New root growth in relation to nitrogen reserves of young Gala/M26 apple trees

One-year-old Gala/M26 apple trees were grown in containers in a lath house during 1998. At the end of the season, the trees were treated by manual defoliation (November 3); 3% urea foliar spray twice (October 16 and 24) followed by manual defoliation (November 3); the urea sprays without manual defoliation; manual defoliation (November 3) followed by 3% urea spray to the bare stem twice (November 3 and 10); or natural defoliation with no urea sprays (control). These treatments were designed to create a range of different levels of internal reserve nitrogen (N). Trees were harvested on December 15, 1998. Five plants from each treatment were destructively sampled for analyses of reserve N and carbohydrates, and the remainder was stored in a cold room at 0-2C for regrowth studies. On April 13, 1999, the plants were taken out of the cold room and potted into N-free medium. Each plant received 300 mL Hoagland solution without N at 5-day intervals during the experiment. Five plants from each treatment were harvested on May 14, June 3, June 20 or July 7, 1999. The number of new feeder and extension roots was counted and the biomass of new shoot growth (both leaves and new stems) was recorded. The relationship between new root growth and N reserve level was analyzed. New root and shoot growth increased with increasing N reserve levels. Total accumulative number of new feeder and extension roots was positively related to N reserve levels. A closer correlation was found between reserve N and extension root growth than between feeder root growth and N reserves, suggesting that the development of extension roots was more dependent on N level than was development of feeder roots.