The role of nickel (Ni) and drought in serpentine adaptation: contrasting effects of Ni on osmoprotectants and oxidative stress markers in the serpentine endemic, Cleome heratensis, and the related non-serpentinophyte, Cleome foliolosa

Aims: To assess the importance of Ni and drought tolerance in serpentine adaptation, the effects of Ni and polyethylene glycol (PEG)-simulated drought, alone and in combination, on growth, water-status, photosynthetic pigments, osmoprotectants, stress markers, and ROS-scavenging enzymes were compared between the serpentine endemic Cleome heratensis and the closely related non-serpentinophyte C. foliolosa. Methods: C. heratensis and C. foliolosa were exposed to combinations of Ni (0, or 20 μM) and PEG (0, −0.3, or −0.9 MPa) in a full factorial, replicated randomized block design. Results were analyzed using 3-way ANOVA. Results: Ni improved growth, enhanced osmoprotectant synthesis, and decreased oxidative stress in C. heratensis, but had insignificant, or deleterious effects on the same parameters in C. foliolosa. The second order Ni*PEG*species interactions were usually significant, demonstrating that the species responded distinctly to combined exposure to PEG and excess Ni. In most cases this was due to additive deleterious effects of Ni in PEG-treated C. foliolosa, i.e. on growth, shoot water status and stress markers, and beneficial effects or no effects of Ni in PEG-treated C. heratensis. Conclusions: In the absence of Ni exposure, the species were approximately equally tolerant to PEG-simulated drought. Under PEG exposure, Ni positively affected the performance of C. heratensis, but had additive deleterious effects in C. foliolosa. C. heratensis does not only exhibit Ni hypertolerance, associated with a strongly decreased root-to-shoot translocation, but also an enhanced requirement for Ni, particularly when under PEG exposure, in comparison with C. foliolosa.