ABSTRACT. This study was developed to evaluate the effects of salinity on the growth and gas exchange of cassava plants, cultivar Verdinha. The four concentrations of NaCl (mM) were as follows: 0, 20, 40, and 60. Under salinity, the lowest concentration of Na + ions was observed in the tuberous roots; however, the dry matter of tuberous roots was reduced with an application of just 20 mM NaCl. The harvest index was reduced 50% with the highest salt concentration. Salinity reduced carbon assimilation (A), stomatal conductance (g s ), transpiration, and the instantaneous water use efficiency. The correlation between g s and A was high and positive, showing that stomatal movement was one of the responsible for the lower A. Under salt stress, there was an increase in intercellular CO 2 concentration, indicating the impairment of carbon metabolism. Based on the reduction of dry matter of the tuberous roots (reduction of 81% under 60 mM NaCl), it was concluded that cassava is sensitive to salinity. The growth of shoots and the absorbing roots were minimally affected by salinity, even in the situation where A was reduced; therefore, the sensitivity of cassava was related to the high sensitivity of the tuberous roots to the ionic and/or osmotic effects of salinity. Thus, tuberous roots can be the target organ in studies that aim to improve the tolerance of cassava to salinity.Keywords: photosynthesis, dry matter, water use efficiency, harvest index, root:shoot ratio, salt stress.Salinidade reduz a fotossíntese e o índice de colheita da mandioca (Manihot esculenta Crantz) RESUMO. Esse estudo foi desenvolvido para avaliar os efeitos da salinidade sobre as trocas gasosas e crescimento da mandioca, cultivar Verdinha. Quatro concentrações de NaCl, em mM, foram utilizadas: 0, 20, 40 e 60. A menor concentração de Na + foi observada nas raízes tuberosas; entretanto, a matéria seca dessa parte da planta foi reduzida pela aplicação de apenas 20 mM de NaCl. O índice de colheita foi reduzido em até 50% pela salinidade. A aplicação de NaCl reduziu a assimilação de carbono (A), condutância estomática (g s ), transpiração (E) e eficiência no uso de água (EUA). A correlação entre g s e A foi alta e positiva, evidenciando que o movimento estomático foi um dos responsáveis pela menor A. Sob salinidade também existiu aumento na concentração interna de CO 2 , indicando que o NaCl pode ter inibido o metabolismo de carbono das plantas. Com base na redução do acúmulo de matéria seca das raízes tuberosas (redução de 81% sob 60 mM de NaCl) é concluído que essa cultivar de mandioca é bastante sensível à presença de NaCl na solução do solo. A sensibilidade da mandioca esteve relacionada à alta sensibilidade das raízes tuberosas aos efeitos iônicos e/ou osmótico do NaCl. Assim, as raízes tuberosas podem ser o órgão alvo em estudos que visem melhorar a tolerância da mandioca à salinidade.Palavras-chave: fotossíntese, matéria seca, uso eficiente de água, índice de colheita, relação raiz:parte aérea, estresse salino.

The objective of this research was to evaluate the influence of salinity on the absorption and utilization of nutrients by cassava. For the study, cassava was submitted to four saline concentrations: 0, 20, 40, and 60mM NaCl. Results showed that the absorption of all nutrients, except nitrogen (N), was reduced by salinity, with highest reduction for potassium (K). However, all nutrients were maintained at concentrations which did not indicate mineral deficiency problem. The abnormal concentration of calcium in the tuberous roots may have been one of the factors that contributed to the lower growth of this organ and of the plant as a whole. Transports of nitrogen, potassium, phosphorus and sulfur from root to the aerial part was higher under salinity treatment. Efficiency in the use of all the nutrients, mainly N, was reduced due to salinity. Given that: (i) the absorption of K was the most impaired, (ii) there was abnormal accumulation of Ca in tuberous roots, and (iii) the efficiency in the use of N was the most affected, it is suggested to prioritize studies on these three issues, as a way to better understand the aspects related to the tolerance/sensitivity of cassava plants to salinity.

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