Plant Nutrition of Greenhouse Crops
Essential Plant Elements
Plants require certain nutrients to grow properly. Sixteen elements are considered to be essential for their growth and development. They are: carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, iron, manganese, copper, zinc, boron, molybdenum, and chlorine. Plants are nonselective in absorbing nutrient elements from the growing medium. This means that the presence of a particular element in a plant tissue does not indicate that the element is essential for growth. For example silicon, chromium and cobalt have been found in many plant species but it is not known if they are essential for growth. Out of the 16 essential elements, carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur are required in relatively large amounts and that is why these elements are referred to as macro or major elements. The remaining elements in the above list are micro-nutrients. They are required in small amounts to carry out different essential functions in the plant.
Nutrient Mobility within the Plant
All nutrients move relatively easily from the root to the growing portion of the plant through the xylem. Interestingly, some nutrients can also move from older leaves (the source) into developing flowers or leaves (the sink) if there is a deficiency of that nutrient. Knowing which nutrients are “mobile” (i.e., able to move) is very useful in diagnosing plant nutrient deficiencies because if only the lower leaves are affected, then a mobile nutrient is most likely causing the deficiency.
Nutrient Mobility in the Soil
The previous section discussed the mobility of each nutrient within the plant. The list of mobile and immobile nutrients is somewhat different in soil than in the plant, yet is very important in understanding why some nutrients limit growth more than others. Specifically, nutrient mobility affects how the grower fertilizes the plants. Table 15.2 illustrates the relative mobility of each of the 13 essential mineral nutrients. The form of each nutrient is very important in predicting its mobility; therefore, the actual ions are shown rather than just the general nutrient.
Nitrogen plays a major role in the growth and development of all parts of the plant. Plants use nitrogen to build essential compounds including proteins, enzymes, amino acids, nucleic acids, and pigments including chlorophyll and anthocyanins of fruit.
Nitrogen Deficiency Symptoms
A nitrogen deficiency (lack) causes the leaves of plants to lose their dark green color. The resulting chlorosis (loss of green color) usually shows first on the lower, older basal leaves of the plant. Purple to red discoloration may develop in older leaves and stems as they turn chlorotic in some species, such as begonia, marigold, and pansy.
More often, there is a problem from excess nitrogen application. Plants receiving excess nitrogen usually results in succulent growth, very dark green color, weak spindly growth, and not much fruit.
Phosphorus has several important functions. It must be available in sufficient quantities early in the life of the plant to assist in cell division and differentiation of the cells into the tissues of the reproductive parts of plants. It is also required for root growth and formation of buds. It is also required for sugar metabolism. Both the respiratory and photosynthetic processes require phosphorus for high-energy phosphate bonds.
Phosphorous Deficiency Symptoms
The plant becomes severely stunted, and at the same time the foliage becomes deeper green than normal. Phosphorous is very mobile in the plant. Deficiencies therefore show up on the older leaves of the plant because phosphorous is translocated out of these leaves to satisfy needs in the new growth.
Excess phosphorus can induce nitrogen and micronutrient deficiencies of zinc and iron.
Potassium plays an important role in the regulating of the opening and closing of stomata and in water retention.
Potassium Deficiency Symptoms
Once in the plant, potassium is very mobile and is transported to young tissues rapidly. Deficiency symptoms for potassium show up first on older leaves first with a marginal necrosis, or browning. As the deficiency becomes more severe, the leaf begins to lose some of its green color and turns yellow.
Excess potassium can cause nitrogen deficiency in plants and may affect inhibit the uptake of cations such as magnesium or calcium.
Calcium appears to influence the growth of the apical meristems of the plant. The calcium prevents the leaching of mineral salts from the cells. Much of the stiffness of plants is due to calcium. Calcium is immobile and is not translocated from older to younger leaves.
Calcium Deficiency Symptoms
Calcium is immobile and is not translocated from older tissue to younger regions of growth. As a result, a calcium deficiency first appears at the youngest growing points of the plant, the shoot tips, and the root tips. The edges of immature tissues become necrotic. Young leaves may develop variable patterns of chlorosis and distortion, such as dwarfing or crinkling.
Excessive calcium content will produce magnesium or potassium deficiency in plants, although this depends on the concentration of these elements.
Magnesium has several functions in the plant. It is the central component of the chlorophyll molecule— the green pigment responsible for photosynthesis in green plants.
Magnesium Deficiency Symptoms
Magnesium is somewhat mobile within plant tissues. Thus, symptoms of a lack of magnesium show up first on older, lower leaves in the form of interveinal (between the veins) chlorosis, which proceeds toward the younger leaves as the deficiency becomes more severe.
Excess magnesium interferes with calcium uptake. Excessive potassium may require the addition of magnesium to balance these nutrients for optimum uptake.
