Abstract
Climate change is likely to lead to an increased frequency of droughts and floods, both of which are implicated in large-scale carbon allocation and tree mortality worldwide. Non-structural carbohydrates (NSCs) play an important role in tree survival under stress, but how NSC allocation changes in response to drought or waterlogging is still unclear. We measured soluble sugars (SS) and starch in leaves, twigs, stems and roots of Robinia pseudoacacia L. seedlings that had been subjected to a gradient in soil water availability from extreme drought to waterlogged conditions for a period of 30 days. Starch concentrations decreased and SS concentrations increased in tissues of R. pseudoacacia seedlings, such that the ratio of SS to starch showed a progressive increase under both drought and waterlogging stress. The strength of the response is asymmetric, with the largest increase occurring under extreme drought. While the increase in SS concentration in response to extreme drought is the largest in roots, the increase in the ratio of SS to starch is the largest in leaves. Individual components of SS showed different responses to drought and waterlogging across tissues: glucose concentrations increased significantly with drought in all tissues but showed little response to waterlogging in twigs and stems; sucrose and fructose concentrations showed marked increases in leaves and roots in response to drought but a greater response to drought and waterlogging in stems and twigs. These changes are broadly compatible with the roles of individual SS under conditions of water stress. While it is important to consider the role of NSC in buffering trees against mortality under stress, modelling this behaviour is unlikely to be successful unless it accounts for different responses within organs and the type of stress involved.
Highlights
Extreme weather events, such as summer drought or heavy precipitation and flooding, are expected to occur more frequently in the future as a result of anthropogenic climate change [1,2].Widespread drought-induced tree mortality has been widely documented in recent years [3,4,5,6], and more such events are anticipated by climate models in the future [7,8]
In the extreme drought treatments, the reduction in root biomass was proportionally less than the reduction in the aboveground tissues, and in some cases (e.g., W30 in both years), decreased biomass in the aboveground tissues was accompanied by an increase in root biomass (Figure 1b and Table S1)
Our results indicated that the optimal growth conditions for R. pseudoacacia seedlings occurred when soil water content (SWC) was 70% (W70), resulting in increased biomass over the 30-day experiment and maximum concentrations of starch in all plant tissues
Summary
Extreme weather events, such as summer drought or heavy precipitation and flooding, are expected to occur more frequently in the future as a result of anthropogenic climate change [1,2].Widespread drought-induced tree mortality has been widely documented in recent years [3,4,5,6], and more such events are anticipated by climate models in the future [7,8]. Isotopic studies indicate that most NSCs are used for respiration [12], providing a rapid link between assimilation, allocation, metabolism, and defence mechanisms [13,14]. They act as a crucial reserve that can be used in times of stress [15,16]. There is evidence that NSCs can be stored for many years: Richardson et al [17] found that the NSCs in the trunks of temperate forest trees were, on average, 7–14 years old Some of this stored material may be quarantined and unavailable for future utilization [18], but other studies show that stored NSCs can be re-mobilized and used during times of stress [19,20,21,22]. There is still much to be learned about the mechanisms of NSC allocation to different structural components of the plant, the different forms of NSC, and changes in allocation and recycling under stress conditions [29]
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