Most of the pre green revolution rice varieties were tall and leafy with weak stems and lodging habit. Their harvest index was only around 0.3 indicating their low yield potential. During the course of development of rice varieties suitable for green revolution, the harvest index and biomass production were increased mainly by the reduction of plant height through the incorporation of the recessive gene sd-1 for short stature from a Chinese variety Deo-Geo-Woo-Gen and also by the development of high tillering varieties. However, further studies indicated certain drawbacks in the case of the high tillering varieties like non synchronized flowering of tillers resulting in non uniform maturity. Rice germinates as a single culmed seedling but soon it starts tillering. Tillering potential of rice is a varietal character and the tillering phase of a variety depends upon its duration and morphology. The first produced tillers in rice are primary in nature followed by secondary and tertiary tillers. However, late emerging tillers may not contribute towards the yield of mature grains at harvest. Khush (1994) has conceptualized a new plant type in rice with lower tillering capacity, absence of unproductive tillers, higher number of grains per panicle, medium height, sturdy stems, dark green, thick and erect leaves, thickened, deepened roots, multiple disease and insect resistance and acceptable grain quality. Under these circumstances a study was carried out presently to analyse the tillering potential and the behaviour of tillers in terms of their emergence and performance in the case of 10 native rice cultivars of Kerala state of India (Table 1).

The present experiment was carried out in the experimental net house of the Genetics and Plant Breeding Division of the Department of Botany of University of Calicut, Kerala, India in the first crop season of 2003 using 10 native rice cultivars of Kerala with 12 plants per variety (Table 1). The plants were grown in 20 cm pots filled with paddy soil + sand + enriched compost in 4:1:1 proportion under wetland condition with one plant per pot and applying 1 g Factamfos (N:P:K:S :: 15:15:0:15) per plant at monthly intervals starting from 30th day onwards till flowering. Performance of tillers of different ranks has been analysed based on mean, standard deviation and comparison of means by t test in relation to the characters studied.

Table 1:	Tiller related analysis of plant and yield characters in rice
**: Significant at 1% level; *: Significant at 5% level (t-test)

Rice germinates as a single culm known as mother tiller. Subsequently it gives rise to primary, secondary and tertiary tillers. As a general rule, primary tillers develop first, followed by secondary and tertiary tillers. In the case of ten rice varieties studied for the purpose, seven produced primary, secondary and tertiary tillers where as three, produced only primary and secondary tillers. In all the cases tertiary tillers emerged significantly late indicating the possibility of their inability to contribute mature grains at the time of harvest (Table 1). Tertiary tillers flowered in the case of three varieties only and they flowered 14-29 days after the first flowering of the plants, thus further indicating their unripe condition at the time of harvest. Tertiary tiller height was significantly low in the case of all the four varieties which developed healthy tertiary tillers. Seeds per panicle and panicle density were found to be low in the case of tertiary tillers. Panicle length, spikelets per panicle and seeds per panicle were also significantly low in tertiary tillers in most of the cases. The above study indicates the relative non contributing nature of tertiary tillers thus emphasizing the need for developing rice varieties with optimum numbers of primary and secondary tillers and with out tertiary tillers so that uniform ripening, harvest and maximum yield is possible. Increasing the number of primary tillers and reducing the number of secondaries should also be considered a breeding priority in rice for better synchronized flowering and maturity of tillers.

Khush (1994, 2000) has indicated the importance of high density planting of optimum or even low tillering rice varieties so as to improve the yield potential of rice. Reduced tillering facilitates synchronized flowering and maturity, uniform panicle size and heavier grains. According to Rao (1987), number of grains produced per unit area determines grain yield. However, the modern high yielding varieties that have replaced the traditional varieties have got higher tiller and panicle numbers, the additional tillers becoming unproductive as opined by Khush (2000). Hence the approach should be to increase the number of grains per panicle rather than number of panicles per plant. In other cereals like maize and jowar also yield potential has been increased by increasing the ear size (Khush, 1993).