Sugarcane (Saccharum spp.), an important food and bioenergy source, is a significant industrial crop suited for cultivation in many countries with tropical and subtropical climatic conditions. Owing to the C4 photosynthetic pathway for carbon assimilation, sugarcane is highly productive, efficiently using farm inputs such as water and fertilizers. Post-harvest processing of sugarcane yields economically valuable products such as sugar and jaggery, alongside its byproducts-molasses, ethanol, and energy. Sugarcane is produced by nearly 100 countries over an area of 25.97 Mha, which is approximately 1.5% of total arable land. India alone contributes to 16% of world sugarcane production (approximately 306 Mt), with an average productivity of 70 t·ha−1 1. The major abiotic stresses drastically affecting sugarcane production include water deficits, water logging, temperature extremes, and soil properties such as nutrient deficiency, salinity, sodicity, and alkalinity. The biggest challenge for developing abiotic stress-tolerant crops is to pinpoint specific traits that confer a substantial yield advantage under stress conditions.
Several aspects of sugarcane physiology are poorly understood, including the root-shoot relationship, which drastically affects cane productivity. Sugarcane root is not as well studied as shoots, although the different types, such as sett roots, shoot roots, and adult roots, may be developmentally distinct with varying functions. Genotypic differences have been observed with regard to the number and length of sett roots emerging during germination2. Sett roots are implicated in the germination of sugarcane buds, ensuring early crop establishment, and are later replaced by shoot roots, which are robust, emerging from the base of the developing shoot3. Fine branches observed in the sett roots help in anchoring the young plants and aid in the absorption of water and nutrients until they are replaced by shoot roots. Similar to sett roots, shoot roots also arise from the root primordia present in the lower, unexpanded internodes of the cane.
As shoot roots persist in the plant for a longer duration, they are 4x-10x thicker than sett roots. Shoot roots constitute the sole root system of sugarcane, with an important role in further growth and development. The vigor of the shoot roots is positively associated with the overall vegetative vigor of the plant. The continuous development of roots resulting from the turnover of sett roots and shoot roots gives rise to the "adult root system" of sugarcane, which is ever-adapting to the prevailing environmental conditions. In general, a deeper, more prolific root system makes more water and nutrients available for the crop than does a shallower distribution of roots. Periodic dissections revealed that, when the soil moisture content was high, shallow root systems were observed, whereas a much deeper root system developed as the water table dropped2. The root system in sugarcane remains active even after harvesting of the crop, contributing to the growth of the ratoon crop until new shoot roots emerge from underground buds4. Root angle and the level of root branching are two important factors determining the volume of soil explored by plant roots. Root angle, a genotypic trait, may be altered through conventional breeding or molecular approaches to improve tolerance to biotic and abiotic stresses. On the contrary, the level of root branching is mostly influenced by the environment, necessitating periodic monitoring of root development and its response to localized soil conditions.
Anatomical features of sugarcane roots have been examined to ascertain differences with regard to genotype and environment. The anatomy of sett roots in sugarcane resembles that of mature roots in other grasses such as maize, wherein the cortex comprises well-differentiated cell layers in a regular pattern. The endodermis is suberized, followed by a single-layered pericycle. Metaxylem elements are the main conductors or water and nutrient ions, radially arranged and interspersed with groups of phloem, the latter comprising a sieve element with two companion cells. The large central mass of undifferentiated cells forms the root pith. Distinct anatomical features of sugarcane cultivars correspond to root hydraulic properties, thereby influencing water movement. Early studies on the differences in the root anatomical traits of sugarcane revealed that, under low moisture stress conditions, pronounced thickening of the cell wall was observed in the layers immediately inside of the endodermis, between the pith and the vascular region, and around the vessels5. Such thickened cells may be an adaptation to prevent the backward flow of sap and for mechanical strength during stress.
Some important traits implicated in the drought resistance of sugarcane include the relative thickness and number of exodermal layers, the ratio of cortex to stele, intercellular spaces in the cortex, and thickened root hair tips. The ratios of the area occupied by cortical cells to the area occupied by the stellar tissues of shoot roots are significantly different among sugarcane cultivars, with wide variability with respect to the area of the stele6. The hydraulic conductivity of sugarcane roots is related to the size and number of metaxylem elements in the shoot roots. Hydrophobic cell layers within the root are likely to define zones of apoplastic water movement. Suberized Casparian bands are found in the endodermis and in the hypodermis (termed as exodermis), which serve as hydrophobic barriers. The disintegration of cortical cells leads to the formation of lysigenous aerenchyma in older roots and in plants subjected to hypoxic conditions, irrespective of developmental age. The formation of aerenchyma during waterlogging stress is correlated with the maintenance of growth in resistant varieties7.
The morphology and anatomy of Erianthus arundinaceus [Retzius] Jeswiet (genera related to Saccharum spp.) roots are implicated in its strong tolerance to environmental stresses8. Erianthus arundinaceus roots exhibit nodal roots distributed at steep angles, with dense roots hairs to facilitate the uptake of water and nutrient ions from deeper soil zones. The deep-root system consists of many nodal roots growing with steep growth angles. The diameter of the nodal roots correlates with the size and number of large xylem vessels, the former varying widely from 0.5 mm to 5.0 mm. These nodal roots also form a rhizomatous sheath, with a hypodermis showing lignified sclerenchyma in the outer cortex (exodermis), lysigenous aerenchyma in the mid-portion of the cortex, and starch granules in the stele. In addition to architecture and morphological traits, root-exuded organic compounds play an important role in determining plant germination, establishment, and survival, with plausible allelopathic effects and/or affinity for microbial symbiosis.
Root enzymatic activity and the finer details of the morphology, including root cap pigmentation and rejuvenation potential upon injury, were documented in sugarcane varieties grown under hydroponic culture9. Root growth shows a highly plastic response to changes in the soil environment, both in terms of the form and size of the root system. The most efficient sugarcane variety would be one that has few or an optimal number of shoots, with a correspondingly lower number of roots, aiding better survival during stressful conditions. The systematic study of the root system should, thus, form an important component of any crop improvement program10. The majority of the experiments focusing on roots rely mostly on developmental aspects, while a focus on functional plasticity is often lacking11. Apart from the structural distribution, functional root plasticity plays a crucial role in survival under stress and would, therefore, support breeders in their efforts to include root system traits in the selection pipeline for abiotic stress tolerance and improve the robustness of sugarcane.
Considering its importance in sustaining growth and yield under stressful environments, it is essential to explore and utilize the inherent variability in root system traits of sugarcane. An emphasis on the selection of component traits and mechanisms imparting superior root systems is the way forward for better crop performance under changing climatic conditions. Phenotypic evaluation is a long and costly process; however, the integration of multipronged approaches would add tremendous value to its utility in crop improvement. In this manuscript, four different approaches for root phenotyping in sugarcane are described, each with its own set of merits and demerits, implying that a concerted effort is required to arrive at comprehensive and holistic results.