चावल में फाइलोक्रोन गतिशीलता (ओरिज़ा सातिवा एल.)

Phyllochron, defined as the interval time between appearances of successive leaves on a shoot, is an important measurement to know the developmental state of a shoot apex in rice. Shoot development of rice was divided into three stages, regardless of environment and genotype:

• Maintenance of short phyllochron in the early developmental stage
• Drastic increase of phyllochron depending on leaf number from the base in the late stage and
• Decrease of phyllochron before final leaf stage.

Origin

The modeling of the phyllochron was first published in 1951 when Katayama presented the growth rules, he had worked out for leaf emergence on the main stem and tillers of rice, wheat and barley.

The numbers of tillers that emerged from the main tiller and subsequent tillers according to respective tiller orders (main, primary, secondary and tertiary) reflecting their sequence of emergences.

The number of total tillers and tertiary tillers increased ex-potentially as the number of phyllochrons (often referred to as leaf number) advanced. It was the growth in number of tertiary tillers that drove the total number up so sharply.  

Production of tillers in rice

The number of primary and secondary tillers increased more linearly with the advance of phyllochrons (the number of periods of tiller emergence). The first primary tiller emerges from the main tiller in the fourth phyllochron, with additional primary tillers up to six in the next five phyllochron periods.

The first secondary tiller comes out later, from the base of the first primary tiller, in the seventh phyllochron, while the first tertiary tiller emerges from the base of the first secondary tiller in the tenth phyllochron.

Phyllochrons are a regular interval or period of plant growth observable for all gramineae species. These ‘growth cycles’ can be as short as 5 days (under ideal growth conditions) or up to 10 days (with less favorable conditions).

During each phyllochron beyond the third, one or more phytomers (a unit of tiller, leaf and root) are produced from the plant's apical meristematic tissue.

They are particularly important for rice, which is a potentially high-tillering plant provided that its root system is intact and the root system and canopy are not constrained by crowding. Between 6 and 12 phyllochrons may be completed before panicle initiation, when vegetative growth stops and the plant goes into its reproductive phase.

The detailed work on phyllochrons by Fr. De Laulanie, who developed System of Rice Intensification (SRI), found somewhat different numbers. He agrees that the first primary tiller emerges in the 4^th phyllochron, but according to his observations, the first secondary tiller emerges in the 6^th  phyllochron, and the first tertiary tiller in the 8th phyllochron. A total of 84 tillers should be possible by the end of the 11th phyllochron according to Laulanies calculations.

The percentage of primary tillers among total tillers decreased with the increase of phyllochron. The peak percentage of secondary tillers occurs in the tenth phyllochron; after that, the percentage decreases slowly. The percentage of tertiary tillers increases from the ninth phyllochron by about 5% in the increase of each phyllochron.

Therefore, after the ninth phyllochron, the percentage of tertiary and secondary tillers among total tillers increases. Theoretically, with the advance of phyllochrons, especially after the ninth period, small and late emergent tillers will occupy a large portion of the total number of tillers in the rice plant. Reasonable tiller number should be a consideration in assessing the components of panicle performance for high yielding cultivation of rice.

Tillers are produced from vegetative shoot branching and tillering of rice plant is an important agronomic trait for more grain production to achieve higher yield and also a model system for the study of branching in cereals.

Tillering can also be considered as a prime yield component due to the production of more numbers of panicles per unit area and often contributes to phenotypic plasticity in small-grain cereals like wheat and rice especially under inclement climatic conditions.

Tillering is initiated on the main shoot at five-leaf stage. During early vegetative growth, rice plant continuously forms new leaves in regular spatial and temporal patterns.

The emerging tillers develop from axils of these leaves on the un-extended nodes of the main shoot. The early initiated tillers on the main shoot also give rise to secondary tillers and secondary tillers produce tertiary tillers on their stem nodes.

Morphogenetic processes in sequentially growing leaves and tiller buds are highly synchronized in rice and the appearance of successive leaves in the main culm acts as the “pace maker” for the whole shoot system development.

