Griebel Anne, Peters Jennifer M R, Metzen Daniel, Maier Chelsea, Barton Craig V M, Speckman Heather N, Boer Matthias M, Nolan Rachael H, Choat Brendan, Pendall Elise
Hawkesbury Institute for the Environment, Western Sydney University, Locked Bag 1797, Penrith, NSW 2571, Australia.
Climate Change Science Institute & Environmental Science Division, Oak Ridge National Laboratory, 1 Bethel Valley Rd, Oak Ridge, TN 37831, USA.
Tree Physiol. 2022 Mar 9;42(3):523-536. doi: 10.1093/treephys/tpab113.
Mistletoes are important co-contributors to tree mortality globally, particularly during droughts. In Australia, mistletoe distributions are expanding in temperate woodlands, while their hosts have experienced unprecedented heat and drought stress in recent years. We investigated whether the excessive water use of mistletoes increased the probability of xylem emboli in a mature woodland during the recent record drought that was compounded by multiple heatwaves. We continuously recorded transpiration ($T_{SLA}$) of infected and uninfected branches from two eucalypt species over two summers, monitored stem and leaf water potentials ($\Psi $) and used hydraulic vulnerability curves to estimate percent loss in conductivity (PLC) for each species. Variations in weather (vapor pressure deficit, photosynthetically active radiation, soil water content), host species and % mistletoe foliage explained 78% of hourly $T_{SLA}$. While mistletoe acted as an uncontrollable sink for water in the host even during typical summer days, daily $T_{SLA}$ increased up to 4-fold in infected branches on hot days, highlighting the previously overlooked importance of temperature stress in amplifying water loss in mistletoes. The increased water use of mistletoes resulted in significantly decreased host $\Psi _{\rm{leaf}}$ and $\Psi _{\rm{trunk}}$. It further translated to an estimated increase of up to 11% PLC for infected hosts, confirming greater hydraulic dysfunction of infected trees that place them at higher risk of hydraulic failure. However, uninfected branches of Eucalyptus fibrosa F.Muell. had much tighter controls on water loss than uninfected branches of Eucalyptus moluccana Roxb., which shifted the risk of hydraulic failure towards an increased risk of carbon starvation for E. fibrosa. The contrasting mechanistic responses to heat and drought stress between both co-occurring species demonstrates the complexity of host-parasite interactions and highlights the challenge in predicting species-specific responses to biotic agents in a warmer and drier climate.
槲寄生是全球树木死亡的重要共同促成因素，尤其是在干旱期间。在澳大利亚，槲寄生在温带林地的分布正在扩大，而其寄主近年来经历了前所未有的高温和干旱胁迫。我们调查了在最近由多次热浪加剧的创纪录干旱期间，槲寄生的过度水分利用是否增加了一片成熟林地中木质部栓塞的可能性。我们在两个夏天连续记录了两种桉树种受感染和未受感染枝条的蒸腾作用（$T_{SLA}$），监测了茎和叶的水势（$\Psi$），并使用水力脆弱性曲线来估计每个物种的导水率损失百分比（PLC）。天气变化（水汽压差、光合有效辐射、土壤含水量）、寄主物种和槲寄生叶面积百分比解释了每小时$T_{SLA}$的78%。虽然即使在典型的夏日，槲寄生在寄主中也充当了不可控的水分汇，但在炎热的日子里，受感染枝条的每日$T_{SLA}$增加了多达4倍，突出了温度胁迫在加剧槲寄生水分损失方面此前被忽视的重要性。槲寄生水分利用的增加导致寄主的叶水势（$\Psi _{\rm{leaf}}$）和树干水势（$\Psi _{\rm{trunk}}$）显著降低。这进一步转化为受感染寄主的PLC估计增加高达11%，证实了受感染树木更大的水力功能障碍，使其面临更高的水力故障风险。然而，纤脉桉（Eucalyptus fibrosa F.Muell.）未受感染的枝条对水分损失的控制比粗皮桉（Eucalyptus moluccana Roxb.）未受感染的枝条严格得多，这将水力故障的风险转向了纤脉桉碳饥饿风险的增加。这两种共生物种对高温和干旱胁迫的不同机制反应展示了寄主 - 寄生虫相互作用的复杂性，并突出了在更温暖和更干燥的气候中预测物种对生物因子的特定反应的挑战。
