Influence of water limitation on the competitive interaction between two Cerrado species and the invasive grass Brachiaria brizantha cv. Piatã.

Invasive grasses inhibit the growth of other plant species, and water deficit is one of the major competition problems for native vegetation. We evaluated whether the presence of Brachiaria brizantha cv. Piatã has a negative influence on the competition for water and nutrients between Anadenanthera macrocarpa and Anadenanthera colubrina (Angico species). The interspecific competition was evaluated using a randomized experimental design with the following treatments: 1) free competition (FC), in which the native species were cultivated without the grass presence and 2) under competition (UC), in which the native species grew together with the invasive grass for 120 days. We analysed the water relationships in the two species, the effect of water limitation on the antioxidant stress, the nutritional content of shoots and roots, the relative competition intensity (RCI) and growth. The presence of Piatã grass reduced the soil moisture causing a decrease of 21.9% and 29.5% in the relative water content (RWC) of leaves for A. macrocarpa and A. colubrina, respectively. For the two Angico species, the quantum efficiency of Photosystem II (ΦPSII) decreased with reduction of RWC leaf, resulting in the H₂O₂ increase (57.5% at day 30 for A. colubrina and 38.8% at day 120 for A. macrocarpa). The oxidative stress was evidenced by the increase in the superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX) activities in leaves and roots of both young native trees. In the UC treatment, reductions in water uptake also led to a decrease in root absorption of N, P, K, a Mg and low transport of these nutrients to the leaves of both Angico species. A. macrocarpa and A. colubrina showed less growth caused by limitation of water uptake, but the joint activity of the physiological and biochemical adjustments provided competitive ability.