短稈

An indigenous variety of Job's tears was exposed to radiation to induce mutation, and short-culmed and shattering-resistant mutant strains were selected from the progenies. (1) Air-dried seeds of an indigenous variety of Job's tears (Okayama zairai) were exposed to γ-ray radiation (20 to 35 KR) from^60Co. The irradiated plants were grown in the field, from which about 10,000 M_2 seeds were collected. (2) From the M_2 plants, only semi-fertile plants totaling about 1,000 were selected to obtain M_3 seeds. Low-fertile plants were avoided because of possible chromosome abberation, and high-fertile individuals, because of possible low mutation rate. (3) From each of these M_3 Seed groups, eight individuals were grown to from a segregation generation consisting of about 8,000 plants, of which the growing behavior, plant height, natural shattering habit, and seed weight were determined. On the basis of these data, about 850 plants were selected to collect M_4 seeds. The selected plants included 240 short-culmed ones with plant-height of 80 to 110 cm, about 500 shattering-resistant ones with natural shattering rate of 5 % or below and about 100 heavy-grain ones with 100-grain weight of 12g or over. (4) On approximately 850 M_4 strains, measurements were made of the growing behavior (heading date), plant-height, grain weight. In order to evaluate the shattering seed retention, tensile strength were also measured using a strain meter. Short culmed strains showed approximately15 % decrease in plant height with an average height of 124 cm as well as reduction in the number of nodes compared to the control strain. Measurements of tensile strength were made on almost all the plants belonging to shattering-resistant strains. The ears used for the measurement were those collected after ripening of grains and then air-dried indoors to reduce possible error owing to difference in measurement time. The distribution of shattering-resistant strains in terms of tensile strength showed a deflexion toward greater values; 85 strains (18%) presented tensile strength of 44g or over (somewhat resistant), and 6 strains (1.3%), 88g or over (resistant). The effect of selection was confirmed in breeding both large-grain and early-ripening strains. (5) In this study we have emphasized that breeding of short-culmed and shattering-resistant strains leads to easier mechanical havesting and less grain loss in havesting work. At the same time we have pointed out that the above traits should be combined with other traits such as early maturation and large grain; early maturation is beneficial to both preceding and succeeding crops, and large grains offer high grain yield.