Nitrate Transport into Chara corallina Cells using36ClO−3as an Analogue for Nitrate

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Abstract
The use of chlorate as an analogue for NO−3 during nitrate uptake into Chara corallina cells has been investigated. NO−3 inhibits 36C1O−3 influx into Chara over the concentration range 0–1000 mmol m−3. Lineweaver-Burke plots of the data are characteristic of competitive inhibition by NO−3 in the low concentration range (0–300 mmol m−3 ClO−3) and apparent KINO3 is 140 mmol m−3 which is of a similar order of magnitude as apparent KmCIO3- 180 mmol m−3. At higher substrate concentrations the inhibition by NO−3 was not characteristic of competitive or uncompetitive inhibition. 36C1O−3/NO−3 influx was dependent on K+ and Ca2+ in the external medium and inhibited by FCCP. NO−3 pretreatment or N starvation increased subsequent 36C1O−3/NO−3 influx into Chara. A comparison between rates of net NO−3 uptake and 36C1O−3/NO−3 influx supported the previous hypothesis that NO−3 efflux is an important component in the determination of overall uptake rates.