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

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Introductiontwo different rice diversity panels during exponential growth indicated a highly significant correlationbetween DR and dry weight growth parameters, notably RGR which is known as a measure of earlyvigor (Poorter 1999). This correlation was stronger in Exp2 on japonica panel than in Exp1 on indicapanel. The data do not permit attributing this difference to panel composition or to environment.Early vigor is not a simple trait and can be conveyed, in a given resource situation and environment,by several component traits that are physiologically interacting and may involve environmentdependent trade-offs (W. ter Steege et al. 2005). It is thus difficult to isolate the effect of a singlecomponent trait of early vigor. In fact, in the present study tiller number and leaf size alsocontributed to dry weight accumulation, in addition or in interaction with DR. Evidence for DR per sehaving a positive effect on vigor was in this study derived from the observation that within subsets ofgenotypes having similar leaf length and tiller number, the effect of DR on vigor was conserved andnot weaker than for the entire panel. This result is important and suggests that DR per se is apotential key trait to breed for improved vigor.Rice genotypes having rapid development thus tend to have higher RGR and therefore developground cover and biomass more rapidly during exponential growth, at least under non-limitingresources when their potential can be expressed. This effect is due to the more rapid succession ofphytomer and leaf production (higher DR values). In turn, it accelerates the production of tiller budsassociated with the phytomers, and therefore potentially accelerates tillering (Miyamoto et al. 2004).In fact, DR and tiller production were consistently positively correlated in this study, as previouslydemonstrated by other authors (Rebetzke et al. 1999; Morita et al. 2005; W. ter Steege et al. 2005).The negative correlation observed between leaf size and both tiller number and DR is probably theresult of a physiological trade-off or compensation as commonly observed in dwarfing, which in riceinvolves smaller leaves but higher tiller number (Zou et al. 2005).Strong evidence for the physiological linkage of DR (in terms of plastochron) and tillering was foundin mutants. For example, in recent studies on rice mutants having modified plastochron (Itoh et al.1998; Asai et al. 2002; Miyoshi et al. 2004) or tillering (Komatsu et al. 2003; Zou et al. 2005), thephysiological linkage between tillering and DR was not overcome and the phenotype of the mutantswas systematically modified on both traits when compared to the wild type. Similar evidence onphysiological trait linkages (based on the genetic modification of a single gene) was also shown forleaf size vs. DR (Itoh et al. 1998; Miyoshi et al. 2004): rice mutants affected on a gene controlling therate of leaf primordium initiation systematically presented reduced leaf size compared to the wildtype. In the present study, an important result is that this trade-off can be overcome, which makes it46
Introductionpossible to breed for superior early vigor through high DR while maintaining high number of tillersand leaf size.Nevertheless, genetic linkages among these traits may be involved. While no genetic linkage wasfound among tillering and DR in previous studies on rice (Miyamoto et al. 2004). W. ter Steege et al.(2005) suggested on wheat that the apparent physiological linkages among traits constituting earlyvigor at different scales (cell, leaf, whole plant) could have genetic bases. Tisné et al. (2008)suggested the same for Arabidopsis thaliana, the orientation of the trait linkages being dependent onthe alleles present on different QTLs involved. These recent findings emphasize that the geneticstudy of traits involved in plant morphogenesis and plasticity should not be addressed by looking at asingle trait (Granier et al. 2009). The data presented here for two rice diversity panels will provideanswers once the genomes of the populations have been densely genotyped and association studiesconducted for the traits phenotyped.While in Exp1 (field) tillering made a greater contribution than DR to biomass accumulation, it wasthe opposite in Exp2 (greenhouse, japonica panel). Leaf size had contributed less than tillering or DR.This confirms previous studies showing a stronger effect on plant growth of organ number (leaf ortiller) compared to organ size (Rebetzke et al. 1999; Morita et al. 2005; W. ter Steege et al. 2005).Size does matter, however, as shown by (Rebetzke et al. 1999; Rebetzke et al. 2007) for coleoptilelength in wheat seedlings having different vigor. In our study DR and first leaf length LL1 werenegatively and significantly correlated (p
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Page 8 and 9: IntroductionIntroductionCONTEXTHost
Page 10 and 11: IntroductionRice crop performance u
Page 12 and 13: IntroductionEarly vigor was also re
Page 14 and 15: Introductionconsidering traits in a
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Page 20 and 21: IntroductionFigure 3. Dry-down expe
Page 22 and 23: IntroductionREFERENCESAsch, F., Sow
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Page 26 and 27: IntroductionMcNally, K. L., Childs,
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Page 32 and 33: IntroductionINTRODUCTIONEarly vigor
Page 34 and 35: IntroductionThe underlying hypothes
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Page 44 and 45: Introductionwhether DR affected RGR
Page 48 and 49: IntroductionCONCLUSIONSThis study t
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Page 52 and 53: IntroductionChapter II : Phenomics
Page 54 and 55: Introductiongrowth (leaf appearance
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IntroductionKato, Y., Hirotsu, S.,
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IntroductionPlasto:CR (0.730) > SDW
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