The circadian clock is an endogenous timing mechanism that allows organisms to coordinate physical and behavioural processes with the natural light dark cycle. Synchronisation between the circadian clock and environment in plants provides a fitness advantage (Dodd et al. 2005) and in wheat several circadian clock genes are candidates for flowering time QTLs. In Arabidopsis a number of photosynthetic processes are under circadian control (Dodd, Kusakina, et al. 2014) and the major product of photosynthesis, sucrose, feeds back metabolic timing information to the circadian clock (Haydon, Mielczarek, et al. 2013).

Quantification of circadian rhythms requires non-invasive measurement tools which are lacking in non-model organisms such as wheat. I therefore optimised a chlorophyll a fluorescence imaging system for the measurement of circadian rhythms in wheat leaves (Chapter 3). Using a suite of Arabidopsis mutant lines, I identified chloroplast movement as the most significant driver of chlorophyll a fluorescence oscillations. I then demonstrated that circadian oscillations in leaf temperature could be measured in continuous light (Chapter 3). This led to the design of a low-cost leaf temperature measuring device which uses thermocouples directly attached to the surface of leaves to measure temperature.

To determine the effect of the circadian clock on photosynthesis two tetraploid TILLING lines containing double null mutations of elf3 (Chapter 4) and gi (Chapter 5) were screened using a combination of chlorophyll a fluorescence, leaf temperature and time-course RT-qPCR. In both cases mutation of circadian clock genes caused a loss of rhythmic oscillations of circadian outputs in continuous conditions. Under diel conditions disruption of the circadian oscillator had little effect on photosynthesis but there were small effects on yield component traits. For the first time in wheat the loss of functional GI was shown to cause a delay in flowering time under long days.

To date the effect of sugars on the wheat circadian oscillator have not been reported. Using a combination of RT-qPCR and chlorophyll a fluorescence I showed that the wheat orthologues of AtPRR7 and AtCCA1 respond differently to changes in sucrose levels compared to Arabidopsis (Chapter 6). I showed that sucrose is required for the maintenance of circadian oscillations in prolonged darkness. I also demonstrated the limitations of using chlorophyll a fluorescence for circadian phenotyping in darkness.