In this article we will discuss about the growth and development of rice.
1. Roots:
Rice form a fibrous root system consisting of seminal, nodal and lateral roots. Seminal roots originate from embryo, whereas nodal roots emerge from stem. Both produce numerous lateral roots. On germination, the primary root emerges followed by two additional seminal roots. Adventitious roots are then produced from basal nodes of primary stem and tillers.
Active emergence of roots takes place only when the nitrogen concentration of the stem base is above 1.0 per cent. When grown in flooded soil, rice roots develop aerenchyma that permits diffusion of oxygen from leaves through roots to the soil. This permits root growth in reduced soil. Oxygen diffusion from roots contributes to rhizosphere oxidation and is a source of oxygen used in respiration by rhizosphere microorganisms.
2. Tops:
Tillering in rice begins at 4-5 leaf stage. Primary tiller emerges from the axel of leaf n-3, when n of the main stem elongates. Secondary and tertiary tillers emerge in the same way and all tillers are synchronised with the development of leaves on the main stem. Nitrogen concentration of 3.5 per cent in the plant is necessary for active tillering.
At 2.5 per cent, tillering stops and below 1.5 per cent death of tillers takes place. Similarly, no tillering when phosphorus concentration of the mother stem is below 0.25 per cent. Initial tillers depend on nutrient supply from the mother stem but become autotrophic when they have three leaves and four or five roots. Decrease in tiller number after 70-100 days results from death of last tillers due to their failure to compete for light and nutrients.
3. Leaf area index (LAI): x
The LAI of rice increases as crop growth advances and reaches maximum at about heading. The optimum LAI for upland rice is 2 to 3 at 85-100 days after sowing (DAS) and 4 to 7 for lowland rice. Rice under upland conditions is often subjected to moisture stress and in general has fewer tillers and less leaf area then that under lowland conditions.
4. Harvest index (HI):
It is the ratio of the grain yield to the above ground biological yield. Grain yield can be increased either by increasing total dry matter production or by increasing the HI. The HI varies with variety and environment and a negative correlation exists between plant height and HI.
The HI of semi-dwarf Indica cultivars is in the range of 0.45-0.55, where as that of Japonicas 0.40-0.49. In general, HI decreases slowly with increasing total plant dry weight. However, records of high yielding crops indicate that high rice yields are achieved by increasing total dry weight and by maintaining a high harvest index.