It has been a long time since I was in school, but I still made an attempt at reading a textbook on fruit tree physiology by T. M. DeJong: “Concepts for Understanding Fruit Trees”. I did enjoy the ways that tree physiology reminded me of human physiology and the circular rhythms of living beings. Wanting to share, this is a summary of my gleanings from the book.
Fruit tree productivity depends on resources being available, such as sun, water, and nutrients.
Like most all plants, fruit trees harvest the sun, through photosynthesis. Chloroplasts within leaves are the solar cells that provide energy to the plant. Optimally, a tree’s shape is such that the leaves are arranged for maximum sun capture. Photosynthesis produces carbohydrates (sugars) as an energy source for the tree. Leaves and other structures of the tree use and store sugars for use during both day and night for the process of respiration. Respiration in the leaves occurs through conversion of sugars, with oxygen, into chemical energy for repair and building of tree structures. Respiration also happens in the roots, with sugars combining with oxygen from the soil, unless the soil is too waterlogged to provide the necessary oxygen.
Together, photosynthesis and respiration provide the conversion of solar energy to sugars and then to chemical energy (ATP). This process of energy conversion happens best between 70 and 100 degrees. The warmth of the day brings the chemical energy to the above ground parts of the tree, and the relative warmth of the soil at night brings the chemical energy to the trunk and roots of the plant. This image reminds me in a way of the pattern of human respiration, with a breath in and breath out.
Water, macronutrients, and micronutrients for the process of photosynthesis are provided by the roots. Water is vital for growth and productivity of the tree, with chloroplasts functioning in an aqueous solution. Water and nutrient transport happens through the xylem (ascending) and the phloem (descending and distribution). Now I am imagining the human circulatory system! Some nutrients are more mobile and able to be distributed in the phloem, such as nitrogen, phosphorus, potassium, sulfur, and magnesium. These can be transported to where they are needed in the whole plant, though proximity to resources can have an influence. Other nutrients are much less mobile, including iron, calcium and manganese among others, and those deficiencies can show in the local leaves or fruit when the tissues have used up those resources.
Roots, shoots, and fruits all demand energy. The phloem flow of energy from the leaves and roots through the tree is governed by proximity and by the need for energy from the various organs of the tree. Demands from the different parts of the tree (energy sinks) change over the course of each day and the time of year. Winter dormancy allows for storage of carbohydrates from the energy produced by the leaves and roots. Early spring when trees are in their grand awakening, roots, and then shoots and flower development are the primary sinks for energy. Later as flowers are pollinated, fruit development requires considerable initial energy. Pruning and fruit thinning are points of influence to change the distribution of energy.
Heading cuts often stimulate heavy regrowth. The water sprouts (epicormic shoots) that develop from dormant buds after heading cuts demand a huge amount of energy and are not productive, growing longer all season long. Often, when they themselves are headed back, they produce more epicormic growth. Heading cuts on vigorous growth near the top of a tree, produces more water sprouts near the top of the tree. Unless needed for rebuilding a tree, water sprouts should be removed (pulled rather than pruned) when 12 to 18 inches long, in late spring. Thinning cuts are generally preferred, keeping in mind the shape of the tree for maximum sun exposure. Fruiting wood generally develops on lateral shoots (proleptic) which emerge from buds off of similarly growing shoots, rather than water sprouts. Pruning can be performed any time of year, with summer pruning removing some solar panels, and slowing growth. Stored energy in branches and roots promotes rapid growth in the spring, when dormant pruning is utilized.
Fruit thinning is best done as early as possible, to manage energy demands and carbohydrate availability for remaining fruits. June drop of immature fruits is generally related to insufficient water, nutrition, and therefore chemical energy for the developing fruits. Leaves (and their solar cells) that are close to the fruits provide energy to that developing fruit, rather than transporting energy throughout the tree. Fruit can also be dropped if insufficient chill prevents normal flower development or if weather prevents normal pollination. Budwood for future fruiting (proleptic) shoots can also be aborted due to lack of chill, leaving blank areas along young branches.
Initiation of floral budwood begins in the summer at the same time the tree is producing its fruit. If there is too much energy being required by developing shoots and fruit, there may be inadequate energy available to produce the floral bud initiation. There may be little fruit production then in the following year, because factors influencing flowering and fruit happen over 2 growing seasons.
Trees have a supply side of photosynthesis and carbohydrates, and a demand side requiring distribution of carbohydrates. Watering more frequently keeps the process in the xylem and phloem moving. The author is a strong proponent of regular water application especially during periods of rapid growth. One side point on dwarfing rootstock mentions that the difference in xylem sizes may be part of the dwarfing influence. I can imagine a limitation of upward flow impacting the vigor of the scion.
Like humans, trees require water and nutrition, and probably more sun than most of us should get.