Ecotypic and genetic variation in poplar bark storage protein gene expression and accumulation

Bark storage proteins (BSP) store nitrogen (N) translocated from senescing leaves in autumn, and supply reduced N for spring growth. Expression of bsp and BSP accumulation are associated with short day photoperiod. To determi ne if photoperiod-associated bsp expression varies among poplars native to different latitudes, Populus deltoides Bartr. clones originating from six l atitudes were grown under natural conditions at a common location. Relative amounts of BSP mRNA in these clones were measured at 2-week intervals from August 7 to October 16. The date of maximum BSP mRNA accumulation was corr elated with latitude of origin, and maximum accumulation of BSP mRNA occurr ed earlier in clones native to northern latitudes than in clones native to southern latitudes. This pattern of variation is consistent with photoperio dic responses of plants native to temperate climates. Genotypic variations in BSP accumulation, bark protein concentration and bark N concentration we re compared among clones of six hybrid poplar (Populus trichocarpa Torr. an d Gray x P. deltoides) full-sib families (three F-2 families, two F-1 famil ies and one BC1 family) after 6 weeks in a short day photoperiod and at mid winter. Significant differences in BSP accumulation occurred among clones w ithin four of the six full-sib families after 6 weeks in a short day photop eriod and also at midwinter for outdoor-grown plants. Bark protein and bark N concentrations also varied significantly among clones within certain fam ilies. In general, the greatest variation was found in F-2 and BC1 families . Within several families, relative BSP amounts were positively correlated with bark protein concentration and total bark N concentration. These resul ts indicate a role of photoperiod in regulating bsp expression and demonstr ate a genetic component underlying seasonal BSP accumulation. The results c ould have significance in selecting for clones with improved N storage capa city and N-use efficiency.