How Do Plants Grow?

From: www.maximumyield.com

By J. Benton Jones, jr.

The simple answer is one cell at a time! But it is not quite that simple. Cellular materials are made of carbohydrates that are formed in the process called photosynthesis. Photosynthesis converts solar energy into chemical energy. The two starting ingredients, water (H 2 O) and carbon dioxide (CO 2 ), are combined to form a simple carbohydrate (see Figure 1). This process takes place in green chlorophyll-containing plant leaves when they are in light, mainly within the chloroplast-containing cells located around small openings in leaves known as stomata. The rate of photosynthesis depends on external factors, such as temperature, movement, and CO 2 content of the air surrounding the plant, and also by plant factors, such as water content and nutritional status and whether the stomata are open or closed.

PLANT TYPES

There are two types of plants based on the number of carbons in the "first product of photosynthesis." For C3 plants, the first product is a carbohydrate containing three carbons and for C4 plants, you guessed it, a carbohydrate containing four carbons. There is actually a third category of plants that are able to produce carbohydrates containing both three and four carbons, unusual plants mostly found in desert climates. Most vegetable crops are C3 plants, whereas grasses and grain crops are C4. There are significant differences between these two C types: C3 plants are light-saturated at relatively low radiation levels and are highly responsive to increasing CO 2 levels in the atmosphere, whereas C4 plants are continuously responsive to increasing light levels but less responsive to increasing CO 2 levels.

EARLY INVESTIGATORS

The ancient thinkers began to wonder about how plants grow, so they began to conduct experiments to find answers. In the 16th century, one of the more interesting experiments was that conducted by van Helmont, who grew a willow twig in a carefully weighed tub of soil. After several months he dismantled his experiment and weighed the tube of soil and willow plant. The plant had increased in weight 30-fold, while there was only a two-ounce weight loss in the tube of soil. Up to that time, it was believed that the soil contained a "particular juyce," the factor that contributed to plant growth. Some scientists at this time began to question that theory and suggested that water was the plant growth factor. Indeed, water is the major plant constituent - a fully turgid plant's wet weight will be between 80 and 95 per cent water. But what about the remaining 5 to 20 per cent? Where did that portion come from? Beginning in the middle of the 1800s, scientists began to sort out what was happening, finding that plants in air and light absorb CO 2 and release O 2 (photosynthesis) and that certain elements are required for plants to grow normally. In this same time period, the "humus theory" was proposed, the first suggestion that the source of an element absorbed through the plant's roots determines its "health" - the basis for today's organic concept of plant growing.

COMPLEX PLANT FUNCTIONS

The plant is a multi-functioning biological system whose healthy growth is determined by both internal and external factors. Growth chamber studies have shown that plants grow best in consistently maintained conditions within a particular temperature range and that plant roots require an aerobic environment, adequately supplied with water and the essential elements in order to sustain healthy plant growth. Plants grow poorly in stagnant air, when exposed to temperature and light extremes, when less than fully water-turgid, and when not adequately supplied with the known essential elements. Plant growth occurs by a series of events: the absorption of water and essential elements through the roots, the movement of these up the plant through xylem vessels, and their redistribution into the outer portions of the plant through phloem vessels. Carbohydrates formed photosynthetically are distributed throughout the plant through the phloem. A portion of these carbohydrates are converted into other compounds, amino acids and complex sugars, and those other substances required to carry out functions that generate continuing plant growth and fruit or seed yield. Water absorption through the roots is an aerobic process, and water movement up the plant is by the "pull" of a continuous water column, the result of water loss from leaf surfaces. This water loss, known as "transpiration," cools the plant (the leaf surfaces feel cool to the touch), and the rate of water loss is determined by the radiation level, the surrounding air's temperature and relative humidity, as well as the rate of air movement over the leaf surfaces. In addition, water loss is highest when the leaf stomata are open.

ROOT VERSUS TOP GROWTH

Plant roots function only in aerobic conditions; their size and form determines their ability to absorb water and other substances, and the characteristics of the water solution around the roots affect the root's absorptive ability. Some plant characteristics are determined by root function and growth - the bonsaiing of a plant by periodic root pruning is an example. Does a plant grow based on the extent of its root development (size) or does top growth determine the extent of root development? Neither is totally true. In soil, root development can be critical, but plants grown hydroponically or in most soilless media are less influenced by root size.

