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Photoperiodic competency for dormancy induction in Populus balsamifera Zhang, Li


Bud dormancy is an important overwintering mechanism for woody perennials and is induced in most species during late summer by short days (SD) and/or low temperature. Adaptively, however, it is important for new growth not to respond to similar photoperiods in early spring. To investigate this matter, two growth chamber experiments were conducted on four genotypes of balsam poplar (Populus balsamifera L.) originating from two latitudes. Rooted cuttings were moved into SD conditions at weekly intervals after flushing at either 15°C or 20°C. Plant heights were measured every other day. Plants were harvested weekly and total RNA was extracted for quantitative reverse transcription polymerase chain reaction (qRT-PCR) on genes known to be involved in dormancy induction, and for transcriptome sequencing followed by qRT-PCR validation to identify additional genes marking photoperiodic competency. Height growth cessation (HGC) data showed that before a certain age, no matter how soon plants were transferred to SD conditions, they continued to grow until they became competent to respond to photoperiod. The different genotypes became competent at different times (18-40 days since flush), indicating possible genetic variation in this trait. Once competency was attained, it took plants 7-20 days under 20°C to cease height growth under SD, depending on genotype, experiment and time since competency acquisition. Leaf number data revealed that competency acquisition and the transition from preformed leaf emergence to neoformed leaf production, are two independent processes. Temperature did not appear to influence the development of competency, though it increased the speed of height growth cessation. RT-qPCR results revealed three promising gene markers for competency: Potri.017G051100 (G6), Potri.001G222000 (G7) and CONSTANS 2. Increased expression for G6 and G7 was observed post-competency relative to pre-competency, and changes in expression varied between leaves and stem tissue. In contrast, CONSTANS 2 mRNA peaked 32 days after bud flush, coinciding with competency acquisition. This research offers new insight into the molecular mechanisms that underlie the acquisition of photoperiodic competency, which, in a warming climate, may cause phenological mismatch in deciduous boreal tree species, causing premature HGC and loss of productivity.

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