Rice Genetics Newsletter, Vol. 18

Rgn18

17.	Genic female sterility as a tool for analyzing the mechanism of photosynthate translocation in rice
	M. YOKOO and M. KATO Institute of Agriculture and Forestry, University of Tsukuba, Tennoudai 1-1-1, Tsukuba, 305-8572 Japan

Sterile rice plants continue their vegetative growth vigorously by producing more tillers even after heading, but these plants have few fertile grains on their panicles regardless of their normal photosynthetic activity. Plant physiologists know that sterile lines offer better materials for the analysis of photosynthate translocation compared with normal fertile plants, if plants with stable sterility are readily available. Unlike cytoplasmic male sterility, genic male or female sterility is easily recovered from selfed seeds to the succeeding generation, and can be used as appropriate experimental materials.

Recently, we conducted physiological studies on photosynthate translocation, using a rice line FS1 with genic female sterility that was controlled by two recessive genes, fes1 and fes2 (Yokoo 1984). The line has normal fertile pollen but almost sterile embryos in spikelets, and

maintains two to five percent of spikelet fertility in successive generations of selfing (Fig. 1). Fujisaka 5, the recurrent parent for backcrossing in the breeding of FS1, was used in the expriment as control.

The vegetative growth of FS1 was similar to that of Fujisaka 5 before heading, but after heading, FS1 grew different from Fujisaka 5 in a unique physiological process due to its low spikelet fertility. Fujisaka 5 showed a rapid decrease in SPAD value (green index of leaves), stomatal conductance and photosynthetic rate in flag leaves as normal plants came to maturity, but FS1 showed higher SPAD value, stomatal conductance and photosynthetic rate even in the late period of maturity. A continuously higher photosynthetic activity showed by FS1 caused a significantly high level of dry matter production.

The photosynthates were mainly distributed to the panicles of Fujisaka 5, but the panicles of FS1 did not function as a sink organ for phtosynthates. In the early period of maturity, FS1 stored photosynthates in the stems and sheaths, while in the late period FS1 showed more late tillers and increased root dry weights (Fig. 2).

The high level of photosynthetic activity and dry matter productivity in sterile plants, such as FS1, can be used to increase total biomas production in paddy fields. We are seeking for environmental factors to solve FS1's low spikelet fertility, to secure enough seeds for practical use in agriculture. We also plan to incorporate a line with genic male sterility controlled by tms2 (Maruyama et al. 1991) into our physiological and agronomical studies.

References

Maruyama, K., H. Araki and H. Kato, 1991. Thermosensitive genetic male sterility induced by irradiation. Rice Genetics II: 227-232.

Yokoo, M., 1984. Female sterility in an Indica-Japonica cross of rice. Japan. J. Breed. 34: 219-227.