Root relative water content is a potential signal for impending mortality of a subtropical conifer during extreme drought stress

Abstract

Adaptation to future climates characterized by more frequent severe droughts requires enhanced mechanistic understanding of tree mortality. However, our knowledge of the physiological limits to withstand extreme drought, and how the coordination between water and carbon traits enhances survival, is still limited. Potted seedlings of Pinus massoniana were dehydrated to three target droughts (percentage loss of stem hydraulic conductivity of ca. 50%, 85%, and 100%; PLC50, PLC85 and PLC100) and then relieved from these target droughts by fully rewatering. Predawn and midday water potentials (Ψ), relative water content (RWC), PLC and nonstructural carbohydrates (NSC) were monitored. During drought, Ψ and RWC declined as PLC increased. Root RWC declined more rapidly than other organ RWCs, particularly after PLC50 stress. All organ NSC concentrations were above predrought values. During rewatering, water trait recovery declined as drought increased, with no mortality at PLC50 but 75% mortality at PLC85. The observed stem hydraulic recovery at PLC50 following rewatering was not correlated to NSC dynamics. Collectively, our results highlighted the primary role of hydraulic failure in Pinus massoniana seedling mortality by assessing mortality threshold and links among water status and water supply. Root RWC can be considered as a potential warning signal of P. massoniana mortality.

The Composite Physiological Response of Hydraulic and Photosynthetic Traits and Nonstructural Carbon in Masson Pine Seedlings to Drought Associated with High Temperature

“…Unlike species that dominate in arid or semi-arid regions and have adapted to dry conditions, those tree species dominating in humid areas may be more susceptible to changes brought by flash drought due to less investment in hydraulic safety [12,13]. So far, the response and adaptation mechanisms of tree species in subtropical China have drawn the attention of governments and scientists [16][17][18], but we still need more in-depth studies on more species to uncover the inter-and intraspecies differences, which will provide a more accurate prediction of the drought impact on forests.…”

“…Masson pine (Pinus massoniana Lamb.) is a dominant coniferous species in southern China, known for its rapid growth, abundant yield, and extensive utilization [17,19]. Despite the robust adaptability of Masson pine and the rich annual precipitation in its distribution areas, this species still faces the mortality risk caused by the recurrent high temperatures in summer and low precipitation in subtropical regions [20], which may worsen the drought threat due to the above-mentioned climate change [7,11,15].…”

“…Despite the robust adaptability of Masson pine and the rich annual precipitation in its distribution areas, this species still faces the mortality risk caused by the recurrent high temperatures in summer and low precipitation in subtropical regions [20], which may worsen the drought threat due to the above-mentioned climate change [7,11,15]. To explore the response of Masson pine to drought, previous studies have emphasized the leaf gas exchange [21], photosynthetic characteristics [22,23], biochemical parameters [18,24,25], hydraulic conductance and nonstructural carbon [17], root tip structure and volatile organic compounds [26], mycorrhizal fungus [22,25,27], and stress-induced defensive secondary metabolites (such as flavonoids and terpenoids [28]).…”

“…Briefly, these previous studies aimed to address how Masson pine adjusted its physiological response to different drought levels, but only one study [21] designed the drought treatment under a higher daytime temperature (35 • C) to simulate a flash drought. Additionally, among these studies, only one study tried to explore the mechanisms of droughtinduced mortality from the interaction of hydraulic failure and carbon starvation [17], the two most recognized physiological mechanisms leading to tree mortality [14,29]. In this study [17], the researchers did not find clear synchronization between hydraulic damage (denoted as the percentage loss of hydraulic conductivity, PLC) and carbon depletion (denoted as the dynamic of nonstructural carbohydrate, NSC) as demonstrated in other species [30][31][32] and determined the decisive role of PLC but not NSC on the mortality of Masson pine seedlings [17].…”

“…Additionally, among these studies, only one study tried to explore the mechanisms of droughtinduced mortality from the interaction of hydraulic failure and carbon starvation [17], the two most recognized physiological mechanisms leading to tree mortality [14,29]. In this study [17], the researchers did not find clear synchronization between hydraulic damage (denoted as the percentage loss of hydraulic conductivity, PLC) and carbon depletion (denoted as the dynamic of nonstructural carbohydrate, NSC) as demonstrated in other species [30][31][32] and determined the decisive role of PLC but not NSC on the mortality of Masson pine seedlings [17]. As the depletion of NSC is one part of the carbon budget, its role may need to be determined by incorporating the dynamics of carbon input by photosynthesis, which usually would be constrained within the drought process.…”