How Plants Grow

The ancient thinkers wondered about how plants grow. They concluded that plants obtained nourishment from the soil, calling it a "particular juyce" existent in the soil for use by plants. In the 16th century, van Helmont regarded water as the sole nutrient for plants. He came to this conclusion after conducting the following experiment:

Growing a willow in a large carefully weighed tub of soil, van Helmont observed at the end of the experiment that only 2 ounces of soil was lost during the period of the experiment, while the willow increased in weight from 5 to 169 pounds. Since only water was added to the soil, he concluded that plant growth was produced solely by water.

Later in the 16th century, John Woodward grew spearmint in various kinds of water and observed that growth increased with increasing impurity of the water. He concluded that plant growth increased in water that contained increasing amounts of terrestrial matter, because this matter is left behind in the plant as water passes through the plant.

The idea that soil water carried "food" for plants and that plants "live off the soil" dominated the thinking of the times. It was not until the mid- to late-18th century that experimenters began to clearly understand how, indeed, plants grow.

A book entitled The Principle of Agriculture and Vegetation, published in 1757 by the Edinburgh Society and written by Francis Home, introduced a number of factors believed to be related to plant growth. Home recognized the value of pot experiments and plant analysis as means of determining those factors affecting plant growth. His book attracted considerable attention and led experimenters to explore both the soil and the plant more intensively.

Joseph Priestley's famous experiment in 1775 with an animal and a mint plant enclosed in the same vessel established the fact that plants will "purify"

rather than deplete the air, as do animals. His results opened a whole new area of investigation. Twenty-five years later, DeSaussure determined that plants consume CO2 from the air and release O2 when in the light. Thus, the process that we today call "photosynthesis" was discovered, although it was not well understood by DeSaussure or others at that time.

At about the same time, and as an extension of earlier observations, the "humus" theory of plant growth was proposed and widely accepted. The concept postulated that plants obtain carbon (C) and essential nutrients (elements) from soil humus. This was probably the first suggestion of what we would today call the "organic gardening" concept of plant growth and well-being. Experiments and observations made by many since then have discounted the basic premise of the "humus theory" that plant health comes only from soil humus sources.

In the middle of the 19th century, an experimenter named Boussingault began to carefully observe plants, measuring their growth and determining their composition as they grew in different types of treated soil. This was the beginning of many experiments demonstrating that the soil could be manipulated through the addition of manures and other chemicals to affect plant growth and yield. However, these observations did not explain why plants responded to changing soil conditions. Then came a famous report in 1840 by Liebig, who stated that plants obtain all their C from CO2 in the air. A new era of understanding plants and how they grow emerged. For the first time, it was understood that plants utilize substances in both the soil and the air. Subsequent efforts turned to identifying those substances in soil, or added to soil, that would optimize plant growth in desired directions.

The value and effect of certain chemicals and manures on plant growth took on new meaning. The field experiments conducted by Lawes and Gilbert at Rothamsted (England) led to the concept that substances other than the soil itself can influence plant growth. About this time, water experiments by Knop and other plant physiologists (a history of how the hydroponic concept was conceived is given by Steiner [1985]) showed conclusively that K, Mg, Ca, Fe, and P, along with S, C, N, H, and O, are all necessary for plant life. It is interesting to observe that the formula devised by Knop for growing plants in a nutrient solution can still be used successfully today in most hydroponic systems (Table 2.1).

Table 2.1 Knop's Nutrient Solution

Reagent

g/L

Potassium nitrate (KNO3)

0.2

Calcium nitrate [Ca(NO3)2^4H2O]

0.8

Monopotassium phosphate (KH2PO4)

0.2

Magnesium sulfate (MgSO4^7H2O)

0.2

Ferric phosphate (FePO4)

