*2.2. Grain Yield Heterosis is the Result of Hybrid Vigor in Grain Number Per Panicle*

To obtain ideal agronomic characteristics of heterosis, the paternal line itself is usually an excellent inbred variety with superior agronomic performance. In addition, the hybrid F1 plant should surpass its paternal parent, particularly the traits related to yield components. Interestingly, the F1 plants did not exhibit greater plant height compared with the parental lines in the present study (Figure 2A). We surveyed the yield components of the parental lines and F1 plants (Figure 2). The panicle length of the F1 plants was significantly longer than that of Sasanishiki and Habataki (Figure 2B). Sasanishiki exhibited a short round grain shape, whereas the F1 and Habataki exhibited a slender grain shape. However, the 1000-grain weight of Habataki was significantly lower than that of the F1 plants, whereas the 1000-grain weight of Sasanishiki was similar to that of the F1 plants. The results showed that the F1 plants exhibited an advantage in grain number per panicle and panicle number compared with Sasanishiki and Habataki. A similar setting rate was observed in F1, Sasanishiki, and Habataki. The slenderest grain shape of Habataki had the lowest 1000-grain weight in the parent line and F1 plants, whereas Sasanishiki had similar 1000-grain weight as the F1 plants. Taken together, the advantage in panicle number and grain number per panicle makes the F1 plants exhibit heterosis in grain yield per plant.

**Figure 2.** Heterosis in F1 plants. (**A**) The plant architecture of F1 plant and parental lines. Scale bar = 10 cm. (**B**) The panicle of F1 plants and parental lines. Scale bar = 1 cm. (**C**) Grains of F1 plants and parental lines. Scale bar = 10 cm. (**D**–**I**) the yield-related traits of F1 plants and parental lines.
