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 bu d break. We used 1-year-old 'Fuji' Malus domestica Borkh on M26 bare-root a pple trees to determine whether the onset of N uptake in spring is dependen t solely on the growth stage of the plant or is a function of soil temperat ure. Five times during early season growth, N uptake and total amino acid c oncentration were measured in trees growing at aboveground day/night temper atures of 23/15 degreesC and belowground temperatures of 8, 12, 16 or 20 de greesC. We used (NH4NO3)-N-15-N-15 to measure total N uptake and rate of up take and found that both were significantly influenced by both soil tempera ture and plant growth stage. Rate of uptake of N-15, increased creased with increasing soil temperature and changed with plant growth stage, Before bu d break, N-15 was not detected in trees growing in the 8 degreesC soil trea tment, whereas N-15 uptake increased with increasing soil temperatures betw een 12 and 20 degreesC. Ten days after bud break, N-15 was still not detect ed in trees growing in the 8 degreesC soil treatment, although total N-15 u ptake and uptake rate continued to increase with increasing soil temperatur es between 12 and 20 degreesC, Twenty-one days after bud break, trees in al l temperature treatments were able to acquire N-15 from the soil, although the amount of uptake increased with increasing soil temperature. Distributi on of N-15 in trees changed as plants grew. Most of the N-15 absorbed by tr ees before bud break (similar to5% of N-15 supplied per tree) remained in t he 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 abo ut two-thirds of the N-15 taken up by the roots. Total amino acid concentra tion and distribution of amino acids in trees changed with plant growth sta ge, but only the amino acid concentration in new growth and roots was affec ted by soil temperature. We conclude that a combination of low soil tempera ture 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 appl ication in the spring when soil temperatures are low or when the abovegroun d portion of the tree is not actively growing may be ineffective in promoti ng N uptake.