Physiological studies on low-tillering rice: an ideotype for increasing grain yield potential.

A low tillering, large panicle IR25588 was compared with a high tillering, small panicle IR58. Both varieties have similar plant type and growth duration but differ in tillering ability and panicle size. High tillering IR58 showed not only higher tiller number but also earlier, higher rate and longer period of tillering compared with low tillering type IR25588. Percent effective tiller was higher in IR25588 (85%) than IR58 (67%) and decreased with increase in nitrogen level and plant density. The six heaviest panicles in both IR25588 and IR58 were the main culm; the first, second, third and fourth primary tillers; and the first secondary tiller from the second primary tiller. This suggests a new rice ideotype of six tillers per plant for increasing grain yield potential. The top six tillers were initiated and flowered earlier, had bigger culm and leaves per tiller to support many spikelets and better grain filling, and had more inner (IVB) and outer vascular bundles (OVB) at just below the neck node of the panicle which may give better translocation of assimilates. The top six panicles based on grain weight was mainly due to the higher spikelet number per panicle with little differences in 1000-grain weight and percent fertility. Spikelet number per panicle for the top six tillers in low tillering IR25588 was around 200 to 250 spikelets to compensate for low tillering. IR25588 had more IVB and OVB, and had bigger culm diameter and thickness than IR58. The number of IVB on the main culm was highest, followed by the primary, secondary and tertiary tillers. IVB decreased at the rate of two per tiller order. Two IVB corresponded with one primary branch of the panicle regardless of tiller order and variety. IR25588 had higher proportion (71%) of bigger panicles than that of IR58 (42%). Tertiary tillers and late primary and secondary tillers contributed little to grain yield. Selection of breeding lines without these kinds of tillers is recommended.