Morphoanatomy and histochemistry analyses of cassava roots do not discriminate resistant from susceptible genotypes to soft root rot

Abstract

Cassava is an important culture in Brazil and in the North of the country, and soft root rot has affected root production. The aim of this work was to identify root morphoanatomic and histochemical characters associated with root rot resistance. In areas with no occurrence of the disease, nine cassava genotypes were tested, four of which were resistant, and five were susceptible to root rot. Root harvest was carried out twelve months after sowing, and thickness of suber, suber and cortex, and secondary xylem were measured. Moreover, texture, suber cell layers, lignin content, and lignin monomers content of the portion suber and cortex were analyzed. Also, histochemical tests and scanning electron microscopy (SEM) analyses of the roots were performed. Results were subjected to analysis of variance, and means were compared by the Scott-Knott test (p ≤0.05). The characters thickness of suber and cortex at proximal and medium portions, thickness of secondary xylem at medium and apical portions, number of cell layers of suber, thickness of suber, and root texture showed differences among genotypes; however, the characters could not be associated with resistance or susceptibility. There were no differences for: vessel elements, lignin and lignin monomers, and for the histochemical tests with Sudan IV and Lugol. Images of SEM showed differences among genotypes, which could not be associated with resistance or susceptibility. Thus, the characters evaluated cannot be used in selection for cassava resistance to soft root rot in genetic breeding programs. KEYWORDS: Anatomy; morphological characterization; genetic breeding; Manihot esculenta Morfoanatomia e histoquímica de raízes de mandioca não diferenciam genótipos resistentes e suscetíveis a podridão mole da raiz RESUMO A mandioca é uma cultura importante no Brasil e no Norte do país, e a podridão mole da raiz tem afetado a produção. O objetivo deste trabalho foi identificar caracteres morfoanatômicos e histoquímicos da raiz associados à resistência à podridão da raiz. Em áreas sem ocorrência da doença, nove genótipos de mandioca foram testados, quatro resistentes e cinco suscetíveis. A colheita da raiz foi realizada doze meses após o plantio e medidas de espessura do súber, súber e cortex e xilema secundário foram realizadas. Além disso, foram analisados a textura, as camadas de célula do súber, o conteúdo de lignina e os monômeros de lignina da porção súber e cortex. Ainda, testes histoquímicos e microscopia eletrônica de varredura (MEV) das raízes foram realizados. Os resultados foram submetidos à análise de variância e as médias foram comparadas pelo teste de Scott-Knott (p≤0.05). Os caracteres: espessura de súber e córtex nas porções mediana e proximal, espessura do xilema secundário nas porções mediana e apical, número de camadas de células do súber, espessura do súber e textura da raiz apresentaram diferenças entre os genótipos, mas não associadas à resistência ou suscetibilidade. Não houve diferenças para: elementos de vaso, lignina e monômeros de lignina...