Tillering continues up to the panicle initiation stage and thereafter some of the late-formed tillers die because of premature senescence. Tiller geometry follows a particular pattern in rice plant.

On the main shoot, the coleoptile leaf and the first leaf do not produce any tiller. There is a definite relationship between tiller and leaf emergence on a rice culm.

Emergence of fifth leaf coincides with that of the tiller at the axil of second leaf; this is the first tiller of the plant called P1. Emergence of subsequent tillers follows the same spatial synchrony; emergence of nth leaf of a culm coincides with that of the tiller of n-3 leaf.

Tillers are also produced on the early initiated primary and secondary tillers on a free-tillering cultivar and spatial synchrony as evident in the main shoot is also retained on the tillers.

Tillers have been classified in to different groups according to their spatial origin on successive internodes. The tillers of the main shoot are called primary tillers whereas those produced from primary tillers are named secondary and tillers emerged out of secondary are called tertiary tillers.

Effect of abiotic factors on phyllochron in rice

The phyllochron is primarily sensitive to temperature and leaf appearance is often linear when plotted against the thermal time of the shoot apex. In addition, many other environmental factors, such as nutrient or water availability, light, and CO₂ concentration, affect phyllochrons, together with plant morphology.

In cereals, development and growth are restricted to a specific zone at the base of the growing leaf. Humidity has different effects on the phyllochrons, depending on the temperature regime.

The effects of decreasing day length have been controversial, but it seems likely that day length itself has little effect on the phyllochrons unless it induces precocious inflorescence initiation (Table 1). 

Table 1. Influence of environmental factors on phyllochron in rice

(+) means increase in phyllochron with increase in factor,

(-) means decrease in phyllochron with increase in factor and

(0) means no change in phyllochron with increase or decrease in factor.

Conclusion

In grasses, construction of a growing plant is determined primarily by the rate of leaf development in the shoot apex and the timing of tillering and rooting of individual phytomers relative to leaf development. The development of the leaf, tiller bud, and adventitious roots of each phytomer proceeds in order, at a rate that depends on leaf initiation rate at the shoot apex.

Consequently, initiation, leaf emergence, tillering, and rooting of each tiller are closely synchronized with leaf emergence on the main stem. Leaves that emerge simultaneously are similar in size. Tillers whose leaf emergence is retarded from this synchronization will generally die before anthesis.

Inflorescence development is closely correlated with development of the subtending leaves in each tiller, resulting in a highly predictable phenology in relation to leaf emergence.

Because of these relationships, the leaf number concept provides researchers with an effective index for studying development of shoot and root systems in rice, including the prediction of tiller morphology, the estimation of potential tiller increases and analysis of root system dynamics.

References

Cook, M.G. and Evans, L.T. 1983. Some Physiological Aspects of the Domestication and Improvement of Rice (Oryza spp.). Field Crops Research, 6, 219-238.

Li, X., Qian, Q., Fu, Z., Wang, Y., Xiong, G., Zeng, D., Wang, X., Liu, X., Teng, S., Hiroshi, F., Yuan, M., Luo, D., Han, B. and Li, J. 2003. Control of Tillering in Rice. Nature, 422, 618-621.

Nemoto, K., S. Morita and T. Baba. 1995. Shoot and root development in rice related to the phyllochron. Crop Sci., 35(1): 24-29.

Veeramani, P., R. Duraisingh and K. Subrahmaniyan. 2012. Study of phyllochron - System of Rice Intensification (SRI) technique. Agric. Sci. Res. J., 2(6): 329-334.

Authors

¹Pradeesh Kumar T and ²Nandhakumar MR

1. Assistant Professor (Agronomy), Department of Agronomy,

VIT School of Agricultural Innovations and Advanced Learning (VAIAL), Vellore - 632 014, Tamil Nadu, India.

2. Associate Professor and Head, Department of Agronomy,

Vanavarayar Institute of Agriculture (VIA), Pollachi - 642 103, Tamil Nadu, India

Corresponding Author E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.