How tree roots respond to drought

Abstract

The ongoing climate change is characterized by increased temperatures and altered precipitation patterns. In addition, there has been an increase in both the frequency and intensity of extreme climatic events such as drought. Episodes of drought induce a series of interconnected effects, all of which have the potential to alter the carbon balance of forest ecosystems profoundly at different scales of plant organization and ecosystem functioning. During recent years, considerable progress has been made in the understanding of how aboveground parts of trees respond to drought and how these responses affect carbon assimilation. In contrast, processes of belowground parts are relatively underrepresented in research on climate change. In this review, we describe current knowledge about responses of tree roots to drought. Tree roots are capable of responding to drought through a variety of strategies that enable them to avoid and tolerate stress. Responses include root biomass adjustments, anatomical alterations, and physiological acclimations. The molecular mechanisms underlying these responses are characterized to some extent, and involve stress signaling and the induction of numerous genes, leading to the activation of tolerance pathways. In addition, mycorrhizas seem to play important protective roles. The current knowledge compiled in this review supports the view that tree roots are well equipped to withstand drought situations and maintain morphological and physiological functions as long as possible. Further, the reviewed literature demonstrates the important role of tree roots in the functioning of forest ecosystems and highlights the need for more research in this emerging field.

Keywords: abscisic acid; avoidance; carbon sequestration; hydraulic signals; molecular responses; mycorrhizas; tolerance; tree root traits.

FIGURE 1

Mechanisms of drought resistance and selected examples of tree root traits that respond to drought with avoidance or tolerance. indicates a positive effect, indicates a predominantly positive trend, indicates a negative effect and indicates a predominantly negative trend. Categories of tree root traits correspond to those given in Table 1. ABA, abscisic acid.

FIGURE 2

Schematic diagram illustrating how water moves into mycorrhizal roots. (A) Water movement under non-drought and (B) water movement under drought conditions. Non-drought conditions: water transport through the roots involves a combination of apoplastic (1), symplastic (2), and transcellular (3) pathways. Drought conditions: water flow is mainly transcellular (3), which causes a high hydraulic resistance as water passes across many membranes via aquaporins. Aquaporins then may act as valves to reversibly increase the hydraulic conductivity, and the suberisation of roots minimizes water loss to the dry soil (4) (according to Steudle, 2000). External mycelium of the fungal mantle facilitate water uptake (5), and melanised fungal hyphae prevent cortical cells from desiccation (6) (according to Lehto and Zwiazek, 2011; Fernandez and Koide, 2013). c, Casparian strip; cc, cortical cells; e, endodermis; em, external mycelium; h, Hartig net; m, fungal mantle; p, plasmodesma; pe, pericycle; ph, phloem; w, water molecules; x, xylem.

FIGURE 3

Schematic diagram illustrating how soil moisture deficit results in physiological damage and ultimately in tree death. The balance between growth and survival is tightly regulated, and specific acclimation mechanisms have evolved to allow growth under drought conditions. When the intensity of drought increases, steady state conditions of water transfer may be irreversibly disrupted, resulting in hydraulic failure and, thus, in premature death of roots or root systems (based on Waring, 1987; Breda et al., 2006; Anderegg et al., 2013a; Claeys and Inzé, 2013).