Diurnal and annual rhythms in trees

Trees, perennial phanerophytes, display a rich variety of rhythmic phenomena. These are either due to exclusive environmental entrainment or due to the functioning of endogenous oscillators independent of the environment. Both types of rhythms are covered in this review. Purely environment controlled rhythms may be considered as a prelude to endogenous rhythms. Environment controlled rhythms discussed are (i) the diurnal rhythms of nyctinastic and heliotropic leaf movements and oscillatory phenomena of photosynthesis, such as the midday depression and Crassulacean acid metabolism (CAM), and (ii) the annual rhythms of annual growth ring formation, autumnal leaf senescence, over wintering mechanisms and flowering. Among the diurnal rhythms, nyctinastic movements and CAM are also free-running endogenous rhythms showing the operation of circadian clocks in trees. In leaf senescence, over wintering, and flowering control, photoperiod sensing is involved which suggests the participation of endogenous clocks. A question asked is if diurnal and annual rhythms are mechanistically correlated. Evidently, phenological phenomena based on photoperiodism (as dependent on measurement of night length) are co-ordinately regulated by the phytochrome system and the circadian clocks and many aspects of annual developments and over wintering are linked to photoperiodism. The existence in trees of circadian clock genes as known to be anchored in the genome of A. thaliana can be assessed by attempts of alignment with the sequenced genome of Populus or by isolating cDNA clones from trees to check them against the genome of A. thaliana. At extreme latitudes near the equator and north of the polar circle trees also display photoperiod-independent phenological phenomena. In the polar region, total irradiance of red and far red light could possibly be involved and the signalling pathway then involves phytochrome, and thus, may still be similar to that of photoperiodism. At the equator, total daily light irradiance received or sensing the dynamics of daily changes in solar irradiance are essential and it remains enigmatic whether signalling cascades are either attached to the circadian clocks in a still unknown way or totally independent of circadian clocks.