Role of Primary Nutrients in Vegetative Growth of Plants
Authors: SIRAZUDDIN and ASHWINI KUMAR
All plants must obtain a number of inorganic mineral elements from their environment to ensure successful growth and development of both vegetative and reproductive tissues. These minerals serve numerous functions: as structural components in macromolecules, as cofactors in enzymatic reactions, as osmotic solutes needed to maintain proper water potential, or as ionized species to provide charge balance in cellular compartments.
Primary minerals and vegetative growth
Depending on the plant species, developmental stage, and organ, the nitrogen content required for optimal growth varies between 2 and 5 % of the plant dry weight. As a constituent of all amino acids and proteins (and thus all enzymes), nitrogen serves a central role in cellular metabolism. Additionally, as a component of nucleotides and nucleic acids (deoxyribonucleic acid (DNA) and ribonucleic acid (RNA)), nitrogen is critical for the transcription, translation, and replication of genetic information. Nitrogen is obtained from the soil environment either as the ammonium or nitrate ions, with nitrate being chemically reduced within the plant to ammonium prior to incorporation into organic molecules. Nitrogen is also a major structural component of chlorophyll. When the supply of nitrogen is suboptimal, growth is retarded; nitrogen is mobilized in mature leaves and retranslocated to areas of new growth. Typical nitrogen-deficiency symptoms, such as enhanced senescence of older leaves, can be seen. An increase in nitrogen supply not only delays senescence and stimulates growth but also changes the plant morphology in a typical manner, particularly if the nitrogen availability is high in the rooting medium during early growth. An increase in shoot-root dry weight ratio with an increase in nitrogen supply takes place in both perennial and annual plant species (Levin et al., 1989; Olsthoorn et al., 1991). This increase in shoot-root ratio might even be larger in terms of shoot and root length (Klemn, 1966), a shift which is unfavorable for the acquisition of nutrients and water from soil at later growing stages. Yoshida et al. (1969) reported that the length, width and area of the leaf blades increase, but the thickness decreases. In addition, the leaves become increasingly droopy which interferes with the light interception. Nitrogen-deficient plants are characterized by the enhanced growth of the root system and retarded shoot growth, which are closely related to high cytokinin/ABA ratio in roots and low ratio in shoots (Mardanov et al., 1996). In cereals, the enhancement of stem elongation by nitrogen increases the susceptibility to lodging. Zhao et al. (2004) N deficiency suppressed plant growth and DM accumulation and allocation. Decreased plant biomass production due to N shortage was associated with reductions in both leaf area and leaf photosynthetic capacity (Sinclair, 1990) and was mainly attributed to a smaller leaf area in sorghum. Sergio and Andrade reported that nitrogen deficiency delayed both vegetative and reproductive phenological development, slightly reduced leaf emergence rate, and strongly diminished leaf expansion rate and leaf area duration.
Phosphorus is a structural component of numerous macromolecules, including nucleic acids, phospholipids, certain amino acids, and several coenzymes. It has a significant role in energy transfer via the pyrophosphate bond in ATP, and the attachment of phosphate groups to many different sugars provides metabolic energy in photosynthesis and respiration. Phosphorus is absorbed by plants largely as the primary or secondary orthophosphate anions, H2PO4- and HPO42-. The phosphorus requirement for optimal growth is in the range of 0.3-0.5 % of the plant dry matter during the vegetative stage of growth. The probability of phosphorus toxicity increases at contents higher than 1% in the dry matter. However, many tropical food legumes are rather sensitive and toxicity may occur already at phosphorus contents in the shoot dry matter of 0.3-0.4 % in pigeon pea and 0.6-0.7 % in black gram. An adequate supply of P is essential from the earliest stages of plant growth. Early season deficiencies of P can lead to restrictions in crop growth from which the plant will not recover, even when P supply is increased to adequate levels. Moderate P stress may not produce obvious deficiency symptoms. However, with more severe P deficiency, plants become dark green to purplish in colour (Hoppo et al., 1999). Phosphorus deficiency can reduce both respiration and photosynthesis but, if respiration is reduced more than photosynthesis carbohydrates will accumulate, leading to dark green leaves (Glass et al., 1980). A deficiency can also reduce protein and nucleic acid synthesis, leading to the accumulation of soluble nitrogen (N) compounds in the tissue, and ultimately resulting in cell growth being delayed and potentially stopped. As a result, symptoms of P deficiency include decreased plant height, delayed leaf emergence, reductions in tillering, secondary root development, and dry matter yield and seed production.
