Rice (Oryza sativa. L) genetic diversity for early vigor and drought ...

Introduction(Rebetzke et al. 1999; Rebetzke et al. 2004). This conceptual model does not take into accountdevelopmental dynamics of the plant, namely the rate of organogenesis (leaves, tillers) constitutingthe demand sides of the sink-source system.Recently, (Dingkuhn et al. 2007) suggested that organogenetic processes and the resulting sinkdynamics are a driving force for growth. (Luquet et al. 2006) presented a rice growth model(EcoMeristem) simulating growth-development interactions, namely competition for assimilatesamong sinks that feed back on both organogenesis and photosynthesis. According to this concept,early vigor should be strongly related to developmental rate (DR) i.e. leaf appearance rate(1/phyllochron). It is important to note that DR is defined here on an organogenetic basis (rate ofphytomer production) and not earliness of flowering, two phenomena that are not necessarily linked.Branching (tillering), organ size and organ abortion (senescence) would then have complementaryeffects to that of DR on vigor, or compensatory effects in the case of resource limitation. In addition,transitory carbohydrate reserves act as a buffer for sink-source imbalances in the case of excessive orlimiting, aggregate sink capacity (Luquet et al. 2006; Dingkuhn et al. 2006, Legros et al. 2009).According to this conceptual framework, high DR, or rapid organogenesis, favours an efficientconversion of assimilate gains into plant structure and thus acts as a pacemaker for growth. Acrossgenotypes in a given environment, growth rate tends to be positively correlated with tillering ability(Goto et al. 1989; Yin et al. 1996; Dingkuhn et al. 1999; Miyamoto et al. 2004). Tillering controls thenumber of axillary buds that can potentially produce other tillers (Nemoto et al. 1995; Itoh et al.1998; Miyamoto et al. 2004). As this would lead to excessive competition for resources within theplant (Dingkuhn et al. 2006; Luquet et al. 2006) followed by organ senescence and restriction ofelongation processes, outgrowth of new tillers is regulated by apical dominance (Fetene et al. 1993;Bangerth et al. 2000). Such adjustments thus set a limit to growth rate and vigor (Granier et al. 1999;Luquet et al. 2005a; Dingkuhn et al. 2006). Consequently, organ number and size are generallynegatively correlated across genotypes (W. ter Steege et al. 2005; Luquet et al. 2005a), but can beexpected to be positively correlated across environments.The compensatory relationship among the different components of early vigor necessarily involvesstrong genotype x environment interactions (GxE) on trait expression. This, as well as unknownphysiological and/or genetic linkages among traits, makes the study and improvement of vigor traitscomplex (Itoh et al. 1998; Miyamoto et al. 2004; W. ter Steege et al. 2005). An important question isthus which of the components of vigor show wide genetic diversity that may be selected for (Poorteret al. 2000; Fuentes et al. 2005) and thus warrants genetic studies (Itoh et al. 1998; Li et al. 2003;Miyamoto et al. 2004; Morita et al. 2005; W. ter Steege et al. 2005).33