The importance of iron supply during repetitive harvesting of Aster tripolium

Aster tripolium L. is a salt marsh halophyte that has recently gained interest as a cash crop vegetable. Leaf yield and quality were investigated in plants grown with salinity in experiments with Perlite in pots and in plots on dune sand. Plants were repetitively harvested in a 14-day cycle. A. tripolium irrigated with 50mM NaCl exhibited the highest yield when grown in pots, whereas in the plot experiment no significant differences in biomass accumulation occurred up to 80mM NaCl in the irrigation water. Chemical leaf composition changed with salinity, exhibiting higher levels of electrical conductivity, total soluble solutes and the non-enzymatic antioxidant compounds ascorbic acid and polyphenols compared with control plants grown without NaCl supplementation. Using the repetitive harvest regime, leaf chlorosis occurred, a symptom shared by deficiencies in either nitrogen or iron. Comparative applications of five iron chelate formulations in plants grown with 50mM NaCl in pots revealed improved leaf colour and chlorophyll content for only two of the applied Fe-chelates. Concomitantly with leaf colour restoration, the activity of nitrate reductase, the first enzyme during nitrate assimilation, which requires heme-iron for its proper function, increased 3-fold as a result of the iron treatment in the plot experiment. Importantly, the enhancement of nitrate reductase activity was associated with a considerable decrease in the leaf nitrate concentration. Therefore, we concluded that iron deficiency, in addition to leaf chlorosis, reduces A. tripolium leaf quality as a vegetable by increasing the leaf nitrate content. Furthermore, nitrate reductase (NR) activity levels in A. tripolium leaves may act as an indicator of iron deficiency that manifests itself as reduced nitrate content owing to the higher NR activity upon proper iron nutrition. These results demonstrate the importance of salinity level and the application of an appropriate iron-chelating formulation to generate marketable yields of Aster tripolium leafy vegetable when grown commercially on dune sand.