Molecular Soil Biology 2024, Vol.15, No.4, 172-182 http://bioscipublisher.com/index.php/msb 174 3.2 Impact of soil composition on water uptake Soil texture, porosity, and organic matter significantly influence root-soil water interactions. Soil characteristics such as texture and porosity determine the availability and movement of water within the soil profile, affecting how efficiently roots can absorb water. For example, soil with higher porosity and organic matter content can retain more water, providing a more consistent supply for root uptake. Conversely, heterogeneous soil conditions can limit RWU, especially during drier states (Jackisch et al., 2019). The presence of symbiotic soil microbes also plays a role in enhancing water uptake by improving soil structure and increasing water holding capacity (Shoaib et al., 2022) (Figure 1). These factors collectively impact the efficiency of water absorption by tree roots, influencing their overall water use strategies. Figure 1 Beneficial roles of root exudates, avascular mycorrhiza (AM), and rhizobacteria in plant drought adaptation (Adopted from Shoaib et al., 2022) Image caption: (a) Exudates effects soil aggregation, water holding capacity, and nutrient mobilization. The plant preferentially selects microbes through exudation, which assists in drought adaptation. Exudates also influence the soil nitrogen (N) cycle. (b) AM increases phosphorus (P) and water uptake, affecting root hydraulic conductivity and reducing drought stress by producing antioxidants. AM also increases soil carbon. Rhizobacteria release exopolysaccharides, volatile compounds, osmolytes, ACC-deaminase, and phytohormones. These compounds increase soil aggregation, lateral root formation, and plant growth; mediate stomatal closure, reduce ethylene’s harmful effect, and ultimately increase drought resistance (Adopted from Shoaib et al., 2022) 3.3 Hydraulic redistribution Hydraulic redistribution is a process where tree roots transfer water within the soil profile, redistributing it from wetter to drier areas. This phenomenon, also known as "hydraulic lift," allows trees to maintain root viability and facilitate growth in dry soils. For instance, Grevillea robusta and Eucalyptus camaldulensis have been observed to transport water from deeper soil layers to dry surface horizons, and vice versa, depending on soil moisture conditions (Burgess et al., 1998). This redistribution helps in maintaining water availability in the root zone, especially during dry periods. Additionally, trees can shift water uptake among existing roots rather than growing new ones, as seen in conifers during prolonged droughts (Mackay et al., 2019). This strategy is crucial for sustaining water uptake and ensuring tree survival under adverse conditions. 4 Drought Adaptation Mechanisms in Tree Roots 4.1 Root plasticity and drought response Tree roots exhibit significant plasticity in response to prolonged dry conditions, adapting both structurally and functionally to enhance survival. Under drought stress, roots may adjust their biomass, with some species increasing root length and surface area to explore a larger soil volume for water (Brunner et al., 2015; Nikolova et
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