H+Efflux and Trans-Root Potential Measured while Increasing the Temperature of Solutions Bathing Excised Roots ofZea mays

Abstract

Kennedy, C. D. and Gonsalves, F. A. N. 1988. H+ efflux and trans-root potential measured while increasing the temperature of solutions bathing excised roots of Zea mays.—J. exp. Bot. 39: 37-49. Novel temperature-ramp procedures have been used to measure H + efflux and trans-root potential of excised roots of Zea mays (var. Fronica). Two types of experiment were performed: (1), increasing temperature from 17°C, and (2), pre-cooling the roots to 1 °C before starting the temperature ramp. The rates of increase of temperature for H+ efflux and trans-root potential experiments were 0-5 and 2-1 °C min-1 respectively. The H + scans revealed strong sharp maxima at 30 °C and 32 °C, for non-pre-cooled and pre-cooled roots respectively, the latter being significantly smaller. The trans-root potential scans for the pre-cooled roots showed a corresponding maximum at 30 °C, which was inhibited by KCN (10 mmol dm-3) with or without SHAM (10 mmol dm-3), or Hg2+ (1, 10, 100 /?mol dm-3) in the bathing solutions. Some of the evidence suggests that these maxima are associated with electrogenic H + pumping, mediated by a plasma membrane-bound ATPase. However, no corresponding maximum was observed in the trans-root potential scans for non-pre-cooled roots, the potential remaining at about — 75 mV from 20 °C to 35 °C. As there is a 7-fold increase in H + efflux between 20 °C and 30 °C, the relationship between net H + efflux and electrogenic proton pumping in these roots is by no means clear. Some possibilities are considered here. Pre-cooled and non-pre-cooled roots show clear maxima in the trans-root potential scans at about 46 °C, at which temperature there is a slight net H+ influx. This, and other less prominent features observed, are briefly discussed.