Effect of Root Cooling on Photosynthesis of Artemisia tridentata Seedlings under Different Light Levels

Root chilling has been shown to inhibit shoot photosynthesis yet the mechanism for such an action is not clearly understood. A study was designed to elucidate the mechanism by which root cooling may affect net photosynthesis. Roots of Artemisia tridentata seedlings were cooled from 20°C to 5°C while their shoot temperature remained at 20°C. This was conducted at two light levels (700 and 1300 μmol m −2 s −1 ). The time course of shoot net photosynthesis (A), stomatal conductance to water vapor (g s ), intercellular CO 2 concentration (C i ) and root respiration (R s ) were determined on a whole‐plant basis. Root cooling caused a 25% reduction in A at high PPFD, which was preceded by more than 50% reduction of g s and about 10% reduction in C i . A versus C i curves for single branches showed no difference between cold and warm soil temperatures, although stomatal conductance was lower for the lower soil temperature. This suggests that a stomatal limitation may have been involved in the inhibition of A. Furthermore, a concomitant decrease of as much as 23% in leaf relative water content (RWC) indicated that root cooling affected stomatal closure due to decreased water supply to the foliage. At lower PPFD, root cooling did not cause a decrease in A of the whole plant despite a moderate drop in g s , C i and RWC. Cold soil also led to a substantial and rapid reduction in root respiration rate (R s ) regardless of the light level.