Root Physiology and Function in Paddy Rice

Structure of the Root System

The rice root system begins with a single seminal root that emerges from the seed, followed by crown roots that develop from the basal nodes of the stem and leaves. Lateral roots branch from these, forming a fibrous network. Roots are part of a developmental unit together with leaves, nodes, tiller buds, and internodes, reflecting the integrated growth of the rice plant.

Root growth is closely linked to leaf emergence. Crown roots from a given node develop in conjunction with the leaf three nodes above it. After germination, the seminal root is responsible for water and nutrient uptake, but from the four‑leaf stage onward, crown roots gradually assume this role. Root elongation and uptake depend on carbohydrate supply from aboveground organs, particularly starch stored in the leaf sheath. The third leaf from the top, known as the active center leaf, accumulates the most starch and is vascularly connected to the node below it, from which vigorous crown roots emerge. As the plant grows, the active center leaf shifts upward, and root development progresses from lower to upper nodes.

Growth Phases and Root Distribution

During establishment, roots grow downward. As development continues, they spread laterally, forming a mesh‑like root mat near the soil surface. The strongest rooting and elongation occur during the peak tillering stage, when photosynthesis is most active. As the plant enters reproductive growth, root vitality increasingly depends on assimilates from the lower leaves. After heading, carbohydrates flow predominantly to the panicle, reducing sugar concentration in the roots, lowering osmotic pressure, and diminishing water uptake.

Types of Roots and Their Roles

By heading, the root system is fully developed and includes old primary roots, new branching roots, deep roots, and shallow roots. Each type plays a distinct role:

• Old roots store sugars and provide much of the water and nutrients for shoot growth.
• New roots absorb nutrients and water while possessing strong oxidative capacity, improving the rhizosphere environment.
• Deep roots absorb elements such as iron, manganese, and silicon, which are prone to leaching in flooded soils.
• Shallow roots form mats near the surface and continue absorbing water and nutrients during ripening, supporting photosynthesis and grain filling.

Oxidative Capacity and Flooded Conditions

Rice roots are adapted to anaerobic environments through well‑developed aerenchyma, which conduct oxygen from the leaves to the roots. This adaptation enables respiration and nutrient absorption even under continuous flooding, ensuring root function in typical paddy conditions.

Irrigation and Root Vitality

After heading and flowering, maintaining root vitality becomes critical for grain production. Continuous irrigation, or saturated water management, supports the flow of starch from lower leaves to roots and stabilizes moisture for photosynthesis. In this method, water is added when the field surface begins to dry and allowed to drain naturally, keeping roots in a state of constant absorption. Intermittent irrigation, by contrast, adds water only after footprints have completely dried, and is typically used from mid‑season drainage through young panicle formation.

Comparison of Rice Root Types

Conclusion

The rice root system is not an isolated organ but part of a tightly integrated physiological network that links leaves, culm, and tillers. Root growth depends on assimilates supplied by the leaves, while culm storage and tiller development in turn rely on the root’s ability to absorb water and nutrients under flooded conditions. Deep and shallow roots together sustain the plant through critical phases, from establishment to grain filling, ensuring resilience against stress and supporting yield formation. Decades of Japanese research have shown that maintaining root vitality through careful irrigation and balanced nitrogen management is essential for sustaining photosynthesis and panicle development.