- In poplar (Populus spp.) a 32-kilodalton bark storage protein (BSP) accumulates
during the fall and winter, and is thought to be a major storage form of overwintering
nitrogen. BSP is induced by short-day (SD) photoperiod, and under natural conditions
BSP accumulation coincides with the induction of dormancy. Because BSP accumulates
to high levels during the fall and winter, this protein may play an important role in
internal nitrogen cycling. In these studies, the relation among plant growth, nitrogen
fertilization and BSP accumulation was examined.
Poplar clones native to different latitudes were grown under natural and controlled
environmental conditions to investigate the relationship between dormancy induction and
BSP accumulation. SDS-PAGE, protein gel blot, and RNA gel blot analysis showed a
seasonal variation in the amount of BSP and BSP mRNA present in bark and buds. The
highest levels of BSP were found during the winter and coincided with bud dormancy.
Differences in the time of BSP accumulation were found among clones. A decline in the
levels of BSP in bark and buds was observed at budbreak. SD photoperiod induced BSP
accumulation in bark and buds of all poplar clones. After 4 weeks in SD BSP levels
increased in the bark tissue, and at the same time plants began to develop dormancy.
Dormancy induction and BSP accumulation occurred at the same time, which suggests
that the environmental factors that trigger these phenomena are probably the same.
The effect of nitrogen fertilization on plant growth, protein content, NH₄⁺ and
NO₃ and BSP accumulation was studied in poplar plants grown under controlled
environmental conditions. Plants were grown under either long-day (LD) or SD
photoperiods, and different levels of NH₄NO₃. High nitrogen (50 mM NH₄NO₃)
fertilization increased the levels of nitrate, ammoniun and total soluble proteins in bark
and leaves of plants grown under both LD and SD photoperiods. Plant growth rate was
negatively affected by the high nitrogen application. When plants were treated with low
and normal levels of fertilization the highest concentration of NH₄ and NO₃ were found
in leaves. Under high nitrogen (50 mM NH₄NO₃), the highest concentration of NH₄ and
NO₃ were found in the bark, and in mature leaves, respectively. During SD exposure,
the levels of NH₄ and NO₃ decline in bark. BSP accumulation was also enhanced by high
nitrogen level, under both photoperiods. However, under SD conditions a higher level
of the protein was detected in the bark of all nitrogen treatments, as compared with the
levels in LD grown plants.
As a result of these experiments, it was concluded that the allocation of the
different nitrogen metabolites, is affected by the nitrogen status of the plants, and also
by photoperiod. BSP accumulation in response to high levels of nitrogen fertilization,
under both favorable (LD) and unfavorable (SD) growing conditions suggest that this
protein may act as a short term, as well as long term nitrogen storage compound,
depending on the growing conditions and the nitrogen status of the plant.