0.1

Keep in mind that the mid-19th century was a time of intense scientific discovery. The investigators named above are but a few of those who made significant discoveries that influenced the thinking and course of scientific biological investigation. Many of the major discoveries of their day centered on biological systems, both plant and animal. Before the turn of the 19th century, the scientific basis of plant growth had been well established, as has been reviewed by Russell (1950). Investigators had proven conclusively that plants obtain carbon (C), hydrogen (H), and oxygen (O) required for carbohydrate synthesis from CO2 and H2O by the process later called photosynthe-sis,1 that N was obtained by root absorption of NH4+ and/or NO3- ions (although leguminous plants can supplement this with symbiotically fixed N2 from the air), and that all the other elements are taken up by plant roots from the soil as ions and translocated throughout the plant — carried in the transpiration stream. This general outline remains today the basis for our present understanding of plant functions. We now know that there are 16 essential elements (C, H, O, S, N, P, K, Ca, Mg, B, Cl, Cu, Fe, Mn, Mo, Zn), and we have extended our knowledge about how these elements function in plants, at what levels they are required to maintain healthy, vigorous growth, and how they are absorbed and translocated.

Although there is much that we do know about plants and how they grow, there is still much that we do not understand, particularly about the role of some of the essential elements. Balance, the relationship of one element to another, and its forms in the plant, may be as important as the concentration of any one of the elements in optimizing the plant's nutritional status. There is still some uncertainty as to how elements are absorbed by plant roots and how they then move within the plant. Elemental form, whether individual ions or complexes, may be as important for movement and utilization as concentration. For example, chelated iron (Fe) forms are effective for control of Fe deficiency, although unchelated ionic Fe, either as the ferric (Fe3+) or ferrous (Fe2+) ions, is equally effective but at higher concentrations.

The biologically active portion of an element in the plant, frequently referred to as the labile form, may be that portion of the concentration that determines the character of plant growth. Examples of these labile forms would be the NO3 form of N, the SO4 form of S, and soluble Fe and Ca in plant tissue — forms of these elements that determine their sufficiency status. The use of tissue tests is partly based on this concept, measuring that portion of the element that is found in the plant sap and then relating that concentration to plant growth (see pages 324-325).

The science of plant nutrition is attracting considerable attention today as plant physiologists determine how plants utilize the essential elements. In addition, the characteristics of plants can now be genetically manipulated by adding and/or removing traits that alter the ability of the plant to withstand biological stress and improve product quality (Mohyuddin, 1985; Waterman, 1993-94; Baisden, 1994). With these many advances, all forms of growing, whether hydroponic or otherwise, are now becoming more productive. Much of this work is being done for growing plants in space in confined environments where the inputs must be carefully controlled due to limited resources, such as water, and control of the release of water vapor and other volatile compounds into the atmosphere around the plant.

Much of the future of hydroponics may lie with the development of plant cultivars and hybrids that will respond to precise control of the growing environment. The ability of plants to efficiently utilize water and the essential elements may make hydroponic and soilless growing methods superior to what is possible today. The genetic yield potential of cultivars in use today is uncertain, and whether that potential can be increased has not been established. A recent report by Moreno et al. (2003) suggests that among 18 tomato cultivars in their study, those that were identified as "least efficient in their uptake of nutrient elements, particularly N," produced highest fruit yields. Therefore, high efficiency in nutrient element utilization may be an undesirable trait — something that may seem counter to what one would expect. It should also be remembered that the adaptability of a cultivar or hybrid to respond to one set of environmental conditions may limit its use to that set of conditions. Therefore, there is still much that needs to be discovered on how plants respond to various sets of conditions and how best to adjust those conditions to achieve high plant performance and yield.

Note

1Process of photosynthesis: the conversion of solar energy into several forms of chemical energy.

Carbon dioxide (6CO2) + water (6H2O) in the presence of light and chlorophyll yields carbohydrate (C6H12O6) + oxygen (6O2)

The photosynthetic process occurs primarily in green leaves, since they have stomata, and not in the other green portions (petioles and stems) of the plant, which do not have stomata. A molecule of CO2 from the air passes into an open stoma, and a H2O molecule, which is taken up through the roots, is split and then combined with CO2 to form carbohydrate, and in the process a molecule of O2 is released. The rate of photosynthesis is affected by factors external to the plant, such as air temperature (high and low), air movement over the leaf surfaces, level of CO2 in the air around the leaves, and light intensity and its wavelength composition. The number of stomata on leaves, and whether they are open or closed, will also determine the rate of photosynthesis. Turgid leaves in a continuous flow of air and with open stomata will have high rates of photosynthesis.

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