Phosphorus Absorption and Accumulation in Apple

Phosphorus fertilization has an important impact on the yield and quality of upland apple production in northwest China. Past study has focused on this relationship, but little information is available on plant uptake, translocation and distribution of P in apple trees. This article outlines a comprehensive study on the dynamics of P uptake and distribution within Fuji (M. micromalus. Makino) apple trees.

A trial was arranged in Qishan county, on the southern portion of the Loess Plateau in Shaanxi, which is well suited to quality apple production and provides a good representation of typical upland apple production in northwestern China. Cultivated apple area in Shaanxi has reached 0.426 M ha producing 6.0 M t — 27% of China’s and 10% of the world’s apple production.

The orchard site was comprised of nine-year old Fuji apple trees with a row spacing of 2 m, 3 m between trees. The trial was carried out during 2004 and 2005. Samples were taken from three trees at similar stages of development on five dates. Sampling in 2004 took place on March 26 (sprouting and foliage growing period), April 30 (young fruit stage), July 30 (fruit expansion stage), and September 21 (fruit maturity), and on January 15, 2005 (tree dormancy). Samples of fruit, foliage, new tops, branches, trunks, and roots were collected each time. Root samples included all those within a 100 cm depth and a radius of 100 cm around the trunk. The cortex and xylem within the trunks and roots were divided and analyzed separately. Enzymatic activity was destroyed by placing plant parts in an oven set at 100 to 105°C for 15 min, and then samples were dried to a constant weight at 70 to 80°C. Samples were ground and digested with concentrated H₂SO₄-H₂O₂. The P content of the resulting solution was measured colorimetrically.

Results from the study showed a sharp increase in biomass from the early growth period in March to fruit maturity and harvest in September (Figure 1a). Tree growth after this period slowed significantly. Apple trees accumulated an average of total 37.1 kg P/ha within the 11-month study period, in which P destined for fruit and foliage amounted to 7.9 kg P/ha that was removed by harvest and defoliation. Very little P was removed by trees between March and late July (Figure 1b). This agrees with previous results (Tagliavini et al., 1998) as initial P demand resulting from new leaf and branch growth is apparently translocated from sources stored the previous season. Trees absorbed the majority of P after July 30. The days between July 30 and September 21, ending at fruit maturity and harvest, represented the peak period of demand for P.. Subsequent plant samples collected until next mid-January suggest continued P uptake and accumulation within the primary storage organs — albeit at a much lower rate (Figure 1b, Table 1).

(A)
(B)
Fig. 1 Annual changes of biomass (A) and P accumulation (B) in apple trees


From July to January, the amount of P accumulated in branches, trunks, and roots increased by 410%, 325% and 397%, respectively (Table 1). Phosphorus accumulation within these storage organs reached a maximum at dormancy in January. At this point, branches, trunks and roots contained 36%, 22% and 34% of the total plant P, respectively. This ranking of P accumulation within the primary storage organs agrees with results reported for N storage (Grassi et al., 2003; Frak et al., 2002).

Table 1. Distribution of P accumulation in organs of apple tree at different sampling times.

Organ	Sampling date, m/d
	3/26	4/30	7/30	9/21	1/15
Fruits, g/tree	—	0.12 ±0.02c	0.42 ±0.38bc	2.84 ±0.76abc
Leaves, g/tree	0.24 ±0.03c	2.02 ±1.04a	1.02 ±0.52abc	1.93 ±0.01bc
Shoots, g/tree	—	0.22 ±0.13bc	0.27 ±0.19c	0.82 ±0.17c	1.39 ±0.40c
Branch, g/tree	2.73 ±0.07a	1.27 ±0.77ab	1.24 ±0.43a	3.53 ±0.39ab	6.33 ±0.38a
Trunk, g/tree	1.19 ±0.19b	0.88 ±0.39bc	0.92 ±0.41abc	2.62 ±1.03abc	3.91 ±0.57b
Roots, g/tree	2.27 ±0.34a	1.19 ±0.50abc	1.19 ±0.52ab	4.33 ±1.50a	5.91 ±0.49a
Total Plant, g/tree	6.43 ±0.41	5.70 ±2.63	5.06 ±0.75	16.07 ±1.10	17.54 ±0.06
Orchard, kg/ha	10.70 ±0.7	9.50 ±4.4	8.40 ±1.2	26.80 ±1.8	29.20 ±0.1

± represents standard deviation
Numbers within sampling dates followed by the same letter are not different at P = 0.05.

The dynamics of annual P concentrations within collected cortex and xylem are provided in Figure 2a and 2b. Phosphorus within the cortex of branches, trunks and roots declined from March to July by 51%, 48% and 55%, respectively. Cortex P content began to increase in all three organs between the fruit expanding period in late July and dormancy in January. Tree branches had the highest cortex P during all growing stage followed by the trunks and roots.

Phosphorus within the xylem of branches, trunks and roots also decreased during early growth (Figure 2b). Concentrations dropped to their lowest levels in April and then rose after July, especially in roots. In contrast to cortex P measurements during all growing stages, xylem P concentrations were highest within the roots followed by the branches and then trunks.

(A)

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Fig. 2 Dynamic changes of P content in organs of cortex (A) and xylem (B)

Measurements of cortex and xylem P concentrations support the conclusion that apple tree roots take up little P in the spring and early summer. The decrease of P concentrations in the cortex and xylem highlights the early spring and summer transfer of P from storage organs to new spring growth, fruits and new tops. This further confirms that P requirements of early growth originate from P absorbed and stored in the cortex and xylem during the previous year.

Total annual net P accumulation in this established apple tree orchard with a yield of 48 t/ha amounted to 28.7 kg/ha, in which 18.4 kg/ha was accumulated from July 30 to Sept. 21, 10.3 kg/ha was taken up from Sept. 21 to January 15. Two distinct periods of plant P demand were identified, the first beginning in early August to support fruit expansion, the second in mid-September after fruit h (A)

(B)
Fig. 2 Dynamic changes of P content in organs of cortex (A) and xylem (B)
arvest to replenishment P reserves in all plant storage organs. Total P uptake during these periods summed to 28.7 kg P/ha. Considering a P fertilizer use efficiency of 25% (Lou Rukun, 1998), the initial recommendation for P application required to offset P removal within the orchard would be 114.8 kg P/ha without considering P supplied from soil. Results from this study indicate that approximately 68.9 kg P/ha (60%) should be applied basally in the autumn after fruit harvested and the remaining 45.9 kg P/ha (40%) should be applied prior to fruit expansion in the early July.

Although it is difficult to quantify the fertilizer recommendation for apple orchard by soil testing at present, it is necessary to make P balance according to apple yield and amount of P removed by fruit and leaves. Due to the facts that initial P demand resulting from new leaf and branch growth is apparently from sources stored the previous season, the appropriate time of P application seems in the autumn after fruit harvested and prior to fruit expansion in the early July.

The authors are with the College of Resource and Environment, Northwest University of Agriculture and Forestry, 712100, Shaanxi, Yangling, P.R. of China. tongyanan@nwsuaf.edu.cn

References
Frak et al., 2002. Plant Physiology, 130:1043.
Grassi, et al. 2003. Tree Physiology, 23:1061.
Lou Rukun, 1998. Soil-Plant Nutrition, Principle and Fertilization, p199.
Tagliavini, et al., 1998. Tree Physiology, 18:203.