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

Introductionthat enable leaf turgor and stomatal conductance maintenance under drought, have been reported.Traits favoring the maintenance of cellular functioning in a situation of dehydration (membraneintegrity, maintenance of the enzyme activity involved in Carbon fixation, low injury caused byreactive oxygen species) were recently reviewed by Blum (2009) and Farooq et al. (2009). Droughtactivates signaling cascades that results in changes in plant metabolism including: the activation andsynthesis of antioxidants, the synthesis and accumulation of osmo-protectors and stomatal closure.All these processes control growth maintenance under drought.Although the physiological role of drought tolerance processes was already reported (Lilley et al.1996), their genetic study is still scarse in rice. Price et al. (2002) insisted on the need to deal withQTL (Quantitative Trait Loci) for rice shoot tolerance traits to improve drought resistance incomplement of QTLs related to root morphological traits and drought avoidance, based on thegenetic study of a mapping population from the cross between Bala and Azucena genotypes.Tolerance and avoidance mechanisms were reported to have different genetic control in rice (Yue etal. 2006), suggesting their association is possible in order to optimize the improvement of plantgrowth under drought (Price et al. 2002).Water and C balance relations along rice shoot growth: implications for growth maintenanceunder droughtIn order to isolate drought tolerance traits and explore their impact on growth maintenance underdrought, it must be first considered that plant growth under drought results from constitutive(capacity or potential expressed under well watered conditions) and adaptive (regulation expressedunder water deficit) processes (Liu et al. 2004; Kamoshita et al. 2008). Both constitutive and adaptivetraits contribute to an effective use of water (Blum 2009) and C assimilates for growth.Luquet et al. (2008) and Parent et al. (2010) reported that the reduction of leaf growth underdrought was associated with a reduction of transpiration rate (thought stomatal closure) in responseto decreasing soil water availability, and Abscisic Acid (ABA) production (Liu et al. 2005). Stomatalclosure under drought reduces water loss and, in the same time, CO 2 absorption and thus Cassimilation. Indeed, its role in equilibrating growth maintenance and water conservation is a majorissue for breeding (Condon et al. 2004) since it controls both; the way plant conserves water andmaintains C assimilation. Water loss through leaf cuticle and the boundary layer can also impact onplant dehydration, thus on C assimilation and growth under drought (Farooq et al. 2009).15