Variation in the use efficiency of leaf iron for growth among rice (<i>Oryza sativa</i> 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 (<i>Oryza sativa</i> 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 &gt; Japonica paddy rice &gt; 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.此前我们已有研究报道，即便是通常被视作耐缺铁的大麦，其不同栽培品种在铁（Fe）缺乏耐受性以及光合功能的铁利用效率方面仍存在显著差异。但目前尚不明确，这一现象是大麦所特有，还是同样适用于其他禾本科物种——例如易受缺铁胁迫影响的水稻（Oryza sativa L.）。为了表征水稻的叶片铁利用效率，我们采用可独立于策略II型（Strategy II-type）根系铁获取机制评估叶片功能的水培混合栽培体系，对粳稻（Japonica）与籼稻（Indica）两个亚种的缺铁胁迫响应展开了研究。在低铁胁迫条件下，各供试材料的地上部生长维持能力呈现如下排序：籼稻＞粳稻水田稻＞粳稻旱稻。对最新完全展开叶的铁浓度与光合速率的检测结果显示：若水稻叶绿体中存在缺铁适应机制，其大概率与大麦中所观测到的机制存在差异。籼稻的相对地上部干重（缺铁组/供铁充足组比值）高于其相对根部干重，而粳稻则无此特征，这表明在缺铁胁迫下，籼稻栽培品种倾向于将更多资源分配至地上部而非根部。然而，籼稻相对较高的地上部生物量无法仅通过亚种间叶片铁浓度的差异来解释，因此同化物向地上部的优先分配或许是其拥有更高叶片铁利用效率的重要原因。