Produção de ácidos orgânicos radiculares de cana-de-açúcar submetida ao estresse por alumínio

Abstract

There are prospects of increases in the productivity of the sugarcane crop with the suitability of the varieties to the edaphoclimatic zoning. Several restrictions on the expression of the genetic potential of the varieties are attributed to the high saturation by phytotoxic aluminum (Al3+) in the soil, which prevents the expansion of the sugarcane root system. This is a typical characteristic of highly weathered acid soils. One of the mechanisms of Al3+ tolerance is associated with the production of organic acids (OAs) by the roots. The aim of this work was to identify and quantify the production of citric, malic and oxalic acids by roots of sugarcane seedlings exposed to Al3+ stress in solution. Mini grinding of two sugarcane varieties (RB928064 and RB855156) were pre-sprouted in vermiculite under greenhouse climatic conditions. After 25 days, seedlings were screened for higher sprout uniformity and better vigor pattern, followed by root system washing. In the laboratory, the seedlings were transferred to 30 L containers for immersion of the root system in complete nutrient solution. The set was maintained at room temperature, with simulated photoperiod of 12 h, light intensity of 4.5 klux and continuous aeration of the solution. After six days of acclimatization, 25 seedlings of each variety were selected. These had the initial root length measurement (Ci) and they were submitted to treatments with increasing doses of Al (0, 500, 1000, 1500 and 2000 μmol L- 1) in individualized tubes, with the same environmental conditions of the acclimation phase. The pH of the solution was adjusted daily (pH = 4.0 ± 0.2) with 0.1 mol L-1 HCl, to guarantee the predominance of the Al3+ species. After six days of exposure to Al stress, the final length (Cf) of the roots was measured. Root growth (cm) was estimated as Cr = Cf - Ci. Samples of fresh root tissue were ground in 80% ethanol, filtered, subjected to ultrasonic treatment and centrifuged. The supernatant was evaporated at 55°C and resuspended in distilled water, before ultrafiltration. Samples of the solution were collected to determine the OAs contents. The extracts were analyzed in a high performance liquid chromatography system for the identification and quantification of citric, malic and oxalic acids. The experiment was arranged in a completely randomized design with factorial scheme 2 (varieties) × 5 (doses of Al3+) and five replicates. The results of Cr and OAs contents were submitted to analysis of variance, with Tukey post-test (p <0.05) and regression analysis as a function of increasing doses of Al in solution. Both varieties produced OAs (malic> citric > oxalic), even in the absence of Al3+. The variety RB928064 produced higher concentrations of citric, malic and oxalic acids. The production of citric acid in the root tissue of RB928064 was induced by 1500 mol L-1 of Al3+. It was adjusted by a quadratic polynomial regression model (R2 = 0.74; p <0.01). The malic acid and oxalic acid production were not induced by the Al3+ stress. Therefore, it was considered a strictly varietal characteristic. The highest proportion (57% to 94%) of the contents of OAs produced by the varieties was maintained in the root tissue, except for the variety RB 855156, which exuded 69% of the total oxalic acid produced. The total contents (root tissue + exudate) of citric acid produced by RB928064 as a function of Al3+ doses was estimated by a linear regression model (R2 = 0.43; p <0.01). Cr of RB928064 was not altered by increased Al3+ stress and Cr of RB855156 variety decreased starting at 1500  mol L-1 of Al. RB928064 variety was considered more tolerant to Al3+ stress than RB855156. Al tolerance allows the expansion of the sugarcane root system to deeper layers of the soil, reducing the susceptibility to water stress and increasing the nutrient exploitation capacity. It is a feature that allows the allocation of RB928064 in more restrictive production environments, maintaining its productive potential.