Response of Winter Wheat Seedlings Respiration to Long-Term Cold Exposure and Short-Term Daily Temperature Drops

The effects of a long-term (6-day) exposure to low positive temperature (4 degrees C, LT treatment) and short-term (3 h per a day for 6 days) daily temperature drops to 4 degrees C (DROP treatment) on leaf respiration in winter wheat (Triticum aestivum L., var. Moskovskaya 39) were studied under controlled conditions. In the case of the LT treatment, the respiration rate in the darkness (R-d) increased by 36%, while the DROP treatment only resulted in a tendency to such an increase. The influence of both DROP and LT treatments on the respiration rate in the light (R-l) and temperature sensitivity of R-d and R-l was insignificant. In contrast to the DROP treatment, the LT treatment increased the level of light-induced inhibition of plant respiration. In both LT- and DROP-treated plants, the rate of gross photosynthesis (A(g)) measured at 12 degrees C increased by 23 and 54%, respectively, as compared to the control plants. The DROP treatment did not affect the R-d/A(g) and R-l/A(g) ratios in plant leaves. The R-d/A(g) value in LT-treated plants measured at 22 degrees C was higher than in control plants, while the R-l/A(g) value measured at 12 degrees C was lower than in the control. The DROP treatment did not affect the sensitivity of respiration to salicylhydroxamic acid (SHAM), an inhibitor of the alternative respiratory pathway. The LT treatment increased the share of a SHAM-sensitive respiration in the total respiration of plant leaves by 25% on average, which indirectly indicates the activation of an alternative pathway of the electron transfer in the respiratory chain. Thus, a long-term continuous exposure of winter wheat seedlings to low positive temperatures provided a modifying effect on respiration manifested via changes in the respiratory rate, activity of the SHAM-sensitive respiration, and the ratio between respiration and gross photosynthesis. In the case of a short-term daily exposure to low temperature high respiration stability was observed.