Comprehensive Analysis of the Morphological Characteristics of Walnut Trees

  I. Root System

  The portion of the walnut below the root crown is collectively called the root system, which is divided into the primary root, lateral roots and fibrous roots. The primary root is well developed, the lateral roots extend far horizontally, the fibrous roots are numerous and the root system is vigorous. Generally speaking, the root system of precocious walnuts is more developed than that of late-bearing walnuts, and this is especially evident in young trees. According to measurements, one-year-old precocious walnuts have nearly 2 times more total roots and 1.8 times greater total root length than late-bearing walnuts. This shows that a numerous root system is an important characteristic of precocious walnuts; a well-developed root system is beneficial for the absorption of water and inorganic salts, for the accumulation of nutrients within the plant and for flower bud formation, enabling early fruiting and early high yields. The walnut root system also has endophytic mycorrhizae, concentrated in the 10-30 cm soil layer; mycorrhizae play a positive role in increasing walnut yields.

  II. Buds, Branches and Leaves

  1. Buds Walnut buds are divided into mixed buds, male flower buds, leaf buds and latent buds.

  (1) Mixed buds A bud from which both branches and leaves and flowers and fruits can grow is called a mixed bud. The bud body is plump and nearly round. After sprouting, it produces a fruiting mother branch, with female buds at the tip, generally a single bud, though sometimes double buds; late-bearing walnuts mostly bear them on the top 1-3 nodes of one-year-old branches, while the axillary buds of precocious walnuts are mostly mixed buds, which first grow a section of branches and leaves before flowering and fruiting.

  (2) Leaf buds Leaf buds are triangular or round, borne singly or stacked with male flower buds, and produce only branches and leaves.

  (3) Male flower buds Short conical in shape, with very small scales that cannot cover the bud body, hence they are called naked buds. After sprouting they produce only male flowers, forming catkins, and do not grow branches or leaves.

  (4) Latent buds Also called dormant buds, flat-round and slender. Under normal circumstances they do not sprout, and as the branch thickens they become buried beneath the bark; they can live for more than a hundred years, sprout easily when stimulated, and the renewal and rejuvenation of walnuts are all accomplished by latent buds.

  2. Branches

  (1) Vegetative branches (also called growth branches or leaf branches) According to their length they are divided into short branches, medium branches and long branches. Medium and long branches can be divided into two types: one is growth branches, developed from leaf buds, which are the basis for expanding the canopy and forming fruiting branches; the other is water sprouts, sprouted from latent buds. If a tree has too many water sprouts, they should be controlled in production; old and declining trees mostly use water sprouts for renewal and rejuvenation.

  (2) Fruiting branches Produced from the mixed buds on fruiting mother branches, divided into long fruiting branches (greater than 20 cm), medium fruiting branches (10-20 cm) and short fruiting branches (less than 10 cm).

  (3) Male flower branches The terminal bud is a leaf bud, and below it only male flowers are borne; they are mostly short, slender and weak. After the male inflorescences fall off, the area below the terminal bud is bare; they are mostly borne on old and weak trees or in shaded, closed areas inside the canopy. Too many male flower branches is a sign of a weak tree and inferior variety; in breeding, varieties with many male flowers should be avoided.

  3. Leaves The leaves are odd-pinnately compound, with the terminal leaflet being the largest and the opposite leaflets below it becoming progressively smaller. Different species and different varieties have different numbers of leaflets; for example, common walnut has 5-9 leaflets, and Manchurian walnut has 7-17 leaflets.

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  III. Flowers and Fruits

  1. Flowers The walnut tree is monoecious with unisexual flowers borne in separate inflorescences. The male inflorescence is 6-12 cm long, with 100-180 male flowers per inflorescence. The female inflorescence is a raceme, with solitary, clustered, spicate or spike-like arrangement; the ovary is inferior, with two carpels and a locule.

  2. Fruit The walnut fruit develops from the ovary. Fruit shapes include elliptical, round, ovate and so on. The fruit skin is green or yellow-green, smooth or downy. When this type of fruit matures, the skin loses water and splits open, revealing the hard seed, which is the so-called nut. Its size varies with species, variety and site conditions. The kernel yield is generally 36%-65%, with some varieties reaching over 75%.