Urban Orchards : Plant Structure and Growth

Contents:
General Structure
Extent of Root Growth
Types of Buds
Bud Differentiation
Plant Growth

General Structure:

Fruit Trees: Central Leader – The main stem of the tree, which gives rise to scaffold branches and determines the tree's primary framework.; Multiple Leaders – Several main stems, arising at the same point low to ground, which each produce scaffold branches and determine primary framework.; Trunk – The aged main stem, growing from ground, graft union, or major low crotch (in the case of multiple leaders).; Graft Union (trunk) – The point at which a scion of the desired variety is joined to rootstock, indicated by bulge low on trunk.; Roots – The feeder/anchoring system, extending from the base of the trunk into the ground.; Scaffold Branch – A main branch off of the trunk, scaffold branches and trunk comprise the primary framework of the tree.; Lateral Branch – A side branch growing off of a scaffold branch.; Twig – A side shoot, 1/4” diameter or less.

Grape Vines: Main trunk – The permanent vertical stem, which grows from ground/root system and supports the foliar/fruiting portion.; Cordon – An arm or main branch off of the main trunk. There are usually four (two tiers of two cordons each).; Cane – A woody shoot off of a cordon.; Sucker – A shoot arising from an underground bud or main trunk, which can be cultivated as new main trunk.

Raspberries: Primocane – A vegetative cane produced in the current season. The top and middle buds will produce fruit the following summer, the 2-3 buds below ground will yield new primocanes in spring (on ever-bearing varieties, the primocanes produce fruit on the top 10-12 buds in fall).; Florocane (seasonal & ever-bearing) – A cane produced last season, which fruit during the current season and then die.; Shoot – Shoots that arise from the crown of the mother plant become primocanes; those that arise from an underground stolon become new plants.

Extent of Root Growth

Reprinted with permission from:
Designing and Maintaining Your Edible Landscape Naturally, by Robert Kourik. Metamorphic Press, 1986. To order write to Box 1841, Santa Rosa CA, 95402. OR call (707) 814-2606. $50 + $5 shipping.

Types of Buds:

1. Dormant Buds – During the growing season (June-December) prior to the year they will break, dormant buds differentiate into shoot, leaf, or flower buds, depending on their exposure to growth hormones and sunlight. There are 2 types or dormant buds:

Terminal Buds – The fat bud at tip of shoot/branch. Each terminal bud produces growth hormones (auxins) that increase its own growth rate while keeping other buds dormant. This is termed apical dominance; the effect diminishes over distance with shoot buds farthest from terminal bud breaking dormancy first. If the terminal bud is removed, the shoot buds immediately behind it will break dormancy.

Lateral Bud – Buds along the length of branch. There are 3 types:

a) Side-branch buds are rounded and oblong. They are not associated with leaf buds.

b) Leaf buds are flat and triangular, and may occur as axillary buds (at the base of the leaf).

c) Flower buds are round and plump, and may occur as axillary buds. They are the first to swell in spring. Apple and pear flower buds contain a few leaves.

Triple buds (Peach, nectarine) – Two fat flower buds on either side of a leaf bud that occur on year-old wood.

Fruiting spurs (apple, pear, cherry, plum, apricot) – Squat twiglets that bear buds singly or in clusters. They occur on old wood, and bear flowers and leaves in alternate years. Their life span varies with fruit type: apple, up to 20 years; pear and cherry, 5 or more years; plum, up to 5 years; apricot, up to 4 years.

2. Latent Buds – Small, under-developed buds that persist on older wood that are strongly connected to the plant's vascular system through bud trace. These act as a plant's insurance – should a branch be cut or broken, a latent bud will quickly yield a new shoot. 3. Adventitious Buds – These develop in response to injury or pruning of mature branch. Because they are not well connected to plant's vascular system, they produce weak branches.

Bud Differentiation into shoot, leaf, or flower buds depends on:

Growth hormones – The closer a bud is to the terminal bud, the more the bud is inhibited by the terminal buds auxins (apical dominance). Auxins travel down a branch in response to gravity (geotropism), but do not move up a branch against gravity, so vertical branches yield less fruit (buds receive full impact of auxin), whereas branches at a 45-60 degree angle to the horizon bear the most fruit (since they are least inhibited by auxins).

Location on tree – Outer canopy (tree fruits on 'new' wood) versus interior (tree fruits on 'old' wood).

Exposure to sunlight – Flower or leaf buds will wither if they do not receive sufficient sunlight during their development. The leaves associated with a developing flower bud must receive a minimum of 30% of full sunlight to supply the energy (carbohydrates) needed for the bud to flower next season.

Plant Growth occurs in two ways:

1) Elongation – The distance a shoot/branch grows, from branch tip to bud scar, in one growing season. Elongation is finite per growing season. It does not occur in the internodes, but only between the terminal bud and the bud scar.: Bud scar: a small bulge or cluster of rings on branch, indicating the site of the terminal bud at the start of a growing season.; Nodes: past bud scars.; Internodes: regions between nodes.

2) Girth – Growth occurs as an increase in thickness, through the deposit of annual growth rings, which are made up of the inactive/dead xylem cells from past years. The xylem together with the phloem comprise a plant's vascular system. A new layer of xylem and phloem develops each season from the vascular cambium, a thin layer of generative cells just under the bark forming a continuous cylinder running from the base of the trunk to the tip of the smallest branch: One annual growth ring is produced each year. The age of a tree can be determined by counting the growth rings in a cross-section of the trunk. The growth conditions for a given year can be determined by the thickness of the annual growth ring produced in that year – the better the growing conditions (water, soil fertility, etc.), the thicker the ring.; Xylem cells transport water and nutrients from the roots to the leaves. Xylem cells divide off the inner face of the vascular cambium, and are woody/hard, do not flatten under pressure and therefore persist throughout the life of the tree as visible annual growth rings.; Phloem cells divide off the outer face of the vascular cambium. They transport carbohydrates produced in the leaves (through photosynthesis) to active growth sites and to the roots for winter storage. Phloem cells are soft and are compressed/crushed over the years into a band just beneath the bark whose layers are not discernable to the naked eye (old phloem looks like the inner face of the bark).