A new morphological marker of the effects of acid rain in the tropics on the stem of Libidibia ferrea var. leiostachya (Leguminosae–Caesalpinioideae)

Some of the largest remaining fragments of the Atlantic rainforest are affected by acid rain. Knowing the responses of its native plant species to acidity may be crucial to use them as bioindicators in affected areas. We tested the effects of pH 3.0 simulated acid rain on the stem of Libidibia ferrea (Mart. ex Tul.) L.P. Queiroz var. leiostachya (Benth.) L.P. Queiroz (ironwood, “pau-ferro”). Visual symptomatology consisted in dark spots on the stem that initiated on and irradiated from lenticels. As they grew larger, adjacent spots coalesced, conferring a blackened aspect to a large extension of the organ. Anatomically, the damage started with hypertrophy and increased accumulation of phenolic compounds in the outermost cell layers of the young, still subepidermal cork, which is what conferred the blackened aspect to the stem. In addition, epidermal cells showed turgor loss and ultimately collapsed, resulting in alteration of cell shape and a consequent alteration of epidermal relief. The stem of L. ferrea var. leiostachya was shown to be a more sensitive organ to the impacts of acid rain than the leaf. The dark spots visualised on the stem may be a promising macroscopic marker to be used in the biomonitoring of acid rain.