Regulation of flowering and canopy structure in timothy (Phleum pratense L.)

Timothy (Phleum pratense L.) is one of the most important forage grass species grown at high latitudes, mainly because of its good winter tolerance and relatively good feeding value. Its harvested biomass is mainly used as silage. Its sward canopy structure, as well as the development of individual tillers, determines the quantity and quality of the yield. Nonetheless, processes behind the transition to flowering, as well as the formation of stems and the connection with forage quality have not been studied in detail for timothy.
Seven experiments were conducted to explore the effect of vernalization, photoperiod and gibberellin (GA3) treatments on stem elongation, flowering and canopy structure in different cultivars or accessions of timothy. The vernalization response (accelerated flowering) or requirement of the accessions was also reported. In addition, the expression of key regulator genes, VRN1 and VRN3, as well as the flowering repressors VRN2 and MADS10 were studied, and the connection between these and flowering induction and stem formation was revealed.
Results showed that photoperiod is the most important regulator of flowering in timothy and that short photoperiod is a strong flowering repressor. In addition, vernalization response was reported in most of the tested accessions, which was seen as faster flowering. Tested accessions were grouped, based on their vernalization response, into groups having obligate vernalization requirement, intermediate response or no response. However, if the photoperiod was long enough, even northern accessions were able to flower without vernalization. Thus, a long-enough period of vernalization or photoperiod removed the differences between accessions in their responses. It was found that the application of GA3 could not replace the LD requirement for flowering. It was hypothesized that GA3-dependent response to flowering in timothy is regulated by photoperiod and genotype.
Results showed that the requirement for flowering and stem elongation vary, and stem elongation could take place without flowering induction. If vernalization and/ or photoperiod were long enough there was a shift from elongating (ELONG) to generative (GEN) tillers. Vernalization released the height growth of tillers, instead of flowering, especially in northern accessions, whereas in southern accessions vernalization was not required either for stem elongation or flowering. More ELONG and GEN tillers were produced after vernalization conditions, whereas at SD conditions more vegetative (VEG) tillers were produced. Moreover, in southern accessions the development of GEN lateral tillers was more synchronized than in northern accessions.
Final leaf number (FLN) has been used in cereals to describe the vernalization saturation, i.e. the time required at vernalization conditions for flowering initiation. In timothy FLN did not function as an indicator of vernalization saturation, because in most of the studied accessions FLN remained unchanged. It is suggested that this is the cause of different growth strategies between forage grasses and cereals. Flowering is also associated with decreased digestibility of grass stems. Our results showed, however, that flowering induction was not required for the development of the lignified sclerenchyma ring in developing stems, but rather lignin accumulation was as a result of stem elongation and requirements for mechanical support.


At molecular level, novel vernalization-related partial cDNAs were identified through sequencing. Both PpVRN1 and PpVRN3 homologs induced the transition to reproductive development, but PpVRN3 was reported to be required for successful flowering in timothy. These results are in agreement with results obtained for other monocots and they support the theory of universal flowering-promoting system between species. The expression of the putative repressor homolog, PpMADS10, was connected to the developmental stage of the apex, so that higher expression and vegetative stage were observed simultaneously. The genetic differences were also seen at molecular level, probably indicating different photoperiod requirements and supporting the result of different requirements for flowering between accessions.
Results obtained in work reported in this thesis shed new light on the regulation of flowering and canopy structure in timothy. It is concluded that large variation exists among timothy accessions in their responses to vernalization and photoperiod. This information can be utilized in breeding for high-yielding new cultivars for different growing conditions at high latitudes and for different harvesting strategies.