Uptake kinetics of arsenic species in rice plants

Abstract

Arsenic (As) finds its way into soils used for rice (Oryza sativa) cultivation through polluted irrigation water, and through historic contamination with As-based pesticides. As is known to be present as a number of chemical species in such soils, so we wished to investigate how these species were accumulated by rice. As species found in soil solution from a greenhouse experiment where rice was irrigated with arsenate contaminated water were arsenite, arsenate, dimethylarsinic acid, and monomethylarsonic acid. The short-term uptake kinetics for these four As species were determined in 7-d-old excised rice roots. High-affinity uptake (0-0.0532 mM) for arsenite and arsenate with eight rice varieties, covering two growing seasons, rice var. Boro (dry season) and rice var. Aman (wet season), showed that uptake of both arsenite and arsenate by Boro varieties was less than that of Aman varieties. Arsenite uptake was active, and was taken up at approximately the same rate as arsenate. Greater uptake of arsenite, compared with arsenate, was found at higher substrate concentration (low-affinity uptake system). Competitive inhibition of uptake with phosphate showed that arsenite and arsenate were taken up by different uptake systems because arsenate uptake was strongly suppressed in the presence of phosphate, whereas arsenite transport was not affected by phosphate. At a slow rate, there was a hyperbolic uptake of monomethylarsonic acid, and limited uptake of dimethylarsinic acid.

Figure 1

As species present in soil solution from a greenhouse experiment when rice was irrigated with arsenate solutions. ▩, Arsenite; ▪, arsenate; ▥, MMAA; □, DMAA. Error bars represent ±se of three replicates.

Figure 2

Concentration-dependent kinetics for high-affinity root arsenite and arsenate influx for eight rice varieties (four from Boro season ○, dashed line; and four from Aman season ●, solid line). Each point is the average of four varieties (each variety is the average of three replicates) and error bars are ±se of the mean of four varieties

Figure 3

Concentration dependent kinetics for high- and low-affinity root arsenite (●, solid line) and arsenate (○, dashed line) influx of Aman rice var. BR11. Each point is an average of three replicates. Error bars are ±se of the replicates. Insert depict the kinetics of arsenite and arsenate at lower substrate concentration (0–0.08 mm).

Figure 4

Uptake of 0.05 mm arsenite (●, solid line) and 0.05 mm arsenate (○, dashed line) by an Aman rice var. BR11 at different concentrations of phosphate (0–0.5 mm). Error bars are ±se of three replicates.

Figure 5

Concentration-dependent kinetics for high-affinity root arsenite (▾, solid line) arsenate (▿, long dashed line), DMAA (○, short dashed line), and MMAA (●, dotted line) influx for an Aman rice var. BR11. Each point is an average of three replicates. Error bars are the ±se of the replicates. Insert, Uptake of MMAA and DMAA.