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ABSTRACT. The purpose of this study was to identify the optimum levels of gibberellic acid and suitable row ratio for maximization the seed yield of hybrid rice.

Two field experiments were conducted at Rice Research and Training Center ( RRTC ) Farm, Kafr-El-Sheikh Governorate, Egypt, during two successive seasons of 2009 and 2010. The main objectives of this study are; identifying both the optimum seeding rates and hill spacing to increase the yield of hybrid rice seed for the hybrid IR69625A/Giza181R (2046H), this study included, three seeding rates for cytoplasmic male sterile line (6kg/fed, 8kg/fed and 10kg/fed) and three hill spacing ( 15×10, 15×15 and 15× 20cm).The treatments were arranged in split plot design with three replications, where seeding rates distributed as main plots while hill spacing were allocated in the sub plots. Data were recorded on different traits relating to yield and its components .The results showed that, the seed rates had highest significant on plant height , panicle weight, panicle length, panicle exsertion, no. of spikelets/ panicle seed set, 1000 grain weight and grain yield . The optimum treatment to produce high quantity of hybrid rice seed from the hybrid combination of IR69625A/Giza181R (SK.2046H) was 8kg seeds/fed, for the seeding rate.Moreover, hill spacing treatments had highest significant on all studied characters,except no. of panicles/m 2 character. The best hill spacing for cytoplamic male sterile was 15 x 15cm.The interaction between seed rates and hill spacing was significant for all studied characters.It could be concluded that the optimum treatments to produce high quantity of hybrid seed from the hybrid combination IR69625A/Giza 181R (SK.2046H) was 8kg seeds/fed for the seeding rate and (15×15cm) for hill spacing, because this treatment was recorded the highest values of panicle exsertion (81.61%),seed set (46.06%) and grain yield (1.32 t/fed).

A field study was conducted at Sakha Agricultural Research Station, Kafr El-Sheikh, Egypt during 2016-17 summer, to estimate general (GCA) and specific (SCA) combining ability effects as well as identify type of gene action controlling the inheritance of the studied traits under normal and infested weed conditions. Seven a diverse rice (Oryza sativa L.) parent’s were crossed using half diallel cross method without reciprocals to produce 21 F1 crosses during 2016 season. The seven parents and their 21 F1 crosses were evaluated under normal and infested weed conditions using a randomized complete block design (RCBD) with three replications during 2017. Highly significant differences were observed among genotypes, parents and crosses for all the studied traits. Moreover, general combining ability (GCA) and specific combining ability (SCA) mean squares were highly significant for all the studied traits under both conditions. The best general combiner for earliness and short stature was Sakha 103 variety. The highest desirable SCA effects were obtained by the crosses; Egyptian Yasmin × Dullar, Sakha 104 × Egyptian Yasmin, Egyptian Yasmin × Rikuto Norin 22 and Giza 177 × Rikuto Norin 22 for flag leaf area, No. of panicles/plant and grain yield/plant. These crosses could be utilized in rice breeding program for improving grain yield trait under normal and infested weed conditions.