Diurnal variation in thermal acclimation of leaf respiration in rice

Rashid FAA1, Asao S1, Taylor NL2, Fenske R2 and Atkin OK1

  1. ARC Centre of Excellence in Plant Energy Biology, Research School of Biology, Building 134, The Australian National University, Canberra, ACT 2601, Australia.
  2. ARC Centre of Excellence in Plant Energy Biology, School of Molecular Sciences, Bayliss Building, The University of Western Australia, Crawley, WA 6009, Australia.

Past studies using plants sampled at a single time-point during the day have shown that cold-acclimated leaves exhibit higher rates of leaf dark respiration (R) at a common temperature than their warm/hot grown counterparts. What is unclear, however, is whether these differences are held throughout the day. To address this, we examined the effect of growth temperature on diurnal variations in leaf R at a set temperature (30°C) and associated metabolite pools of rice (Oryza sativa, IR64). Plants were grown in three temperature controlled glasshouses (25, 30 and 35°C), with leaf R measured at 8:30 am, 1 pm, 5:30 pm, 9:30 pm and 4:30 am; metabolites were also quantified at the same time points. As expected, rates of leaf R measured at 30°C were significantly higher in the 25°C grown plants than those grown at 30°C and 35°C. However, the relative difference in leaf R between the growth temperatures changed through the day, being greatest at the end of the day-light period and least during the night hours. Underpinning this pattern were diurnal variations in leaf R of the 25°C & 30°C grown plants (with rates highest at 5:30 pm, and lowest at 4:30 am), whereas leaf R did not vary diurnally in the 35°C grown plants. Starch concentrations were lowest at the end of the night and highest at the day period – by contrast, soluble sugar concentrations remained constant through the day-night cycle in all three growth treatments. Thus, availability of substrate pools to glycolysis did not account for the divergent effects of time observed among the growth treatments. The effect of growth temperature and time of day on glycolytic and TCA cycle metabolites will be reported. Collectively, the results highlight the importance of considering time of day when assessing thermal acclimation of respiratory energy metabolism in rice.