Diallel Analysis and Growth Parameters as Selection Tools for Drought Tolerance in Young Theobroma cacao Plants

Abstract

This study aimed to estimate the combining ability, of T. cacao genotypes preselected for drought tolerance through diallel crosses. The experiment was conducted under greenhouse conditions at the Cacao Research Center (CEPEC), Ilhéus, Bahia, Brazil, in a completely randomized block design, in an experimental arrangement 21 x 2 [21 complete diallel crosses and two water regimes (control and stressed)]. In the control, soil moisture was kept close to field capacity, with predawn leaf water potential (ΨWL) ranging from -0.1 to -0.5 MPa. In the drought regime, the soil moisture was reduced gradually by decreasing the amount of water application until ΨWL reached -2.0 to -2.5 MPa. Significant differences (p < 0.05) were observed for most morphological attributes analyzed regarding progenies, water regime and their interactions. The results of the joint diallel analysis revealed significant effects between general combining ability (GCA) x water regimes and between specific combining ability (SCA) x water regimes. The SCA 6 genetic material showed high general combining ability for growth variables regardless of the water regime. In general, the water deficit influenced the production of biomass in most of the evaluated T. cacao crosses, except for SCA-6 x IMC-67, Catongo x SCA, MOC-01 x Catongo, Catongo x IMC-67 and RB-40 x Catongo. Multivariate analysis showed that stem diameter (CD), total leaf area (TLA), leaf dry biomass (LDB), stem dry biomass (SDB), root dry biomass (RDB), total dry biomass (TDB), root length (RL), root volume (RV), root diameter (RD) <1 mm and 1 <(RD) <2 mm were the most important growth parameters in the separation of T. cacao genotypes in to tolerant and intolerant to soil water deficit.

The physiological responses of cacao to the environment and the implications for climate change resilience. A review

“…Biomass allocation patterns often change in plants exposed to water deficit causing shifts in root/shoot ratios. In cacao, changes to root biomass production under water deficit vary and appear to be genotype specific with both increases and decreases in root growth reported in response to water deficit (Mohd Razi et al 1992;Moser et al 2010;dos Santos et al 2014dos Santos et al , 2016. The ability to increase root growth under water deficit may be a beneficial trait for conferring tolerance to soil water deficit, by enabling greater access to and uptake of soil water (dos Santos et al 2014Santos et al , 2016.…”

“…In cacao, changes to root biomass production under water deficit vary and appear to be genotype specific with both increases and decreases in root growth reported in response to water deficit (Mohd Razi et al 1992;Moser et al 2010;dos Santos et al 2014dos Santos et al , 2016. The ability to increase root growth under water deficit may be a beneficial trait for conferring tolerance to soil water deficit, by enabling greater access to and uptake of soil water (dos Santos et al 2014Santos et al , 2016. In robusta coffee, root depth was associated with greater tolerance to drought as water limitation could be avoided for longer in varieties with deeper roots (Pinheiro et al 2005).…”

“…These variables were total leaf area per plant, root dry weight, stem dry weight, leaf dry weight, total dry weight, relative growth rate and Ca and Mg concentrations. While in a later study dos Santos et al (2016) included the variables root length, root volume, root diameter (of roots < 1 mm) and root diameter (of roots > 1 mm < 2 mm). The authors suggest that the high heritability of a number of these traits has the potential to be used in the selection of cacao genotypes with enhanced water-deficit tolerance (dos Santos et al 2016).…”

“…While in a later study dos Santos et al (2016) included the variables root length, root volume, root diameter (of roots < 1 mm) and root diameter (of roots > 1 mm < 2 mm). The authors suggest that the high heritability of a number of these traits has the potential to be used in the selection of cacao genotypes with enhanced water-deficit tolerance (dos Santos et al 2016). It is worth noting that most of these traits are measures of vigour.…”

Bayesian decision theoretic design of two-founder experimental crosses given diallel data

“…Diallels have been used to study numerous traits in a diverse range of organisms, including: mating speed, female receptivity, and temperature preference in flies (Parsons 1964;Casares et al 1992;Yamamoto 1994); immune function, polyandry, and genetic-environment interactions in crickets (Rantala and Roff 2006;Ivy 2007;Nystrand et al 2011); and heterosis and reciprocal effects in poultry (Fairfull et al 1983). Additionally, the diallel has a long history in plant breeding (Gilbert 1958) and numerous recent applications (Bahari et al 2012;Ghareeb Zeinab and Helal 2014;Dos Santos et al 2016).…”