TEMPERATURE EFFECTS ON THE ACTIVATION AND INACTIVATION OF PYRUVATE, P-I DIKINASE IN 2 POPULATIONS OF THE C-4 WEED ECHINOCHLOA-CRUS-GALLI (BARNYARD GRASS) FROM SITES OF CONTRASTING CLIMATES

Five-week-old plants of Echinochloa crus-galli (L.) Beauv. (barnyard grass) from Mississippi (MISS) and from Quebec (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 26/20 degrees C day/night regime to 14/8 degrees C for 2-3 days the rate of activation of the C-4 photosynthetic enzyme pyruvate, P-i dikinase (PPDK; EC 2.7.9.1), following illumination of plants at 26 degrees C, 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 (26/20 degrees C) even after 2 h illumination. After 3 days at 14/8 degrees C 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 26/20 degrees C regime. Lesser but qualitatively similar effects were observed when plants were exposed to only cooler nights (26/8 degrees C) or cooler days (14/20 degrees C). When plants at the 14/8 degrees C regime were transferred back to 26/20 degrees C 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 (26/20 degrees C) was carried out at 8 degrees C instead of 26 degrees C. However, a significant finding was that the rate of activation at 8 degrees C 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 (2 degrees C) than the enzyme from QUE plants and only 80% of the initial activity was recovered upon rewarming at 25 degrees C. The physiological significance of these results are discussed in relation to the cold sensitivity of C-4 photosynthesis and the previously reported differences in the growth and physiological performance of MISS and QUE plants at lower temperatures.