Temperature Effects on the Activation and Inactivation of Pyruvate, P_i Dikinase in Two Populations of the C₄ Weed Echinochloa crus-galli (Barnyard Grass) From Sites of Contrasting Climates

Abstract

Five-week-old plants of Echinochloa crus-galli (L.) Beauv. (barnyard grass) from Mississippi (MISS) and from Québec (QUE) grown under controlled conditions were subjected to cold temperature acclimatory treatments for periods of up to 3 days. After plants were transferred from a 26/20ºC day/night regime to 14/8ºC for 2-3 days the rate of activation of the C4 photosynthetic enzyme pyruvate, Pi dikinase (PPDK; EC 2.7.9.1), following illumination of plants at 26ºC, was substantially reduced in both ecotypes. This effect was far more pronounced in MISS plants and the PPDK activity of these plants remained at less than half that of control plants (26/20ºC) even after 2 h illumination. After 3 days at 14/8ºC the half-time for PPDK activation was more than 120 min in MISS plants and 35 min in QUE plants compared with about 3 min for plants remaining in the 26/20ºC regime. Lesser but qualitatively similar effects were observed when plants were exposed to only cooler nights (26/8ºC) or cooler days (14/20ºC). When plants at the 14/8ºC regime were transferred back to 26/20ºC for 2 days there was a substantial recovery of the capacity for rapid PPDK activation but recovery was slower in MISS plants. Predictably, the rate of activation of PPDK was reduced when activation in control plants (26/20ºC) was carried out at 8ºC instead of 26ºC. However, a significant finding was that the rate of activation at 8ºC was more strongly affected in MISS plants, with a reduction to about 15% of the rate of control plants and a final steady PPDK level after 60 min of only half that in control plants. PPDK extracted from MISS plants underwent more rapid in vitro cold inactivation (2ºC) than the enzyme from QUE plants and only 80% of the initial activity was recovered upon rewarming at 25ºC. The physiological significance of these results are discussed in relation to the cold sensitivity of C4 photosynthesis and the previously reported differences in the growth and physiological performance of MISS and QUE plants at lower temperatures.