Sulfur serves many functions in plants. It is used in the formation of amino acids, proteins, and oils. It is necessary for chlorophyll formation, promotes nodulation in legumes, helps develop and activate certain enzymes and vitamins, and is a structural component of two of the 21 amino acids that form protein.
Sulfur Deficiency Symptoms
Resembles nitrogen deficiency in that older leaves become yellowish green and the stems become thin, hard, and woody. Some plants show colorful orange and red tints rather than yellowing.
High sulfur levels in the growing medium can compete with and induce nitrogen deficiency.
Iron plays an important role in chlorophyll formation, in photosynthesis and respiration, and cell strengthening.
Iron Deficiency Symptoms
Iron deficiency symptoms appear as interveinal chlorosis on young leaves that can progress throughout the shoot and result in a shot tip death in severe cases.
The symptoms of excess iron include bronzing and stippling of leaves. The leaf discoloration is caused by the plant creating enzymes to control free radicals that are present in high iron levels. Some plants that are prone to iron toxicity include tomatoes, basil, phlox, and impatiens.
Boron is used by plants during cell division and is required for development of tissue near the tips of shoots and roots. It is also required for the growth of the pollen tube during flower pollination and thus fruit and seed production.
Boron Deficiency Symptoms
As with other immobile elements, deficiency symptoms with boron first appear on new leaves. Often new leaves will develop necrotic spots and will be wrinkled. Frequently, the buds at the ends of stems (apical buds) will die. In severe cases, older leaves may become irregularly shaped. The growing point often aborts (apical meristem), which leads to proliferation of branches known as witches' broom.
Boron can be highly toxic to some plants at low levels. Boron toxicity most often occurs either as a result of excess boron containing fertilizer use or because the soil is naturally high in boron.
Manganese is needed in chloroplast formation and photosynthesis, nitrogen metabolism, and synthesis of various enzymes.
Manganese Deficiency Symptoms
A manganese deficiency appears first in the new growth. Manganese deficiency is often characterized by interveinal chlorosis of young leaves, sometimes followed by the formation of tan spots in the chlorotic areas between the veins.
Toxicity very often begins with interveinal chlorosis of young leaves due to iron deficiency caused by high manganese antagonism of iron uptake. Manganese toxicity takes the form of burning of the tips and margins of older leaves or formation of reddish-brown spots on older leaves.
Copper is needed for the proper function of enzyme systems. It stabilizes chlorophyll and delays its breakdown. Thus, copper helps to increase the effective life of leaves.
Copper Deficiency Symptoms
Typically the symptoms start as cupping and a slight chlorosis of either the whole leaf or between the veins of the new leaves. Within the chlorotic areas of the leaf, small necrotic spots may form, especially on the leaf margins.
Reduced growth followed by symptoms of iron chlorosis, stunting, reduced branching, abnormal darkening and thickening of roots. This element is essential but extremely toxic in excess.
Zinc activates enzymes that are responsible for the synthesis of certain proteins. It is used in the formation of chlorophyll and some carbohydrates, conversion of starches to sugars and its presence in plant tissue helps the plant to withstand cold temperatures.
Zinc Deficiency Symptoms
Like most micronutrients, zinc is immobile, meaning the deficiency symptoms occur in the new leaves. Symptoms vary depending on the crop. Typically they are expressed as some varying pattern of chlorosis of the new leaves (often interveinal) and necrotic spots may form on the margins or leaf tips.
Toxicity symptoms are expressed as smaller leaf size, chlorosis of the newer leaves, necrotic leaf tips, retarded growth of the entire plant, and/or reduced root growth.
Molybdenum is an essential component in two enzymes that convert nitrate into nitrite (a toxic form of nitrogen) and then into ammonia before it is used to synthesize amino acids within the plant. Plants also use molybdenum to convert inorganic phosphorus into organic forms in the plant.
Molybdenum Deficiency Symptoms
Molybdenum is closely linked to nitrogen that, its deficiency can easily resemble nitrogen deficiency. When a plant lacks molybdenum, nitrates are not absorbed properly, even when there are large amounts of nitrates in the soil.
Marginal leaf scorch and abscission as found in typical salt damage are typical symptoms. Because of the intensity of interactions, toxic symptoms will normally manifest themselves as deficiencies of other nutrients such as copper.
Chlorine occurs predominantly as chloride (Cl-) in soil and plant. It is an essential micronutrient of higher plants and participates in several physiological metabolism processes. Its functions in plant growth and development include osmotic and stomatal regulation, evolution of oxygen in photosynthesis, and disease resistance and tolerance.
Chlorine Deficiency Symptoms
Symptoms of a chlorine deficiency are wilting followed by a mild chlorosis and bronzing of older leaves.
Excess chloride causes scorching or firing (necrosis) of leaf tips or margins, bronzing, premature yellowing, and abscission (dropping) of leaves. When excessive, chloride can be as a major component of salinity stress and toxic to plants.
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