ABSTRACT
Plants are composed of 80%–95% water� Plant dry matter is 10%–20% of the fresh weight� Over 90% of the dry matter of plants is composed of carbon (C), hydrogen (H), and oxygen (O)� We are all familiar with the term photosynthesis� This is the process whereby light supplies energy, water from the growing medium provides hydrogen and oxygen, and carbon dioxide (CO2) from air produces carbon and oxygen that become the building blocks (sugars) for plant growth (Figure 5�1)� All of the other elements needed in photosynthesis, making up 1�5% of the fresh weight, are from the soil or nutrient solution� These are the essential elements that we discuss in Section III of this book�
Photosynthesis may be expressed as an equation as follows:
Light 6CO2 + 6H2O → C6H12O6 + 6O2 Carbon dioxide Water Sugar Oxygen
The sugar is a form of chemical energy that is used to drive all the plant’s processes� Plants are the basis of almost all life on our planet and photosynthesis the source of energy for nearly all life on Earth� Photosynthesis uses light visible to our eyes (Figure 5�2)� The light is absorbed by chlorophyll, the green pigment, in all plant parts, especially in the leaves where most organelles called chloroplasts are located� The chloroplasts contain chlorophyll-a, chlorophyll-b, and carotenoid pigments� Most absorption of light is in the violet-blue and red light of the visible spectrum as shown by the absorption spectrum of these pigments (Figure 5�3)� When we use supplementary lights for our plants indoors, we want light that gives off most energy in this part of the visible light� There are many complex processes that take place within the plant to convert the sugar into carbohydrate products by carbon fixation whereby carbon is taken from sugars and combined to form sucrose and starch� The carbon from photosynthesis is used to form other organic compounds such as cellulose, lipids, and amino acids or others to fuel respiration�
In respiration, metabolic reactions take place in the cells of plants (and animals) to convert biochemical energy from nutrients into high-energy molecules that can later break down into smaller molecules releasing energy in the process� Respiration provides the energy to fuel cellular activity� The nutrients used by animal and plant
cells in respiration include sugar, amino acids, and fatty acids� The energy is stored in the high-energy molecule adenosine triphosphate (ATP) and during oxidation (use of molecular oxygen), the energy stored in ATP is released to drive energy processes such as biosynthesis, locomotion (movement in animals), or transportation of molecules across cell membranes�
A simplified reaction for respiration is as follows:
C6H12O6 + 6O2 → 6CO2 + 6H2O + Heat Sugar Oxygen Carbon Water dioxide
Because respiration requires oxygen in plants, it is termed as aerobic respiration� It is the main process by which both fungi and plants break down organic compounds into energy needed for their growth� These organic compounds are produced during photosynthesis� In plants, respiration occurs during the dark� Consequently, at night the plants use oxygen and give off CO2 and water�
It is important to understand these simplified basics of plant growth in order to know the key factors of the environment that plants require for healthy development� When these factors are not at optimum levels, they will limit plant growth and therefore can be termed limiting factors� During the day, plants need the correct quality and intensity of light to drive photosynthesis� The quality refers to the color of light (determined by its wavelength)� Plants require light between 400 and 700 nm wavelength, which is in the visible spectrum as shown in Figure 5�2� A nanometer is 10−9 or 1/1,000,000,000 m (one billionth of a meter) in length� This light source that plants utilize is termed photosynthetically active radiation (PAR)� This designates the solar radiation from 400 to 700 nm that plants actively utilize in photosynthesis (Figure 5�3)�
Increasing light energy in the PAR range increases photosynthesis� Each crop has an optimum light intensity that maximizes plant growth� If there is insufficient light, plant growth slows down and if excess light is given, plant growth will not increase (Figure 5�4)� As a result, when using lights you must be sure to give sufficient, but not excess as the cost of the additional light will not result in increased production� The quantity of light is the intensity that can be measured� In the United States, the unit for measuring light intensity is the foot-candle, whereas lux is used in Europe� An argument against the use of foot-candles is that it primarily measures visible light detected by the human eye and not necessarily the amount of light a plant receives� Most horticulturists use a unit that measures light at any instant in micromoles (µmol) per square meter (m−2) per second (s−1) of PAR� This unit measures the number of photons (individual particles of energy) used in photosynthesis that fall on a square meter of surface every second� Because this is an instant reading, the better unit to use is the daily light integral (DLI), which is the amount of PAR received each day (moles per day)� In greenhouses, the values are normally less than 25 mol/m/day� To grow plants in your home using artificial lights you need to get sufficient light for optimum yield� Researchers have
developed DLI levels for groups of plants classifying them as low-light, mediumlight, high-light, and very-high-light crops� Fruit-bearing crops such as tomatoes, peppers, and European cucumbers would lie in the very-high-light crops� Although this is a little more technical, it shows you what amount and type of light is best for your crops�
Another environmental factor that affects plant growth and yields is temperature (Figure 5�5)� Again all crops have different optimum temperature ranges� Crops are divided into cool-season crops and warm-season crops� Cool-season crops include cabbage, cauliflower, broccoli, and lettuce, whereas warm-season crops include fruiting crops such as tomatoes, peppers, cucumbers, and eggplants� Normally, cool-season crops require night temperatures in the 50s F (10-15°C) and 60s F (16-21°C) to low 70s (22-23°C) during the day, whereas warm-season crops like 65°F (18°C) or higher at night and 75-80°F (24-27°C) during the day� When you browse through seed catalogs searching for varieties of crops to grow, information will be available on their optimum temperatures� If not, simply look up on the Internet search engines for crops and their ideal temperatures� Of course, temperature can only be regulated in greenhouse or indoor gardening, not outside in prevailing weather conditions� This, however, is significant with hydroponic growing as in most cases hydroponic culture is most applicable to greenhouse or indoor growing�
Under very-high temperatures and especially with low relative humidity (RH) (percentage of moisture in the air) plants will slowdown in growth due to their inability to keep their tissues at optimum temperatures� This causes the closing of stomata (small pores particularly numerous on the lower sides of leaves) to partially or fully close� The closing of the stomata blocks the entrance of CO2 into the leaves and restricts water loss that in effect reduces cooling of the plants through evapotranspiration (loss of water by evaporation and transpiration)� It will then reduce water and resultant nutrient uptake slowing growth further� As was pointed out earlier, plants receive CO2 from the air as part of the photosynthesis process� Any environmental factors that are not at optimum levels for the specific crops, will restrict photosynthesis and subsequent plant growth and development (Figure 5�6)� When these environmental factors are restricting or limiting growth, they are termed “limiting factors�”
With hydroponic gardening in greenhouses and indoors, you must be aware of the optimum levels of light, temperature, CO2, and RH for your crops, monitor and regulate them at levels best for crop growth to maximize yields� This is discussed in more detail in Section V under greenhouse and indoor growing�
