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Alkaline stress is caused by accumulation of alkaline salts (such as NaHCO3 and Na2CO3) in the soil. This problem, common in arid and semiarid areas of Spain, results from intensive agriculture, the use of hard water, and cultivation in areas of calcareous rock. The present study examines plant metabolic responses against this stress in Brassica oleracea L. cv. Bronco in order to better understand the systems of resistance that could promote plant growth in alkaline soils, and indicates the most relevant biomarkers of alkalinity toxicity. Alkaline stress causes a sharp reduction in biomass, an accumulation of Na+, and a nutritional deficiency. With this debilitated photosynthetic capacity and the antioxidant activity, superoxide dismutase (SOD) sharply decreases and O-2(center dot-) accumulates, resulting in two of the main causes of stress. Therefore the mechanisms of uptake and compartmentalisation of nutrients and detoxification of O-2(center dot-) systems should be strengthened in order to remedy alkaline stress. In this work, the authors conclude that Na, P, K, Ca, Mg, and Mn ions are the best indicators of alkaline stress, due to their strong correlation with reduced leaf biomass reduction and photosynthetic pigments, together with the greater accumulation of proline and certain antioxidant enzymes such as SOD.