Differential interactive effects of the Ca/Mg quotient and PEG-simulated drought in Alyssum inflatum and Fortuynia garcinii

Background and aims: Serpentine soils typically have low Ca/Mg quotients, low water-holding capacities, and are rich in Ni and Cr, but poor in N and P. To better understand the role of drought and low Ca/Mg quotients in plant adaptation to serpentine soil, we compared the effects of different Ca/Mg quotients (0.18, 2 and 4) and polyethyleneglycol (PEG)–imposed osmotic stress on growth and Ca and Mg uptake and translocation between two Iranian serpentine endemic plant species, the Ni-hyperaccumulator Alyssum inflatum, and the non-accumulator Fortuynia garcinii. Methods: Seedlings were grown in modified Hoagland’s solution with varying Ca/Mg molar quotients (0.18, 2 and 4) and then exposed to PEG (209.7 g L⁻¹) for 8 days. Results: In both species, shoot dry weight was highest at the highest Ca/Mg quotient (4). However, at this Ca/Mg quotient A. inflatum suffered much more from PEG-induced drought (84% mortality and severe growth inhibition in the survivors), compared with the lower Ca/Mg quotients (<20% mortality and no significant growth inhibition). In F. garcinii, the PEG*Ca/Mg interaction was not significant. Shoot Ca concentrations and translocation factors in A. inflatum were higher than in F. garcinii in all the treatments. In both species the Mg translocation factor was highest at the highest Ca/Mg quotient (4) in the PEG treatment. Conclusions: We conclude that a high Ca/Mg quotient stimulated growth in the absence of PEG in both species, but strongly decreased survival and growth in combination with PEG in A. inflatum. In F. garcinii root dry weight and the root to shoot dry weight ratio were significantly decreased at the highest Ca/Mg quotient (4), particularly after PEG treatment, suggesting that F. garcinii also may require a moderate or low Ca/Mg quotient to cope with natural drought.