Shufu Dong

Urea uptake and nitrogen mobilization by apple leaves in relation to tree nitrogen status in autumn

Summary Bench-grafted Fuji/M.26 trees were fertigated with one of seven nitrogen concentrations (0, 2.5, 5, 7.5, 10, 15 or 20 mM), using a modified Hoagland’s solution from 30 June to 1 September. In mid-October, plants in each N treatment were randomly divided into three groups. One group was sampled destructively to determine background tree N status before foliar urea applications. The second was painted with a 3% 15N-enriched urea solution twice, at weekly intervals, on both sides of all leaves. The third group served as the control. All the fallen leaves from both the 15N-treated and the control trees were collected during leaf abscission. The trees were harvested at the end of natural leaf fall. Nitrogen fertigation during the growing season resulted in a wide range of tree N status, as indicated by leaf N (1.3–3.5.g m–2) in the autumn. The percentage of N partitioned into the foliage increased linearly with increasing leaf N content up to 2.2 g m–2, then reached a plateau of 50–55% with further rise in leaf N. 15N uptake and mobilization per unit leaf area and the percentage of 15N mobilized from the leaves decreased with increasing leaf N content. Of the 15N mobilized back to the tree, the percentage of 15N partitioned into the root system decreased with increasing tree N status. For the control trees, N mobilization per unit leaf area increased with increasing leaf N up to 3 g m–2, then levelled off with any further increase in N content. The percentage of N mobilization remained at approximately 60% until leaf N reached 3 g m–2, then declined with a further increase in leaf N content. Foliar 15N-urea applications reduced mobilization of endogenous leaf N regardless of tree N status. On a whole-tree basis, foliar 15N-urea applications increased the total amount of N across N fertigation treatments. We conclude that trees with low N status are more efficient in absorbing and mobilizing N from foliar urea than those with high N status. Likewise, more N derived from foliar urea is partitioned into the root systems of low N trees than of high N trees.

Foliar N application reduces soil NO 3 − -N leaching loss in apple orchards

A comparison of the effects of foliar and soil N application was made in field-grown mature fruiting Gala/M9 apple trees (Malus domestica Borkh) in 2001 and 2002 growing seasons under Pacific Northwest growing conditions in southern British Columbia, Canada. The trees, six years old at the start of the experiment, were treated: (1) with 5 g/l urea sprays supplied every two weeks (7 times) from mid May to mid August (total about 50 g N/tree/year), (2) with the same amount of N applied to the soil with the same timing and quantity as for the foliar treatment, and (3) with no N (control). Leaf color (as SPAD readings) and N concentrations (mg/g), and soil NH4+-N and NO3−-N were measured periodically throughout the two seasons. Leached NO3−-N was monitored monthly via an anion exchange probe from June to October in 2001 and from May to November in 2002. Shoot length was measured in October and N concentration of one-year-old wood and roots was determined in December of each growing s eason. Soil N application significantly increased shoot length relative to control or foliar N application. Leaf color, leaf N, and N concentration of one-year-old wood and roots were similarly increased relative to control by both soil and foliar N application. These treatments also increased fruit yield relative to control. There was no significant difference in yield and fruit quality between soil and foliar N applications. Soil N application increased soil NH4+-N and NO3−-N content in the root zone, and also increased the NO3− leaching loss below the root zone especially late in the growing season. Our results suggested that tree N status and yield and fruit quality could be maintained by multiple urea sprays during the growing season in apple orchards, and foliar N application will reduce the risk of soil NO3−-N leaching.

Root damage affects nitrogen uptake and growth of young Fuji/M.26 apple trees

Summary Effects of root damage during the transplant process on growth and nitrogen (N) uptake were studied with one-year-old bench-grafted Malus domestica Borkh ‘Fuji’ on M.26 rootstock apple nursery plants. Plants were potted after grafting and grown outside for one season. At the end of the season uniform trees were selected and randomly divided into four groups. One group of plants were moved into a 2°C cold room with soil and container intact (IR Treatment). Plants in other groups were removed from pots and stored as bareroot in the same cold room for three months. In the spring, bareroot plants were either: (1) transplanted with about 10% of the root system damaged during transplant (TP Treatment and Control-CK); or (2) root pruned by 25% (by volume) prior to transplant (RP treatment). Five trees from each treatment received 1 g of 15NH415NO3 at 12, 41 and 76 d after repotting. Control (CK) trees received no N. Trees were harvested 10 d after each N application, and plant growth and total N and 15N content of different tissues were determined. Root pruning reduced plant total biomass and root biomass at the first two harvests, but the plants from the RP treatment had highest total plant biomass and root biomass at the third harvest. There was no significant difference in the new stem and leaf growth among IR, RP and CK treatments at harvests but the TP treatment reduced new shoot biomass. Plants with intact roots (IR) had the higher total N content while control plants (CK) had the lowest. Root pruning reduced 15N uptake rate at the first two harvests but promoted it at the third harvest. Our results suggest that plant growth and nutrient uptake was suppressed by root pruning/damage during transplanting only in the early season, and the negative effects on growth and N uptake were offset later in the season by compensative root regeneration.

