Soil temperature and plant growth stage influence nitrogen uptake and amino acid concentration of apple during early spring growth

In spring, nitrogen (N) uptake by apple roots begins about 3 weeks after bud break. We used 1-year-old 'Fuji' Malus domestica Borkh on M26 bare-root apple trees to determine whether the onset of N uptake in spring is dependent solely on the growth stage of the plant or is a function of soil temperature. Five times during early season growth, N uptake and total amino acid concentration were measured in trees growing at aboveground day/night temperatures of 23/15 degreesC and belowground temperatures of 8, 12, 16 or 20 degreesC. We used (NH4NO3)-N-15-N-15 to measure total N uptake and rate of uptake and found that both were significantly influenced by both soil temperature and plant growth stage. Rate of uptake of N-15, increased creased with increasing soil temperature and changed with plant growth stage, Before bud break, N-15 was not detected in trees growing in the 8 degreesC soil treatment, whereas N-15 uptake increased with increasing soil temperatures between 12 and 20 degreesC. Ten days after bud break, N-15 was still not detected in trees growing in the 8 degreesC soil treatment, although total N-15 uptake and uptake rate continued to increase with increasing soil temperatures between 12 and 20 degreesC, Twenty-one days after bud break, trees in all temperature treatments were able to acquire N-15 from the soil, although the amount of uptake increased with increasing soil temperature. Distribution of N-15 in trees changed as plants grew. Most of the N-15 absorbed by trees before bud break (similar to5% of N-15 supplied per tree) remained in the roots. Forty-six days after bud break, approximately one-third of the N-15 absorbed by the trees in the 12-20 degreesC soil temperature treatments remained in the roots, whereas the shank, stem and new growth contained about two-thirds of the N-15 taken up by the roots. Total amino acid concentration and distribution of amino acids in trees changed with plant growth stage, but only the amino acid concentration in new growth and roots was affected by soil temperature. We conclude that a combination of low soil temperature and plant developmental stage influences the ability of apple trees to take up and use N from the soil in the spring. Thus, early fertilizer application in the spring when soil temperatures are low or when the aboveground portion of the tree is not actively growing may be ineffective in promoting N uptake.