Widespread forest mortality worldwide has been linked to drought. Loss of hydraulic function and consumption of non-structural carbohydrate (NSC) pools (carbon depletion) have been implicated as the two primary physiological mechanisms leading to drought-induced mortality (DIM). However, our ability to predict DIM remains limited because plant hydraulic function and stored NSCs interact in complex ways that remain unresolved and the role of NSC storage on water relations and responses to drought is not fully known.
80 two-year-old ponderosa pine seedlings were planted in pairs in five gallon pots and kept well watered in a greenhouse. Both seedlings in each pot were inoculated with ectomycorrhizae which were allowed to establish a network between plants for a full year. In half of the pots, one of the seedlings was subjected to dark for a month to prevent assimilation, reduce NSC pools and reduce carbon sources to the ectomycorrhizae. We expected that the paired seedling in the light would subsidize carbon demands by the ectomycorrhizae and undergo a natural depletion of NSC. Plants were harvested after the dark treatment, when we measured hydraulic conductivity, NSC concentrations (starch and soluble sugars), and osmotic, pressure, and xylem water potentials across tissues.
Results/Conclusions
As expected, both light and darkened paired seedlings exhibited a reduction of NSC (though greater in the darkened seedlings). These results indicate that paired seedlings in the light subsidized carbon supply to the ectomycorrhizal network when the neighboring seedling was darkened. Although pots were watered, depletion of stored NSCs in both paired seedlings impaired plant water relations: osmotic retention capacity and turgor pressure decreased with decreases of NSC concentrations across all tissues and at the whole plant level. NSC-depleted plants showed higher water potential at the turgor loss point than control plants. However, hydraulic conductivity did not change with NSC depletion. Our results suggest that NSC pools are intimately linked to plant water retention capacity and that depletion of NSC pools under drought may lead to loss of water retention and death. More generally, our results suggest that plant resistance to drought is associated to NSC storage. Thus, forested regions expecting longer, more frequent droughts leading to depletion of stored NSCs may become more vulnerable to DIM in the nearby future.