Method of Nitrogen Application in Summer Affects Plant Growth and Nitrogen Uptake in Autumn in Young Fuji/M.26 Apple Trees

Abstract Effects of foliar vs. soil nitrogen (N) application during the summer and the autumn were studied in young Fuji apple trees (Malus domestica Borkh) on Malling 26 (M.26) rootstock. One‐year‐old bench‐grafted trees were potted in 3.8‐liter pots in a mix of perlite, peat moss, and loam soil (1:1:1 by volume) and were treated weekly from late June to early September with 0.5% urea either by foliar or soil application or soil application of water (control). At the end of September, five trees from each treatment were harvested, and shoot and root growth and leaf N concentration were determined. In mid‐October of the same year, trees from each treatment were randomly divided into three subgroups. One group received soil application of water (control), and the other groups received either a foliar or soil application of 3% 15N‐urea. After natural leaf defoliation, trees were harvested, and total N and 15N concentration of stems (all aboveground tissue) and roots was determined. Regardless of application method, N application during the summer promoted plant growth and increased leaf N concentration. Soil N application during the summer stimulated more shoot and extension root growth than foliar N application. In contrast, the foliar N application in the summer promoted more feeder root growth than soil N application. Regardless of application method, autumn N application after terminal bud set had little effect on current‐year growth but increased total plant N concentration. In the autumn, trees that received soil N applications in the summer had more 15N uptake by leaves, while trees receiving foliar N applications in the summer had more 15N uptake by root. These results suggest that the differential influence of N application methods during the summer on growth and partitioning of trees affects tree responsiveness to autumn N applications. The use efficiency of autumn‐applied N depends on the method of N application in the summer.

Timing of urea application affects leaf and root N uptake in young Fuji/M.9 apple trees

Summary Leaf and root nitrogen (N) uptake was compared at different times of the season in young apple trees. One-year-old potted Fuji/M.9 trees were supplied with 1% 15N-labelled urea either by foliar or soil applications in May, July and September. Trees receiving only water served as controls. The trees were harvested 10 d after 15N application, separated into shoots (leaves and current-year stems), stem (previous-year wood) and roots. Biomass, total N and 15N contents of all tissues were determined. New shoot biomass and total tree biomass increased as the season advanced, while root biomass peaked in July. Leaf N uptake was higher than root uptake in May and September, while root N uptake was higher than leaf uptake in July. Leaf N uptake increased as the season advanced, while root N uptake was highest in July. The lowest 15N recovery (11%) was obtained in May with soil N application, and the highest 15N recovery (48%) was obtained in September with foliar N application. Our results suggest that foliar application of N early in the season, followed by soil N application in mid-season, then foliar application again late in the season is an efficient N management strategy for young trees.

N uptake, soil retention and loss of soil-applied 15NH415NO3 in young Fuji/M.26 apple trees with different N status

Summary Nitrogen (N) uptake, soil retention and loss of soil-applied N were studied in young apple trees with different N backgrounds. Bench-graft potted Fuji/M.26 (Malus domestica Borkh) trees were fertigated with 5, 10 or 20 mM N twice a week from June to August, and the trees were removed from soil and bare-root stored in a 2°C cold room in December of the first season. In April of the second season, the trees were washed and replanted in containers with a N-free medium (perlite:vermiculite=l:l v), and received 500 ml Hoagland’s nutrient solution without N weekly through the experiment. The trees received 15NH415NO3 at 1g per plant or no N on June 21. Four trees from each treatment were harvested at one, two and four weeks after 15N application, and 15N and total N in plant tissues and soil were analysed. N fertigation rates during the first growing season increased tree growth and N reserve levels, and N content in trees in the second year. New shoot and leaf growth in the following season was positively related to reserve N. 15N uptake increased during the four weeks after 15N application while soil 15N retention decreased. There was no significant difference in the total 15N uptake per tree. However, trees with the lowest N contents at the end of the first growing season had the highest rate of 15N uptake per unit root dry weight. Four weeks after application of 15N, tree uptake of N accounted for about 60% of applied 15N, while about 20% of the N still remained in the soil, and another 20% of the N was lost. Our results suggested that trees with lower N status are more efficient in N uptake from soil.