Iron deficiency decreases the Fe(III)-chelate reducing activity of leaf protoplasts

Abstract

The ferric-chelate reductase (FC-R) activity of mesophyll protoplasts isolated from Fe-sufficient (control) and Fe-deficient sugar beet (Beta vulgaris L.) leaves has been characterized. Measurements were made in an ionic environment similar to that in the apoplastic space of the sugar beet mesophyll cells. The FC-R activity of Fe-sufficient and Fe-deficient protoplasts was dependent on light. Fe deficiency decreased markedly the FC-R activity per protoplast surface unit. The optimal pH for the activity of the FC-R in mesophyll protoplasts was in the range 5.5 to 6.0, typical of the apoplastic space. Beyond pH 6.0, the activity of the FC-R in mesophyll protoplasts decreased markedly in both Fe-sufficient and Fe-deficient protoplasts. These data suggest that both the intrinsic decrease in FC-R activity per protoplast surface and a possible shift in the pH of the apoplastic space could lead to the accumulation of physiologically inactive Fe pools in chlorotic leaves.

Figure 1

A, Freshly isolated sugar beet protoplasts observed in a chamber-type slide without a cover. B, Sugar beet protoplasts from control, Fe-sufficient leaves. C, Sugar beet protoplasts from Fe-deficient leaves. D, Protoplasts from Fe-sufficient leaves after reduction of Fe(III)-EDTA to Fe(II) in the presence of PDTS observed with phase contrast in a chamber-type slide without a cover.

Figure 2

pH dependence of the FC-R measured in vivo in protoplasts isolated from Fe-sufficient and Fe-deficient sugar beet leaves. Measurements were made with 400 μm Fe(III)-EDTA as a substrate. Samples were pre-illuminated with red light (PPFD 90 μmol m⁻² s⁻¹). Data are means ± se of two experiments with three replications each.

Figure 3

Dependence on the Fe(III)-EDTA concentration and corresponding Eadie-Hofstee plots of the FC-R measured in vivo at pH 5.5 in protoplasts isolated from Fe-sufficient and Fe-deficient sugar beet leaves. Samples were pre-illuminated with red light (PPFD 90 μmol m⁻² s⁻¹). Data are the means ± se of two experiments with three replications each.