Taking Shandingzi as the rootstock, Beijing Agricultural University observed Guoguang (born in 17~ 18) for two years. Under the condition of no irrigation, root growth has 2~3 obvious peaks a year. The first time is from early April to late June or early July, the second time is from mid to late August, and the third time is from late September to 10. The root growth of different soil depths is also different. The upper root (above 40cm) began to move earlier, and the lower root began to move later. In summer, the growth of upper roots is small, while the growth of lower roots is large, and the growth of upper roots is strengthened in autumn. Roots in the upper and lower layers of soil grow alternately within one year, which is related to soil temperature, humidity and soil ventilation.
According to the observation of Hebei Agricultural University 1965, the root system of golden crown apple tree with vigorous growth and first fruit has three growth peaks in one year, which are interdependent and mutually restrictive with the growth and development of aboveground organs.
Wang et al. (1997) studied the dynamics of new root years of young trees, highly stable trees, old trees, young trees, prosperous trees and weak trees under natural conditions.
Annual occurrence dynamics of new roots in different types of apple plants
The results showed that after potted 1~2 years, the total number of new roots after germination of young trees increased continuously, showing a single peak curve. Under the same environmental conditions, the dynamics of new roots of field plants vary with plant types, and both bimodal and trimodal curves exist. Different kinds of trees have the biggest difference in rooting in spring. Young trees and weak trees take root late, with little occurrence, but they rise slowly after germination, and do not decline with the rapid growth of spring shoots, which can last until July, showing a bimodal curve. Spring trees and flourishing trees reach the peak before the vigorous growth of spring shoots, and then decline to form a trough; The number of new roots of high and stable yield trees also decreased with the vigorous growth of spring shoots, but it still maintained a high level. The appearance of new roots in spring is positively correlated with the amount of starch stored in roots. After the growth of spring shoots stopped, the rooting of all kinds of trees reached a peak, which had the largest rooting amount and the longest duration, and ended with the growth of autumn shoots and the high temperature in July and August. However, the peak size of different types of plants is different, the weak tree is the lowest, the stable tree and the young tree are higher, and the strong tree has a late peak but a long time, which can last until the autumn shoot growth period. After the autumn shoot stopped growing, the autumn shoot peak appeared, but the autumn shoot peak of overloaded trees disappeared in the new year, which affected the occurrence of new shoots in the next spring (small-year trees).
Therefore, the periodicity of root growth mainly depends on the growth of new shoots and fruit load. However, the growth of new shoots does not simply inhibit the occurrence of new roots, resulting in the phenomenon of alternate growth of roots and shoots. The vigorous growth of new shoots should also be based on a certain number of new roots. The growth of roots and buds promote each other and contradict each other. The growth of new buds and the competition of roots for nutrients. Excessive growth of new shoots will reduce the occurrence of new roots, but the occurrence of roots, especially growing roots, needs the stimulation of IAA produced by the tips of young leaves. The same is true of the relationship between overload, early defoliation, autumn shoots and new roots. Overloading not only reduces the downstream photosynthetic products, but also reduces the transportation of IAA to the base due to the limited growth of autumn shoots. Overload and premature defoliation not only affect the growth of autumn roots, but also make the occurrence of new roots less and later in the next spring.
(2) Root distribution and density
Tree age, rootstock, soil type, groundwater level and cultivation techniques will all affect the root distribution of apples.
Due to the diversity of media and environment, the roots in orchard soil show different ecological phenotypes. Clay roots tend to be "linear" distribution, with few branched roots and low density, but in the process of extension, if you encounter areas with good air permeability, a large number of branches will occur; The roots with good fertilizer and water conditions are far-reaching, with many branches and large amount of fine roots, which are often evenly distributed; In arid sandy land and coarse bone orchard with good air permeability, the root system is widely distributed, with low density, short and dry absorption roots, poor function and "alienated" root system distribution; In soil orchards in mountainous areas and alluvial plains, gravel layers and cohesive layers often limit the downward expansion of roots, so roots tend to be concentrated in the surface layer and distributed in a "layered" manner.
The vertical distribution of root system is affected by soil structure and layer, and the clay barrier layer and high groundwater level will limit the root system from spreading to the deep layer. For example, the root depth of apples in mountainous areas such as southern Liaoning and Jiaodong is about1m; The roots of loess plateau and alluvial land in northwest and north China are as deep as 4 ~ 6 m; The beaches along the coast and the river, the alluvial plain of the old course of the Yellow River, are often affected by the groundwater level, and the root depth is only about 60cm. However, in most areas, the root system of arbor-stock apple is mostly distributed in the range of 20~60cm, and the root system of shallow-rooted dwarf-stock apple is mostly concentrated in the range of 40cm. Even for arbor species, more than 90% of the fine roots are distributed in the soil layer within 40 cm. Therefore, no matter how deep the root system is, most of the roots, especially the fine roots, are close to the surface of the soil, and the utilization of the surface roots can not be ignored, but the deep roots play an important role in maintaining the tree vigor and the adaptability of plants to adversity such as drought resistance.
The horizontal extension range of apple root system is 1.5~3 times the crown diameter, the rootstock and loose soil are wider, and the dwarf rootstock and clay are narrower. According to the comprehensive investigation data of Papp and Tamasi( 1979), more than 80% of apple roots are distributed in the inner range of the crown edge, but the fine roots with the diameter less than 1mm are mostly distributed in the crown edge and far away from the central trunk. Although the root system has the potential to expand to a wider range, the roots of neighboring plants will limit its expansion. Even in high-density apple orchards, root system interlacing between plants rarely occurs.
(3) Soil environmental conditions affecting root system.
Soil aeration, water content, soil temperature, nutrient status and pH value affect the growth and development of root system. In order to ensure the oxygen demand of root respiration, the soil needs to have a large porosity, and the fine root density is significantly related to the soil porosity. Because of the limited pore capacity in soil, these pores are often occupied by water, and soil oxygen often becomes the limiting factor of root growth and functional activities, so improving soil permeability should be the primary task of soil management.
Generally speaking, soil nutrients will not become the limiting factors of root growth like oxygen, water and temperature, because soil is poor and has certain natural fertility, but the soil nutrient status has a great influence on the shape, distribution range and density of root system. Fertile soil has high root density, small distribution range and concentrated root system; However, in order to obtain nutrients, the roots of poor soil have a wide foraging area, an expanded distribution range and a small density. Planting in poor and arid areas will form such a huge root system, which is conducive to the full use of water and nutrients, but it will consume a lot of photosynthetic products during the construction process.