Resposta de cultivares de Urochloa brizantha à toxidade de alumínio livre durante as fases de germinação e desenvolvimento de plântulas

Abstract

Whereas it is necessary to obtain higher pastures productivity to prevent new areas occupation, agriculture maintenance and sustainability, even in Brazil and in the world, the increasing demand for water use efficiency in agriculture and use of acid soils associated with aluminium. The aim of this study was to evaluate the changes in germination and in the early seedlings development and some biochemical parameters in Urochloa brizantha cultivars subjected to stresses caused by free aluminum toxicity. The treatments were resulting from the combination of six cultivars (Marandú, BRS Piatã, MG4, MG5, Xaraés and Basilisk), with five levels of aluminium toxicity (0; 1; 2; 4 and 8 mmolc Al³+ dm³) obtained using aluminium trichloride solutions (0; 0.3996; 0.7992; 1.5984 and 3.1968 AlCl3 (g. L-¹)) in random design with four replicates. At the preliminary evaluations lots were selected with almost the same seed initial physiological quality (moisture content, germination and viability). During first trial, the selected lots were submitted to germination in simulated stress condition in laboratory quantifying germination considering root protrusion or normal seedling, shoot and root length and dry mass. At the second trial, the evaluations were done at the seventh day after seedling transfer to stress conditions, measuring length and dry mass of shoot and root relative growth-rate and biochemical determinations: membrane stability index (MSI), superoxide dismutase activity (SOD), proline, total antioxidants (DAT) and malondialdehyde amount (MDA). At the seedling studied period, up to 14 days after sowing, the most aluminium tolerant cultivar maintain root growth in that conditions, while the most susceptible one reduced root growth and developed proportionally more shoot, once the growth is done with reserves mobilization from seed tissue. The first trial methodology shows that the initial seed quality interferes in the evaluation and the Al+3 increasing levels were harmful to germination. The MG5 cultivar presented longer and heavier seedlings without stress and following 1 mmolc. dm-3 de Al³+ presented decreasing measurements, showing low stress tolerance. The Basilisk cultivar is not the more tolerant considering germination results, but its development, principally of the root, is lower affected with Al³+levels increasing. The second trial methodology, once do not include the germination period and its influence, achieve cultivar differences clearer. The cultivars, Xaraés (dry mass of shoot relative growth-rate), MG4 and MG5 (length of shoot relative growth-rate), was superior only at the non-stress situation, with more development at control (0) up to 1 mmolc. dm-3 de Al³+ level. The Basilisk cultivar is more tolerant accumulating root dry mass even in high Al³+ levels, while the Marandu cultivar do the same up to 4 mmolc. dm-3 de Al³+. The biochemical analyses, Proline amount, SOD and DAT was not related with Al3+ toxicity, but MSI e MDA was promising.