变化。在旱季后期，当根水势（ψ根）约为-2.6 MPa时，下午根木质部导水率损失达到80%，而在早晨当ψ根约为-1.0 MPa时，导水率损失恢复到25%-40%。随着生根深度的增加，ψ根的日变化减小，根木质部脆弱性和电容增加。然而，所有物种在接近完全水力衰竭时都经历了季节性最低ψ根，这与其生根深度或栓塞抗性无关。当土壤水势（ψ土壤）相对较高（>-0.7 MPa）时，黎明前的ψ根低于ψ土壤，但在旱季后期，其负值小于ψ土壤，这与从夜间蒸腾引起的植物与土壤ψ之间的不平衡状态向深层土壤层水分水力再分配引起的不平衡状态的转变相一致。木质部外部浅的纵向根切口阻止了栓塞在夜间的逆转，这表明根的机械完整性对于恢复是必要的，这与如果栓塞是张力的函数，再填充可能是内部压力不平衡的函数这一假设一致。在旱季过程中，所有物种都有一个共同的关系，即最大日气孔导度随着下午根导水率损失的增加而线性下降。根中的每日栓塞和再填充是常见现象，因此可能是水力信号机制的一个固有组成部分，使气孔能够在茎等长寿结构中维持水力管道的完整性。
