Variation in the use efficiency of leaf iron for growth among rice (Oryza sativa L.) cultivars

Previously, we reported that even barley, generally known as an Fe-deficiency-tolerant species, exhibits substantial variation in iron (Fe) deficiency tolerance and Fe-use efficiency in photosynthetic functions among cultivars. However, it remains unclear whether this finding is specific to barley or also applies to other grass species, such as rice (Oryza sativa L.), a crop susceptible to Fe deficiency. To characterize leaf Fe-use efficiency in rice, we examined the responses of Japonica and Indica subspecies to Fe deficiency using a hydroponic mixed-plant culture system that allows evaluation of leaf functions independently of the Strategy II-type root Fe acquisition mechanism. Under low-Fe conditions, shoot growth was maintained in the following order: Indica > Japonica paddy rice > Japonica upland rice. The Fe concentration and photosynthetic rate of the newest fully expanded leaves suggested that, if Fe-deficiency acclimation mechanisms exist in rice chloroplasts, they are likely different from those observed in barley. The relative shoot dry weight (Fe-deficient/Fe-sufficient) was greater than the relative root dry weight in Indica rice, but not in Japonica. This indicates that Indica cultivars tend to allocate more resources to shoots than to roots under Fe deficiency. However, their relatively high shoot biomass could not be explained solely by differences in leaf Fe concentration among subspecies. Preferential assimilate allocation to shoots may therefore contribute to the higher leaf Fe-use efficiency of Indica rice.