**3. Root Biology**

Roots provide anchorage and facilitate the acquisition of water and nutrients from the soil, hence understanding the multi-faceted aspects of root biology are key to plant water management [15]. Plants exhibit a high degree of root plasticity and can transiently adapt to various environmental stresses. Sugarcane is a deep-rooted crop, owing to its long growth cycle. Sugarcane roots may be highly branched superficial roots, downward oriented buttress roots or deeply penetrating agglomerations of vertical roots known as rope roots. Sugarcane root systems may reach depths of about 1.5 to 6.0 m [16]. Nevertheless, drought stress usually leads to the formation of deeper root systems, which aid in extracting water from Sub-surface soil [17]. Sugarcane genotypes with drought-tolerant mechanisms cope better under water-limited environments by maintaining high water status and investing a higher proportion of assimilates for root growth under stress. Maintenance of high-water uptake under drought stress is also facilitated by improved root/shoot ratio in drought-tolerant cultivars [18]. Namwongsa et al. [19] observed that roots of sugarcane genotypes showed reduced growth in the upper soil layers (0 to 30 cm) in response to drought stress, whereas growth in the lower soil layers (below 30 cm) increased substantially. Under water-limited conditions, assimilated partitioning towards the roots area relatively higher than to shoots [20]. With the decrease in the soil moisture content, the roots alter their distribution patterns, proliferating more into deeper soil layers for extracting and engaging a larger soil volume for water uptake. As moisture at the soil surface and top soil profile is reduced under drought, deep roots take up water from the deeper profile [21]. Such deep root schemes are characteristic features under drought

and is an important consequence to soil drying, allowing some roots to continue their lifecycle under stress. Hence, root architecture and distribution strongly depend upon the moisture content of various soil layers.

Jangpromma et al. [18] noticed that drought stress significantly reduced root length, root surface area, root volume and root dry weight, with negligible effect on the root/shoot ratio. Most of the root morphological traits could fully recover when plants were re-watered following drought stress during the formative phase. The unchanged root/shoot ratio might explain that sugarcane invested in roots under drought conditions to mine more water from deeper soil layers. Drought stress reduced root growth of sugarcane by 50 to 80% when the soil water status reduced to a water potential of −0.07 MPa [22]. Endres et al. [23] reported a droughttolerant sugarcane genotype with higher root length density and better field performance under stress. Vantini et al. [24] observed differentially expressed genes between tolerant and sensitive sugarcane varieties across different time intervals (1, 3, 5, and 10 days after withholding water) in root tissues. At the beginning of the stress (1 and 3 days), genes encoding proteins with protection function (chaperones, heat shock proteins, antioxidant enzymes and protease inhibitor proteins) were induced in the tolerant variety. Gene encoding a protein involved in ABA-response, a trehalose-phosphatase synthase (enzyme involved in the synthesis of trehalose) and serine/threonine kinase receptors also showed higher expression in the tolerant variety, revealing differences between sugarcane genotypes for water stress protection and adaptive mechanisms. Hence, root systems are more important sink as compared to above ground organs under drought stress, especially during the active growth vegetative stages. Root growth might be indicative of a drought resistance mechanism under water-limited conditions. The positive relationship between root length and soil water content at the end of the drought period in 40 cm soil layers re-emphasizes the advantage of deeper roots for extracting water over extended drought periods. Nonetheless, the association between root length and physiological responses to plant water status are very complicated. Several root systems are considered to be essential in sustaining plant productivity under drought. Overall root growth, branching and distribution pattern is crucial to improve the acquisition of water and nutrients from the soil, and are positively associated with drought resistance and yield performance under stress.
