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

Introductionsame panel of 200 japonica genotypes, is used to confirm the genetic diversity of morphogenetic,constitutive and drought response traits reported in chapter 1 and 2. More particularly, this chapteraims at evaluating the genetic diversity of water related traits under drought (plant leaftranspiration rate and efficiency) and their linkage with morphogenetic traits.Finally, the fourth chapter is a general discussion of this work. Considering the results of Montpellierand IRRI experiments in chapters I, II, III, it addresses the opportunities and limits of the presentwork regarding the phenotyping of early vigor diversity and its drought response, as well as theconsequences for genetic studies and finally molecular breeding. Future physiological and geneticstudies are suggested to better understand trait interactions that limit the improvement of rice earlyvigor and the consequence for grain yield, under well watered and drought conditions .Methodological overview: details on the dry-down systemTo our knowledge few studies have focused on drought tolerance traits avoiding the expression ofrooting (i.e. avoidance) differences during the vegetative stage (Kato et al. 2008; Luquet et al. 2008;Parent et al. 2010). For controlled environments, Luquet et al. (2008) presented a dry downmethodology that enables simultaneous monitoring of drought establishment, dynamicallymeasuring plant growth and avoiding the expression of rooting differences by a reduced soil volumein pots. This gravimetric dry down system was adapted (and semi-automated) for a large diversitypanel (200 genotypes). The use of this system to monitor and compare drought impact on severalgenotypes used the Fraction of Transpirable Soil water (FTSW) as an indicator for drought intensity,based on the hypothesis that plants respond similarly to the progressive soil drying across a widerange of conditions (climate, soil type, pot size) when water stress is expressed on the basis of FTSW(Sinclair et al. 1986; Sinclair 2005). To adapt the dry down system detailed by Luquet et al (2008) to alarge panel we calculate FTSW’, by contrast to FTSW, with FTSW’ considering the same wilting pointvalue for all genotypes.19