Floating row covers affect the molybdenum and nitrogen status of Chinese cabbage grown under field conditions

In three consecutive years of field experiments, an uncovered control (T0), and floating row covers treatment (T1: perforated polyethylene, 50 μm; T2: polypropylene, 17 g m^-2) were used to produce different growth conditions for Chinese cabbage [Brassica pekinensis (Lour) Rupr. cv. Nagaoka 50]. Five samplings (whole tops) were made between transplant and harvest, and measurements were made of NO3^-, NH4⁺. organic N, and Mo concentrations, as well as nitrate reductase (NR) activity, and amino acids in the whole shoots. The microclimatic conditions in T0 resulted in plants with higher Mo accumulation, as well as the highest basal NR activity and the lowest NO3^- concentrations. The thermal environment under the T1 treatment induced the highest mean temperature values and resulted in plants with the highest amino acid level, whereas the plants in the T2 treatment with its intermediate thermal conditions, had the lowest levels of Mo and the highest NO3^- concentrations. The floating row covers influenced the Mo status, which in turn affected NR activity. In addition, T1-polyethylene sheet and T2fleece favoured an increase in the levels of NO3^- by the limitation in the NR activity, with the consequent risk of a high NO 3^- level in vegetables for human consumption.