Crescimento reduzido de plantas de milho (Zea mays L.) cv. BR 5011 deficientes em potássio não está diretamente lidado à queda na atividade fotossintética

Abstract

How the deficiency of potassium affects the photosynthesis of C4 plants still unclear, being the mechanisms behind the limitations imposed by the privation of this nutrient not well understood. This study aimed to elucidate how the potassium deficiency affects the photosynthesis of maize plants. To achieve that, accesses of gas exchange, chlorophyll fluorescence, absorbance of P700, carbohydrates, K content, water relative content, osmotic potential of roots and leaves and growth parameters were carried out. The experiments were conducted in greenhouse condition. A hydroponic medium was used to sustain the plants growth; it consisted of a modified Hoagland and Arnon hydroponic solution, in which the plants stayed until the expansion of the third leaf. After that, the treatments were implanted. The treatment consisted of two distinct solutions with different K concentrations: a K sufficient solution, with 6 mM K (+K) and a K absent solution, with ≈0.03 mM (-K). The maize cv. utilized in this study was the BR5011. The results showed that maize plants subjected to K deficiency had reduced growth in comparison to +K plants. The fresh weight of the -K plants was reduced by 59%, besides the leaf emission rate and the total leaf number were not affected. In addition, the relative water content was 16% lower in -K plants. Significant changes to gas exchange parameters was only achieved at 18 days of treatment (DAT) of K privation. At this point, the plants already showed a severe reduction in growth, and the concentration of K in leaves was below the critical level found in the literature. The parameters of chlorophyll fluorescence and P700 absorbance also suffered from K deficiency, but only 18 DAT. Together, the data suggests that the reduced growth of - K plants was probably due to K deficiency negative influencing the capacity of the plant to maintain the turgor pressure inside the growing tissues. In addition, the limitations to photosynthesis found here must be related to metabolic limitations. The limited growth of -K plants must not be linked to reduction in the photosynthesis rate, due to plants be already presenting stunted growth even when photosynthesis was not yet affected by K deficiency.