Genetic parameters for drought-tolerance in cassava

Abstract

-The objective of this work was to evaluate the effect of drought on genetic parameters and breeding values of cassava. The experiments were carried out in a completely randomized block design with three replicates, under field conditions with (WD) or without (FI) water deficit. Yield of storage roots (RoY), shoot (ShY), and starch (StY), as well as the number of roots (NR), and root dry matter content (DMC) were evaluated in 47 cassava accessions. Significant differences were observed among accessions; according to heritability, these differences had mostly a genetic nature. Heritability estimates for genotypic effects ( h g 2 ) ranged from 0.25±0.12 (NR) to 0.60±0.18 (DMC), and from 0.51±0.17 (NR) to 0.80±0.21 (RoY and StY) for WD and FI, respectively, as a consequence of greater environmental influence on WD. Selective accuracy was lower in WD, and ranged from 0.71 (NR) to 0.89 (RoY, DMC, and StY). However, genetic gains were quite high and ranged from 24.43% (DMC) to 113.41% (StY), in WD, and from 8.5% (DMC) to 75.70% (StY) in FI. These genetic parameters may be useful for defining which selection strategies, breeding methods, and experimental designs are more suitable to obtain cassava genetic gains for tolerance to drought.Index terms: Manihot esculenta, breeding, drought stress, germplasm. Parâmetros genéticos da mandioca quanto à tolerância ao deficit hídricoResumo -O objetivo deste trabalho foi avaliar o efeito do deficit hídrico sobre os parâmetros e os valores genéticos da mandioca. Os experimentos foram realizados em delineamento de blocos ao acaso com três repetições, em campo com (CD) ou sem deficit hídrico (SD). A produtividade de raízes (PR), da parte aérea (PA) e de amido (PAM), assim como o número de raízes (NR) e a massa de matéria seca das raízes (MS) foram avaliados em 47 acessos de mandioca. Observaram-se diferenças significativas entre os acessos; conforme a herdabilidade, estas diferenças foram em sua maioria de natureza genética. As estimativas de herdabilidade dos efeitos genotípicos ( h g 2 ) variaram de 0,25±0,12 (NR) a 0,60±0,18 (MS) e de 0,51±0,17 (NR) a 0,80±0,21 (PR e PAM) para CD e SD, respectivamente, em decorrência da maior influência ambiental sobre o CD. A acurácia seletiva foi menor no CD, com variação de 0,71 (NR) a 0,89 (PR, MS e PAM). No entanto, os ganhos genéticos foram elevados, de 24,43% (MS) a 113,41% (PAM) no CD, e de 8,5% (MS) a 75,70% (PAM) no SD. Estes parâmetros genéticos podem ser úteis para definir estratégias de seleção, métodos de melhoramento e delineamentos experimentais mais apropriados, para a obtenção de ganhos genéticos em mandioca quanto à tolerância à seca.Termos para indexação: Manihot esculenta, melhoramento, estresse hídrico, germoplasma.

Genetic parameters and agronomic evaluation of cassava genotypes

“…However, there was no significant difference for NRRP and FRY. Genetic variation for cassava root traits, including root yield, has been commonly identified in different studies in Brazil (Barreto & Resende, 2010;Oliveira et al, 2014Oliveira et al, , 2015 and in Africa (Aina et al, 2007;Ojulong et al, 2008;Tumuhimbise et al, 2015). Possibly, in the present study, the augmented blocks design was not suitable to detect differences for root yield.…”

“…This indicated that SCR had low environmental effect on local conditions, and evaluations of SCR in a higher number of divergent cassava genotypes may guarantee better results of genetic selection for this trait. Higher values of CVe for starch yield were detected by Oliveira et al (2014Oliveira et al ( , 2015. However, estimates of CVg for starch yield were low for a sample of 250 cassava genotypes assessed in the state of Amazonas, Brazil (Barreto & Resende, 2010).…”

“…In the present study, the value of h g 2 was low and selective accuracy was of medium magnitude (Table 4). Oliveira et al (2015), however, obtained high starch yield estimates for 47 genotypes of cassava, whereas Kizito et al (2007) identified h g 2 of 0.42 for SCR in the analysis of genotypes with low hydrogen cyanide and high dry matter contents. Oliveira et al (2015) found that this difference in the results of h g 2 concerning SCR may be due to the use of different methods to determine dry matter in the roots, as well as to the effects of location and harvest time.…”

“…The estimates obtained for genetic gains for PSW, based on the selection of the five best genotypes, varied from 6.0 to 11.08%, when compared with the general mean of the evaluated sample (Tables 4 and 5). Oliveira et al (2015) assessed 47 cassava genotypes and identified higher values of h g 2 (0.67) and selective accuracy (0.93) for shoot yield, with genetic gains that varied from 8.4 to 20.4%, considering the ten best genotypes. The low h g 2 observed in the present study may be an effect of the high CVe (Table 4), which reflected on low values of genetic gains with the selection of the five best genotypes.…”

“…The conservation of cassava germplasm is essential to reduce genetic erosion and to supply part of the genetic diversity for genetic breeding programs (Fukuda et al, 2005). A fundamental action in the process of genetic breeding is the genotypic prediction of the most promising genotypes, which depends on the estimation of genetic parameters, as well as on the correlations among traits under selection (Resende et al, 2001;Oliveira et al, 2015). However, researches on estimates of genetic parameters for cassava quantitative traits and of polygenic heritage (Resende, 2002;Ojulong et al, 2008) are still scarce in Brazilian conditions (Barreto & Resende, 2010;Farias Neto et al, 2013).…”