Plant growth factors or, to be complete, “plant growth and development factors” refer to everything that can affect the expression of plant characteristics. Each factor is anything that controls or influences any aspect of growth and development and contributes to that overall expression.
These characteristics are apparent in those which are visible like, for example, the distinct characters of corn or maize compared to trees so that one can easily identify it and not mistake it for any tree. The yields of two corn plants can also be compared first visually as small vs. big ear, and then by measuring the weights of ears and kernels. So arises the question What makes this one corn yield high compared to the other? This question to be answered comprehensively requires a dissect of the plant growth factors affecting such characteristics.
But the characteristics which plant growth factors control or influence are not restricted only to those external to the plant and are visible or measurable. These factors also affect the internal and chemical features of plants which, to dig deeper, are responsible for the expression of diverse external traits.
Few examples are the number, types, and sizes of cells composing the different plant organs, the thickness of cell walls, the volume of cells which changes with water availability, the presence or absence of vascular bundles (xylem and phloem) and their arrangement, the pigments responsible for coloration, and the structural and chemical composition of the organic compounds in the plant body.
Main Divisions of Plant Growth Factors
Plant growth and development factors control or influence plant characteristics particularly crop productivity. In general, these factors are distinguished and placed under two main divisions: genetic and environmental.
The genetic factor is also called the internal factor because the basis of expression of plant traits (the gene) is located within the cell. The environmental factor is considered external, and refers to all factors other than the genetic factor. Stated another way, the environmental factor includes everything excepting the genetic factor.
It is the genetic factor that dictates what a plant should be. Thus, for example, rice is rice and corn is corn and each one cannot transform into another because each one has a specific gene constitution dictating that it should be so. A dissection of this genetic plant growth factor would show that the two crops differ in the genome, karyotype, and individual genes.
The differences in the adaptation of the two crops are due also to differential genetic makeup. Corn easily succumbs to waterlogging but rice has been grown in lowland fields. Likewise, a crop variety may outyield other varieties because it possesses a genetic constitution that combines desirable genes.
The environmental plant growth factors in turn determine the extent to which the genetically dictated character is expressed. These factors are divided into biotic and abiotic factors. The biotic factors refer to living organisms which include both macro-and micro-organisms. The abiotic factors include the elements of climate, soil, and topography. These biotic and abiotic factors are treated on separate pages.
Plant Growth Factors Interact and Can be Manipulated
The various characters that plants display are not necessarily exactly alike, even within the same species, varieties and strains. The same applies to clones that are genotypically identical. This deviation is more pronounced with plants that are grown at different periods of time, or in different locations, or with different amounts of care.
Both plant growth factors interact in various ways. It is well established that the genetic factor cannot cause a plant character to develop to its maximum potential without a favorable environment. A crop variety may possess the genetic constitution which provides the blueprint for the plant to produce a high yield. But without the proper nutrition and sufficient supply of other inputs, it will produce a dismal yield, or worst, it may even fail to produce. For example, depriving any plant of any one of the essential elements will prevent the plant from completing its life cycle. The absence of magnesium, in fact, will prevent the development of chlorophyll, it being a structural component of the chlorophyll molecule.
By understanding how these plant growth factors interact and affect plant expression it will be possible to increase crop productivity.
Manipulating Environmental Factors
Increased yields can be achieved by growing crops under conditions where they are naturally adapted. In crop farming, this means growing high-yielding varieties on suitable land having favorable conditions including both biotic and abiotic factors. Included among the abiotic factors is the availability the supply of essential plant nutrients and water.
Otherwise, if there is scarcity in any environmental factor, it is corrected or supplied.
Man in fact has learned long ago to manipulate the environment to grow plants away from their natural habitat for the purpose of research, pleasure, or food, or commercial use. An example is the Hanging Gardens of Babylon. One important feature of the Gardens is the provision of irrigation water.
Another example is the Climatron at the Missouri Botanical Garden in St. Louis, Missouri, USA, with several climates under a 70-foot (21.3 m) glass dome, including tropical climate (Went and the Editors of Life, 1963). The tropical banana, which needs at least eight times as much water as that of tomato, has been grown in the desert (Belt, 2010).
Gigantic trees are often seen naturally growing on an open field. But trees of the same species growing under harsh conditions with scant water and nutrient supply tend to become stunted. This interaction of the plant growth factors has been realized many centuries ago and widely applied with awesome effect in the creation of bonsai trees.
Manipulated by human hands through continuous root and shoot pruning and by growing them in shallow pots, large, gnarled, and aged trees in miniaturized form have been created which have lived for generations, even surpassing the longevity of naturally growing trees.
Plants can also be made to adapt to conditions that deviate from their natural habitat through acclimatization, but only up to a certain point. In-plant nursery management, the process of acclimatizing the seedlings in preparation for outplanting is called hardening.
Manipulating the Genetic Factor
Increased yields have been achieved through plant breeding. Essentially it consists of producing new varieties or plant types having a genetic constitution that combines desirable genes.
The primitive man did it, for example, by the continuous selection of bigger ears having more kernels of corn. Now such a technique is called mass selection.
Camerarius performed artificial cross-pollination leading to his “discovery” of plant sex in 1694 but it was already practiced a long time ago to enhance increased fruit production in date palm. Gregor Mendel formulated the basic laws of inheritance in 1805. His discovery provided the fundamentals which led to the advancement of both plant and animal breeding.
Consequently, plant breeding companies have multiplied worldwide. Although only partially correct, the term “plant breeding” has been equated with the production of hybrid seeds especially among farmers.
The plant itself can, and has been, genetically engineered to produce cultivars with desirable characteristics including high yields, improved quality, resistance to pests and diseases, and tolerance to environmental stress.
In the 1970s, there was an outbreak of the grassy stunt virus which prevented rice from flowering and producing grain. The International Rice Research Institute (IRRI) found a gene for resistance to the disease in a wild relative, Oryza nivara, growing in India. The gene has, since then, been introduced into most varieties (Burness Communications, 2010).
With the advances in biotechnology, it has become possible to produce transgenic varieties, such as tomato, corn, and other farm crops through recombinant DNA techniques.
In 1994 the Flavr-Savr tomato, the first transgenic variety, was released in the USA. It does not produce the enzyme polygalacturonase which is responsible for fruit softening. As a result, the tomato fruits remain longer attached to the plant without getting too soft. The tomato fruits also have a longer shelf life. In corn, soybean, cotton, canola, and potato, transgenic varieties have been produced that carry the endotoxin or Bt gene from the bacterium Bacillus thuringiensis for pest resistance (Lantican, 2001).
Another milestone in the manipulation of the plant growth factor is the Golden rice in which the provitamin-A biosynthetic pathway was genetically engineered into the endosperm. It is claimed that it was developed to prevent vitamin-A deficiency in developing countries (Potrykus, 2001).
Both plant growth factors, genetic and environmental, can therefore be modified to serve man’s purpose in relation to crop farming.
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Burness Communications. 2010. Adapting agriculture to climate change: new global search to save endangered crop wild relatives. Retrieved January 2, 2010, from http://www.sciencedaily.com/releases/2010/12/101209201938.htm.
Lantican, R.M. 2001. The Science and Practice of Crop Production. UPLB, College, Los Banos, Laguna, Phils.: SEAMEO SEARCA and UPLB. 330 p.
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