The yield advantage of hybrids in the DS was associated with large sink size compared with indica inbreds
and NPTs (Tables 1 and 3). Sink size is a function of
panicle number per unit area and spikelet number per
panicle. These two parameters are negatively correlated
(Yoshida, 1981). Therefore, their balance is important for
attaining maximum sink size. Despite the lower panicle
number, hybrids had greater sink size than indica inbreds
and NPTs due to large panicles (Table 3). The NPT was
designed to achieve higher grain yield from large panicles
(Peng et al., 1994); however, the NPTs produced smaller
panicles than the hybrids in both the DS and WS. The
NPTs had a grain-filling percentage similar to hybrids
but significantly lower than indica inbreds. An interesting
observation is that high harvest index in hybrids was not
associated with the high grainfilling percentage (Tables
1, 2, and 3). The grainfilling percentage of the hybrids
was significantly lower than that of the indica inbreds,
which was probably due to the difference in panicle size
between the two groups. Large sink size could be responsible for the high harvest index of hybrids.