In plants facing deficiency of phosphorus, reduction in leaf expansion, leaf surface area (Frendeen et al., 1989) and also number of leaves (Lynch et al., 1991) are the most striking effects. Leaf expansion is strongly related to the expansion of epidermal cells and this process might be particularly impaired in phosphorus deficient plants for various reasons, for example low phosphorus content of epidermal cells and decrease in root hydraulic conductivity (Radin, 1990).
In addition, P deficiency has been suggested to reduce tillering (Woodward and Marshall, 1988; Sato, et al., 1996), the rate of individual leaf expansion (Radin and Eidenbock, 1984), and the rate of assimilate production per leaf area (Rao and Terry, 1989; Jacob and Lawlor, 1991). The root:shoot ratio of crops tends to increase with early season P deficiency (Brenchley 1929, Schjorring and Jensen 1984). Growth reduction is generally greater in the shoot than in the root, allowing the plant to maintain root growth and encounter and extract P from the soil. The growth of tops and roots closely paralleled the distribution of P between the plant parts. Where P supply was low, the proportion of P held in plant roots was higher than where the P supply was moderate. At higher P status, there was also a relative increase in root P as compared to shoot P. This may imply P retention by the root to meet its 65 requirements at low concentration, P export to the shoot at sufficient concentrations, and P retention by the root at high concentration to avoid P toxicity in the shoot (Schjorrring and Jensen 1984).
In spring wheat and intermediate wheat grass, maximum tiller production was obtained when P was supplied in the nutrient culture for the first five weeks of growth and longer periods of available P did not increase the number of tillers produced (Boatwright and Viets 1966). In field-grown corn, P deficiency slows the rate of leaf appearance and leaf size, particularly in the lower leaves (Barry and Miller 1989, Pellerin et al., 2000). With less leaf growth and solar radiation interception caused by P deficiency, C nutrition of the plant may fall and so reduce subsequent nodal root emergence, which would have an additional impact on P uptake capacity.
Potassium is absorbed as the cation, K+, which is readily soluble in soil solutions. It is the most abundant cation in the cytoplasm and, because it is not metabolized, K+ and its accompanying anions contribute significantly to the osmotic potential of cells. Next to nitrogen, potassium is the mineral nutrient required in the largest amount by plants. The potassium requirement for optimal plant growth is in the range 2-5% of the plant dry weight of vegetative parts. When potassium is deficient, growth is retarded, and net retranslocation of potassium is enhanced from mature leaves and stems, and under severe deficiency, these organs become chloritic and necrotic, depending on the light intensity to which the leaves are exposed (Marschner and Cakmak, 1989). Lignification of vascular bundles is also impaired (Pissarek, 1973), a factor which might contribute to the higher susceptibility of potassium-deficient plants to lodging. Potassium deficiency causes yellowing and chlorosis to the edge and tip of older leaves, with progressive senescence. Plants may be stunted and exhibit excessive basal tillering. During rapid vegetative growth, the rapid uptake of nitrogen as negatively charged nitrate ions (NO3 -) is normally balanced by a similar uptake of positively charged potash ions (K+) which maintains the electrical neutrality of the plant. If potash supply is limiting, the uptake and utilization of nitrogen will be restricted and plant growth will be affected similarly to nitrogen deficient plants.
About Author / Additional Info:
I am currently pursuing Ph. D in Agronomy from G. B. Pant University of Agriculture and Technology, Pantnagar. I have also served as Assistant Professor for 2 years