ESSENTIAL MINERAL ELEMENTS

Before the end of the 18th century, 10 elements (C, H, O, N, P, S, K, Ca, Mg, and Fe) of the 16 essential elements known today (the other six are B, Cl, Cu, Mn, Mo, and Zn) required for plant growth had been identified. The element last identified and added to the list of essential elements was chlorine (Cl), in 1935. All of the essential elements have specific functions in the plant: some are part of the plant's structural components, some act as catalysts, others have various biochemical functions (see Table 1). These elements are "essential" because when at insufficient concentration in the plant, growth is impaired and there is generally a lack of both fruit and seed production.

PLANT GENETICS

All of the processes occurring in the plant are governed by a DNA blueprint, producing a plant of distinct character and functioning characteristics - why a tomato plant is a tomato plant, why some plants can withstand environmental stress while others cannot, etc. Plant modification by natural selection or selection by man can generate plant types with particular characteristics, such as disease resistance, moisture stress tolerance, fruiting habit, etc. Genetic engineering, on the other hand, deals with changes in the DNA structure, breaking and changing the genetic code, an activity that is highly controversial today. DNA recombination can significantly alter the growth habit(s) of a plant, modify fruit quality factors, and change the plant's response to certain environmental stresses. Although DNA modification is being met with opposition, genetically-modified plants are going to be continually introduced into the marketplace.

ADAPTATION TO STRESS

Plants are able to adapt to changing growing conditions, surviving even under very harsh environmental conditions. However, as stated earlier, plants grow best when growing conditions are constantly maintained within certain optimum ranges. Changing conditions, no matter how small, can significantly affect plant growth. For example, moisture or light-intensity stresses (low or high), may either increase or decrease fruit yields, even though the stressed condition is brief and occurred weeks before fruit was set and then matured. Plants will change their growth habit and physical appearance in order to adjust to less than optimum conditions. Plants bred for specific growing conditions do poorly when placed in unsuitable environments, for example, growing a greenhouse-bred variety in the open field or placing a plant adapted to a neutral soil pH condition into an acid soil.

THE FUTURE

The use of plants, not only for the production of food, but as a source of fibre, industrial chemicals, etc., as well as for beautifying the landscape, will continue to expand based on research being conducted by plant scientists throughout the world. Many of the global issues confronting us today have potential solutions in the wider use of plants here on earth, in space, or in controlled environments. It took about two centuries for scientists to sort out those functions and conditions affecting how plants grow; that sorting process continues today, but at a much faster pace. We have entered an age where our expanding knowledge is being used to improve the lives of the growing world population as well as improve human health and the planet's environment. Stay tuned-exciting times are ahead!

FIGURE 1 . Photosynthesis-the conversion of solar energy into chemical energy.

Carbon dioxide (6CO 2 ) + water (6H 2 O)

in the presence of light and chlorophyll

carbohydrate (C 6 H 12 O 6 ) and oxygen (6O 2 )

A molecule of carbon dioxide from the air passes into the plant leaf through an open stoma. A molecule of water taken up by the roots flows into the leaf, where it is split and then combined with that adsorbed molecule of carbon dioxide, forming a carbohydrate-the process occurring only in the light and in the presence of chlorophyll. In the process, a molecule of oxygen is released through the open stomata back into the atmosphere.

TABLE 1 . The essential elements-their common forms of utilization and functions in the plant.

Essential Elements : carbon (C), hydrogen (H), oxygen (O), nitrogen (N), sulfur (S)

Form utilized : in the form of gases (CO 2 ), water (H 2 O), and as ions (HCO 3 - , NO 3 - , NH 4 + , SO 4 2- ) from the rooting media

Function *: major constituents of organic compounds

Essential Elements : phosphorus (P) and boron (B)

Function : P as esters in energy transfer reactions, B in esterification functions with alcohol groups

Essential Elements : potassium (K), magnesium (Mg), calcium (Ca), manganese (Mn), chlorine (Cl)

Form utilized : as ions (K + , Mg 2+ , Ca 2+ , Mn 2+ , Cl - ) from the rooting media

Function : nonspecific functions, establishing osmotic potential, enzyme activation, balancing anions, controlling membranes permeability, and regulating electro-potentials

Essential Elements : iron (Fe), copper (Cu), zinc (Zn), molybdenum (Mo)

Form utilized : as ions (Fe 2+ or Fe 3+ , Cu 2+ , Zn 2+ , MoO 4 2- ) from the rooting media

Function : enable electron transport by valency change

*generalized functions; some elements also have very specific functions