ABSTRACT
Rice (Oryza sativa L.) is the most important crop in the world in terms of cultivated area and because it is the staple food for more than half of the world population. Breeding programs are necessary to increase rice productivity, at both the international and the local scale. This study is part of the rice breeding program at I.V.I.A., and its main aim is to increase the knowledge on the genetic control of some traits in our growing conditions. We have focused in parameters that could increase lodging resistance, and also grain characteristics required by the market. All these characters are quantitative, as are most of the important agronomic characters, and their study and selection are difficult due to their polygenic control and because they are highly influenced by the environment. Our work has focused in two main topics: on the one hand, we have analyzed the heritability and selection efficiency of these characters, as well as the environmental influence and correlations between them. On the other, we have identified the QTLs (quantitative trait loci) that determine them, as well as their genetic effects and expression in different environmental conditions and populations. Such information could increase the selection efficiency when breeding for new varieties, by allowing the design of more suited strategies, and by identifying QTLs appropriate for marker assisted selection (M.A.S.). 
Lodging is a mechanical accident that causes losses in harvest quality and quantity. Evaluation of lodging resistance in early generations is difficult; therefore we aimed at determining which related parameters would allow an indirect and effective selection. Previous studies concluded that it is possible to increase lodging resistance by reducing the weight of the aerial part of the plant (mainly, using shorter lines), as well as by increasing stem diameter (especially, that of the basal internodes) or the pushing resistance. But selection of some of these characters could reduce productivity. We have also studied tiller angle and days to maturity due to their relation with lodging, and other traits related to grain production (flag leaf morphology, panicle number, and panicle and grain weight), and with grain quality (dimensions and white core presence). 
Two segregant populations have been studied, having the same female parent but two different male parents. F2 plants were evaluated in two environmental conditions (field and artificial rice paddies) in order to analyze the environmental effect on characters and on QTLs expression. F3 lines derived from all F2 individuals were evaluated the following year in a single plot. Selected F4 lines according to lodging related traits showed that selection was effective for reducing height, and increasing diameter and length of basal internodes in both populations, and for increasing panicle number and bending resistance in only one of them. 
Genotype information (using SSR markers), and phenotypic values from F2 plants and F3 lines were used in QTL analysis. We have identified QTLs for all the studied characters, most of them affecting more than one trait, in accordance with correlations found between them. Interesting areas which could be used for M.A.S. have been identified.