Careful observations of the growth of plants can furnish direct evidence of their nutritional conditions. Metabolic disruptions resulting from nutrient deficiencies provide links between the function of an element and the appearance of a specific visible abnormality. Symptoms of disorders, therefore, provide a guide to identify nutritional deficiencies in plants. Careful experimental work and observations are needed to characterize symptoms. For example, nitrogen is needed for protein synthesis and for chlorophyll synthesis, and symptoms appear as a result of the disruption of these processes. Symptoms of nitrogen deficiency appear as pale-green or yellow leaves starting from the bottom and extending upward or sometimes covering the entire plant. Magnesium deficiency also affects protein synthesis and chlorophyll synthesis, but the symptoms may not resemble those of nitrogen deficiency, which affects the same processes. Experience is necessary to distinguish the symptoms of nitrogen deficiency from symptoms of magnesium deficiency or in the identification of the deficiency of any nutrient.
As mentioned above, for example, nitrogen deficiency appears across plant species as chlorosis of lower or of all leaves on plants. Advanced stages of nitrogen deficiency can lead to leaf death and leaf drop. Nitrogen-deficient plants generally are stunted and spindly in addition to showing the discoloration that is imparted by chlorosis. Potassium-deficient plants have marginal and tip necrosis of lower leaves. On the other hand, for elements that are immobile (not transported in phloem) or slowly mobile in plants, the deficiency symptoms will appear on the young leaves first. The symptoms might appear as chlorosis, as with sulfur, iron, manganese, zinc, or copper deficiency, or the symptoms might be necrosis of entire plant tips, as occurs with boron or calcium deficiency. Brooms or rosetting may occur in cases where deficiencies (e.g., copper or zinc) have caused death of the terminal bud and lateral buds have grown or where internode elongation has been restricted by nutrient (e.g., zinc) deficiencies. Accumulation of anthocyanin, exhibited by reddening of leaves, may indicate phosphorus deficiency, although nitrogen deficiency can lead to a similar development.
Some people try to distinguish the two deficiencies by noting whether the symptoms of reddening develop between the veins (phosphorus deficiency) or along the veins (nitrogen deficiency). Stunting is a good indication of nutrient deficiency, but often stunting cannot be recognized unless a wellnourished plant is available as a standard of comparison. A stunted plant may have normal color and not be recognized as being deficient until abnormal coloration develops with advanced stages of deficiency. In some cases, symptoms may not develop during the growth cycle of crops, but yields may be suppressed relative to plants that have optimum nutrition. Hidden hunger is a term applied to cases where yield suppression occurred but symptoms did not develop.
Deficiency symptoms can occur at any stage of growth of a plant. The most typical symptoms are those that appear early in the cycle of deficiency. Early diagnosis of deficiencies may also allow time for remedial action to take place. Generally, however, if symptoms have appeared, irreparable damage has occurred, with quantity or quality of yields being suppressed or diminished with annual crops or with slowing or damaging of growth and development of perennial crops. Also, symptoms that resemble nutrient deficiency can develop on plants as a result of conditions that are not related to nutrient deficiencies, for example, drought, wet soils, cold soils, insect or disease infestations, herbicide damage, wind, mechanical damage, salinity, or elemental toxicities. Deficiency symptoms are only one of several diagnostic criteria that can be used to assess the nutritional status of plants. Plant analysis, biological tests, soil analysis, and application of fertilizers containing the nutrient in question are additional tools used in diagnosis of the status of plant nutrition.
© 2018 Biocyclopedia | All rights reserved.