Influence of UV-B radiation and Cd2+ on chlorophyll fluorescence, growth and nutrient content in Brassica napus

The possible interaction of two stresses, UV-B radiation and cadmium,applied simultaneously, was investigated in Brassicanapus L. cv. Paroll with respect of chlorophyll fluorescence,growth and uptake of selected elements. Plants were grown in nutrientsolution containing CdCl2, (0, 0.5, 2 or 5 M)and irradiated with photosynthetically active radiation(PAR, 400-700 nm, 800 mol m^-2s^-1) with or without supplemental ultraviolet-Bradiation (UV-B, 280-320 nm, 15 kJ m^-2d^-1, weighted irradiance). After 14 d of treatment,the most pronounced effects were found at 2 and 5 M CdCl2 with andwithout supplemental UV-B radiation. Exposure to cadmium significantlyincreased the amount of Cd in both roots and shoots. In addition, increasesoccurred in the concentrations of Fe, Zn, Cu, and P in roots, while K wasreduced. In shoots the S content rose significantly both in the presenceand absence of UV-B radiation, while significant increases in Mg, Ca, P,Cu, and K occurred only in plants exposed to Cd and UV-B radiation.Manganese decreased significantly under the combined exposure treatment.The rise in S content may have been due to stimulated glutathione andphytochelatin synthesis. Cadmium exposure significantly decreased root dryweight, leaf area, total chlorophyll content, carotenoid content, and thephotochemical quantum yield of photosynthesis. As an estimation of energydissipation processes in photosynthesis, non-photochemical quenching(qNPQ) was measured using a pulse amplitude modulatedfluorometer. The qNPQ increased with increasing Cd,while the combination of cadmium and UV-B reduced theqNPQ compared to that in plants exposed only tocadmium or UV-B radiation. The chlorophyll a:b ratioshowed a reduction with UV-B at no or low Cd concentrations (0 M,0.5 M CdCl2), but not at the higher Cd concentrations used (2M, 5 M CdCl2). Thus in some instances there appeared tobe a UV-B and Cd interaction, while in other plants response could beattributed to either treatment alone.Keywords:Brassica napus, cadmium, ultraviolet-Bradiation.