Regulation of root respiration in two species of Plantago that differ in relative growth rate: The effect of short- and long-term changes in temperature

This study investigates the effect of short- and long-term changes in temperature on the regulation of root respiratory O₂ uptake by substrate supply, adenylate restriction and/or the capacity of the respiratory system. The species investigated were the lowland Plantago lanceolata L. and alpine Plantago euryphylla Briggs, Carolin & Pulley, which are inherently fast- and slow-growing, respectively. The plants were grown hydroponically in a controlled environment (constant 23 °C). The effect of long-term exposure to low temperature 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 limited by substrate supply and adenylates. The results suggest that adenylates and/or substrate supply exert the greatest control over respiration 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 that respiration was limited by enzyme capacity alone. The Q₁₀ (proportional increase of respiration per 10 °C) of respiration was increased following the addition of CCCP and/or exogenous glucose. The degree of stimulation by CCCP was considerably lower in P. euryphylla than P. lanceolata. This suggests that respiration rates operate much closer to the maximum capacity in P. euryphylla than P. lanceolata. When P. lanceolata was transferred to 15 °C for 7 d, respiration acclimated to the lower growth temperature (as demonstrated by an increase in respiration rates measured at 25 °C). In addition, the Q₁₀ was higher, and the stimulatory effect of exogenous glucose and CCCP lower, in the cold-acclimated roots in comparison with their warm-grown counterparts. Acclimation of P. lanceolata to different day/night-time temperature regimes was also investigated. The low night-time temperature was found to be the most important factor influencing acclimation. The Q₁₀ values were also higher in plants exposed to the lowest night-time temperature. The results demonstrate that short- and long-term changes in temperature alter the importance of substrate supply, adenylates and capacity of respiratory enzymes in regulating respiratory flux.