Tapping into the physiological responses to mistletoe infection during heat and drought stress

Abstract

Mistletoes are important co-contributors to tree mortality globally, particularly during droughts. In Australia, mistletoe distributions are expanding in temperate woodlands, while their hosts 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 (vapour pressure deficit, photosynthetic 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 $\Psi _{leaf}$ and $\Psi _{trunk}$. 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 had much tighter controls on water loss than uninfected branches of Eucalyptus moluccana, 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.

From leaves to the whole tree: Mistletoe effects on the productivity, water relations, and demography of a Neotropical savanna tree

“…Predawn leaf water potential (Ψ pd ) was marginally less negative in infected individuals compared to uninfected ones irrespective of the season, contradicting previous studies (Ehleringer et al, 1986;Griebel et al, 2022;Scalon et al, 2021). This suggests that V. thyrsoidea leaves under mistletoe influence have virtually similar water status to leaves from trees without mistletoe over the entire year.…”

“…They grow specialized roots (i.e., haustorium) inside their hosts and uptake water and nutrients from the parasitized branches (Aukema, 2003). Consequently, these canopy‐dwelling hemiparasites can create a water and nutritional deficit in the trees they infect (Cocoletzi et al., 2020; Griebel et al., 2022; Lüttge et al., 1998; Meinzer et al., 2004). Host responses to mistletoe infection are often examined at the leaf (Scalon & Wright, 2015; Silva et al., 2021; Wang et al., 2008) or branch level (Reblin & Logan, 2015; Sala et al., 2001; Tennakoon & Pate, 1996).…”

Mitigating the Mistletoe Menace: Biotechnological and Smart Management Approaches

“…Irrigation profiles of the host may variously impact hemiparasite infectivity. In many cases, the higher water status of the host corroborates with heightened mistletoe infectivity, hinting that intermittent drought regimes would offer some protection for the hosts [ 90 , 166 , 167 , 168 ]. Improving the fertility of hosts by the use of nitrogen fertilizers has shown some protection from the mistletoes’ effects, with increments in tree height [ 169 ].…”

“…interaction with their pine hosts [ 272 ]. Climate change, on the other hand, also may variously impact the global distributional shifts in mistletoe [ 166 , 167 , 333 , 334 ]. These factors likely impede field investigations that could otherwise identify genetic controls of mistletoe resistance.…”

Mistletoes could moderate drought impacts on birds, but are themselves susceptible to drought-induced dieback

“…By retaining cations in semi-succulent foliage, mistletoes maintain water balance by passively drawing down a concentration gradient [ 8 , 9 ]. While this enriches tissues, increases water flux and likely within-canopy humidity [ 10 ], their limited control over stomatal closure makes mistletoes sensitive to sudden reductions in moisture availability [ 7 ], with increased evapotranspiration from prolonged hot/dry conditions associated with mistletoe mortality [ 11 , 12 ].…”