Good neighbours: current-year needles in Nordmann fir rely on their 1-year-old neighbouring needles for adequate nutrient supply.

The annual transport patterns of mineral nutrients in conifers remain poorly understood. Calcium (Ca) and magnesium (Mg) deficiencies often occur in planted conifer stands and cannot be easily remedied by simply adding these nutrients to soil. This study examined translocation and remobilization of Ca and Mg in young Nordmann fir (Abies nordmanniana) trees. Our data revealed that Ca and Mg absorbed by the roots do not reach the needles during the current growth season, indicating that internal nutrient remobilization is crucial for the growth of new needles, particularly shortly after the budbreak. Monitoring 1-year-old needles over a full annual cycle revealed steep declines in Ca and Mg around budbreak, coinciding with their increase in the emerging needles on the same branch. This strongly suggests that older needles supply Ca and Mg to new needles. Trees with low Mg concentration in 1-year-old needles in the spring developed visual Mg deficiency symptoms during the summer. For Ca, which is immobile in the phloem, we recorded a substantial increase in the apoplast of older needles just before budbreak. Simultaneously, the immunolocalization of cell wall homogalacturonan in these needles declined, suggesting pectin modification that facilitates Ca release from cell walls, enabling its translocation to the neighbouring, current-year needles. Our findings reveal a previously unknown process that advances understanding of mineral nutrient remobilization in conifer trees.