This study investigates the effect of short- and long-term changesin temperature on the regulation of root respiratory O2 uptakeby substrate supply, adenylate restriction and/or the capacityof the respiratory system. The species investigated were the lowland Plantagolanceolata L. and alpine Plantago euryphylla Briggs, Carolin& Pulley, which are inherently fast- and slow-growing, respectively. Theplants were grown hydroponically in a controlled environment (constant23 °C). The effect of long-term exposure to lowtemperature on regulation of respiration was also assessed in P.lanceolata using plants transferred to 15/10 °C(day/night) for 7 d. Exogenous glucose and uncoupler (CCCP)were used to assess the extent to which respiration rates were limitedby substrate supply and adenylates. The results suggest that adenylatesand/or substrate supply exert the greatest control overrespiration at moderate temperatures (e.g. 15–30 °C)in both species. At low temperatures (5–15 °C),CCCP and glucose had little effect on respiration, suggesting thatrespiration was limited by enzyme capacity alone. The Q10 (proportionalincrease of respiration per 10 °C) of respirationwas increased following the addition of CCCP and/or exogenousglucose. The degree of stimulation by CCCP was considerably lowerin P. euryphylla than P. lanceolata. This suggeststhat respiration rates operate much closer to the maximum capacity in P.euryphylla than P. lanceolata. When P. lanceolata wastransferred to 15 °C for 7 d, respirationacclimated to the lower growth temperature (as demonstrated by an increasein respiration rates measured at 25 °C). In addition,the Q10 was higher, and the stimulatory effectof exogenous glucose and CCCP lower, in the cold-acclimated rootsin comparison with their warm-grown counterparts. Acclimation of P.lanceolata to different day/night-time temperatureregimes was also investigated. The low night-time temperature wasfound to be the most important factor influencing acclimation. The Q10 valueswere also higher in plants exposed to the lowest night-time temperature.The results demonstrate that short- and long-term changes in temperaturealter the importance of substrate supply, adenylates and capacityof respiratory enzymes in regulating respiratory flux.