Aluminum Effects on Nitrogen Uptake and Nitrogen Assimilating Enzymes in Maize Genotypes with Contrasting Tolerance to Aluminum Toxicity

Abstract

The objective of this study was to investigate the effect of aluminum (Al) on NO₃ ⁻ and NH₄ ⁺ uptake and on activity/expression of the N‐assimilating enzymes glutamine synthetase (GS1, GS2, and GSr isoforms, EC 6.3.1.2), glutamate synthase (NADH‐GOGAT, EC 1.4.7.14; ferredoxin‐GOGAT, EC 1.4.7.1), and glutamate dehydrogenase (GDH, EC 1.4.1.2) of maize (Zea mays L.) genotypes contrasting in tolerance to Al toxicity. Seeds were germinated on paper towels, screened for uniformity and allowed to grow for 5–6 d in a modified Steinberg nutrient solution, pH 5.5, in a glasshouse, under natural light. Immediately after transference to a growth chamber (12 h photoperiod, 26°C ± 0.5/18°C ± 0.9 day/night temperature, 72% RH and 540 µ Em⁻² s⁻¹ quantum flux density) Al stress was imposed by lowering pH to 4.5 and addition of 166 µmoles L⁻¹ AlCl₃. Control plants were grown in the same nutrient solution without Al, pH 4.5. Aluminum inhibited NO₃ ⁻ and NH₄ ⁺ uptake and decreased shoot dry matter yield in all genotypes used in this investigation. Irrespective of their tolerance to Al toxicity, Al‐treated plants had less nitrogen (N), phosphorus (P), calcium (Ca), magnesium (Mg), and sulfur (S) in their leaves. In roots, Al treatment did not affect dry matter weight but decreased accumulation of N, Ca, and Mg. Phosphorus and potassium (K) tended to accumulate in roots of Al‐treated plants, suggesting that Al inhibited P translocation to the leaves. In roots, Al induced anaplerotic GDH and phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) activities, but inhibited GS activity. Moreover, in the apex (0–5 mm) and in the 6–15 mm segment of the elongation zone of most genotypes, Al tended to inhibit expression of GSr and favor expression of GS1. This increase in GS1 expression concomitantly with decreased N uptake and elevated deaminating GDH activity are indicative of metabolic changes usually associated with plant senescence. In leaves, Al did not influence GOGAT and GS activities and expression of GS isoforms suggesting it does not disturb assimilation of nitrogenous compounds translocated from roots to leaves. Nonetheless, lower leaf PEPC activity suggests that Al intoxicated plants had lower CO₂ ⁻ fixation rates. The inverse effect of Al treatment on the activities of GDH and PEPC in roots and leaves, indicates that whereas increased activities in roots were induced by stress related stimuli, decreased activities in leaves were caused by a reduced nitrogen supply to the leaves. Because the earliest effects observed in this investigation appeared only after 24 h of exposure to Al and were similar in both tolerant and sensitive genotypes, it is concluded that the changes observed in N uptake and assimilation into plant amino acids were downstream of the initial, primary Al toxicity event.

Keywords:

• NO₃ ⁻ uptake
• NH₄ ⁺ uptake
• GS
• GOGAT
• GDH
• PEPC

Acknowledgments

The authors are thankful to Gislene Cristelli and Edna P. P. de Pinho for excellent laboratorial assistance and to Dr. Antonio Carlos de Oliveira for statistical analysis of the data. During the course of this work CLB was supported by grant ERBIC 18CT 960063, Fitting maize into cropping systems on acid soils of the tropics, and LLL by a post‐doctorate fellowship from FAPEMIG CAG 410/98. Consumables were purchased with funds provided by CNPq 524399/96‐4 to AACP and FAPEMIG CAG 1148/96 to VMCA.