Abstract
Carbon starvation and hydraulic failure are considered important factors in determining the mechanisms associated with tree mortality. In this study, iso/anisohydric classification was used to assess drought resistance and mortality mechanisms in two contrasting poplar species, as it is generally believed that isohydric species are more susceptible to carbon starvation, while anisohydric species are more susceptible to hydraulic failure. However, these assumptions are rarely tested in poplar genotypes with contrasting growth strategies. Thus, we subjected potted poplar genotypes (I-101 (Populus alba L.) × 84K (Populus alba L. × Populus glandulosa Uyeki)) with fast and slow growth rates to drought–rehydration treatments. The slow-growing genotype maintained higher stomatal conductance and lower predawn leaf water potential than the fast-growing genotype, thus exhibiting a near-anisohydric stomatal behavior throughout the treatment period. The nonstructural carbohydrate (NSC) content indicated that the two genotypes had the same trend of carbon change (e.g., the NSC content in the leaves increased with drought and then decreased). However, when NSC content data were combined with the growth and photosynthetic data, it was observed that the slow-growing genotype mobilized carbon to maintain hydraulic safety, while the NSC content of the fast-growing genotype among tissues was static. The percent loss of hydraulic conductivity in the branches during treatments indicated that the fast-growing genotype could recover more quickly from xylem embolism than the slow-growing genotype. The slow-growing genotype with a slow growth recovery after rehydration showed a significant increase in carbon consumption, combined with a significant increase in the hydraulic safety threshold value, indicating that there may be drought tolerance. In comparison, the fast-growing genotype showed a faster hydraulic recovery ability that had no effect on the NSC content in the leaves and roots. Our findings demonstrate intraspecific isohydric behavior in poplar; however, the trade-off between carbon distribution and stomatal regulation should be considered separately within genotypes of the same species. In addition, NSC plays an important role in water–carbon balance in the drought–rehydration cycle.
Highlights
Over the past 20 years, a growing number of climate-related forest mortality events have been observed [1,2]
Under moderate water stress (MS) conditions, there were significant differences in the predawn and midday water potential between the two genotypes, but no differences were observed under the SS treatment, except that the ψm value was lower for the slow- than the fast-growing genotype
The midday leaf water potentials returned to the control level (−1.44267 and −1.35726 MPa, respectively) and predawn leaf water potential of the slow-growing genotype recovered to the control level after moderate drought stress (−0.75733 MPa)
Summary
Over the past 20 years, a growing number of climate-related forest mortality events have been observed [1,2]. These events are related to drought and have been observed in tropical rainforests [3]. Water deficits and drought in forest and woodland ecosystems are thought to be responsible for the increasing mortality of tree species in arid areas [2]. The rate and magnitude of change in the carbon (C) and water balance of plants are considered important factors for determining the mechanisms associated with plant mortality [8]. Trees need to rely on nonstructural carbohydrate (NSC)
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