Analysis of tomatoes showing iron deficiency symptoms in winter under heavy fruit load

Declines in crop productivity that are thought to be due to iron deficiency have been reported even on non-calcareous-alkaline soils. For example, tomatoes exhibit iron deficiency-like symptoms (yellowing of new leaves) if the fruit load is heavy during winter. The objective of this study is to investigate the relationship between fruit load, low temperatures, low sunlight, and iron deficiency, and to explore the possibility of increasing productivity with iron-containing fertilizers. Results of an investigation of the farms experiencing iron deficiency-like symptoms in winter showed low iron concentrations and elevated expression of iron deficiency-inducible genes in yellowing leaves. To investigate the effects of heavy fruit load on iron nutrition, plants were grown in three groups: those with a medium number of fruits per truss (MED), a high number of fruits (heavy fruit load; HEAVY), and a high number of fruits with additional iron fertilization (++Fe). The SPAD leaf-color value of the new leaves and weight of the lower leaves in winter were in the order of HEAVY < ++Fe < MED. In all groups, the iron concentrations in the upper leaves in January were lower than those in October and March. RNA-seq analysis showed that the expression of genes involved in iron absorption was suppressed in the roots in January, which may have contributed to iron deficiency in winter. These results suggest that one of the factors limiting the productivity of tomatoes in winter with heavy fruit load is iron, and that productivity can be restored by iron materials.