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

IntroductionThis equation expresses the change in dry matter between two dates (SDWi and SDWf), normalizedby the current dry matter, under the assumption that relative gain per unit of thermal time isconstant (exponential growth). The assumption of exponential growth is justified by the fact thatpopulations were still far from canopy closure at the time of sampling.In order to compare genotypes sampled at various dates and phenological ages (in terms of HaunIndex) within Exp2 and between Exp1 and Exp2, growth related variables in Exp2 (SDW, tiller numberNBT, total leaf number LN) were normalized by an environmental variable combining the incidentdaily radiation (PAR) and thermal time (TT) accumulated during plant growth. This photothermalvariable (PT) was computed as (Eq.2):PT PARTTn, in MJ °Cd m - ² (Eq.2)Normalized growth variables of Exp2 were multiplied by the PT value of the plants in Exp1 at 35 daysafter sowing (i.e. 2.2x10 6 MJ °Cd m - ²).Since in both experiments LL and LW were measured on leaves of various ranks, observations werenormalized by this rank. Then, assuming linearity of change in leaf dimensions with rank during plantexponential growth (Dingkuhn et al. 2006; Luquet et al. 2006), i.e. until Rank 7, values weremultiplied by 7 to estimate LL7 and LW7.Developmental rate (inverse of phyllochron) was calculated using the thermal time accumulatedfrom sowing (Exp1) and germination in the incubator (Exp2) to final sampling and the number ofappeared leaves on the main stem (LN_MS) measured at the end of the experiment, following (Eq.3):DR= (LN_MS)/ TT, in °Cd -1 (Eq.3),Statistical analysesThe Npar1way procedure of SAS for windows V.9.2 (Statistical Analysis System institute, Cary, NY,USA) was used to compare parameter distributions across genotypes using the Wilcoxon MannWithney Test. Matrices of correlation were performed with the SAS Correlation procedure. XLSTATsoftware (2008 version 3.02, copyright Addinsoft 1995-2008; www.xlstat.com) was used to performmultiple linear regressions. Sigmaplot9 software (for windows version 9, copyright 2004, Systatsoftware inc.) was used for plotting data and non-linear regressions.37