Chapter 37 Plant Nutrition Fig. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Outline of this chapter Nutritional requirement of plants The role of soil in plant nutrition The special case of nitrogen as a plant nutrient Nutritional adaptations: symbiosis of plants and soil microbes Nutritional adaptations: parasitism and predation by plants

Nutritional requirement of plants The chemical composition of plants provides clues to their nutritional requirements. Plants require nine macronutrients and at least eight micronutrients. The symptoms of a mineral deficiency depend on the function and mobility of the element.

The chemical composition of plants provides clues to their nutritional requirements Researchers can analyze the chemical composition of plants after they are dried. More than 50 chemical elements have been identified, but not all of them are essential. Organic substances (for example, carbohydrate) account for about 95% of the dry weight, with inorganic substances making up the remaining 5%. Nonwoody plants contain 80-85% water, and plants grow mainly by accumulating water in the central vacuoles of their cells.

Plants do not get nutrients from one source

Plants require nine macronutrients and at least eight micronutrients Mineral nutients are essential chemical elements absorbed from the soil in the form of inorganic ions. Essential nutrient is a particular chemical element required for a plant to complete the life cycle. Hydroponic culture helps researchers to determine which of the mineral elements are actually essential nutrients.

Hydroponic culture Because most research has involved crop plants, little is known about the specific nutritional needs of uncultivated plants.

Essential nutrients in plants

Macronutrients and micronutrients Macronutrients (C, O, H, N, S, P, K, Ca, Mg) are elements required by plants in relatively large amounts; micronutrients (Fe, Cl, Cu, Mn, Zn, Mo, B, Ni) are elements required in very small amounts.

The symptoms of a mineral deficiency depend on the function and mobility of the element Although iron is not a chemical element in chlorophyll, iron deficiency still results in chlorosis (yellowing of the leaves) because it is required for chlorophyll biosynthesis. Mobility of the particular element will determine where the symptoms first showing up when it is not enough.

Deficiency of iron results in the same symptoms as magnesium deficiency

Diagnosis and treatment of nutrient deficiency Chemical analysis and observation of symptoms help plant physiologists and farmers to diagnose the cause of nutrient deficiency. Usually N, K, and P deficiencies are most common. Treatment of micronutrient deficiencies should be careful because overdose of some micronutrients can be toxic to plants.

Hydroponics can ensure optimal mineral nutrient supplement

The role of soil in plant nutrition Soil characteristics are key environmental factors in terrestrial ecosystems. Soil conservation is one step toward sustainable agriculture.

Soil characteristics are key environmental factors in terrestrial ecosystems. Texture and composition of soils are major factors in terrestrial ecosystems. Availability of soil water and minerals.

Texture and composition of soils Topsoil (see figure 37.5, p. 771) is where plant grows. It is a mixture of weathering solid rock, living organisms, and residue of partially decayed organic material called humus. Weathering of solid rock is achieved by water, temperature, and organisms like lichens, fungi, bacteria, mosses and the roots of vascular plants.

Topsoil is where plant grows

The texture of topsoil depends on the size of its particles The most fertile soils are usually loams, made up roughly equal amounts of sand, silt, and clay.

Loams Loamy soils have enough fine particles to provide a large surface area for retaining minerals and water, which adhere to the particles. But loams also have enough coarse particles to provide air spaces containing oxygen that can be used by roots for cellular respiration. Humus is the decomposing organic material formed by the action of bacteria and fungi on dead organisms, feces, fallen leaves, and other organic refuse. It prevents clay from packing together but is still porous enough for the aeration of the roots. It is also a reservoir of mineral nutrients that are returned gradually to the soil as microorganisms decompose.

The availability of soil water and minerals Soil particles have electrically charged surfaces. Some of the water adheres so tightly to the hydrophilic soil particles that it cannot be extracted by plants. The film of water bound less tightly to the particles is the water generally available to plants.

Minerals are attracted to soil particles by electrical attraction Because the surfaces of clay particles are negatively charged, positively charged minerals like K+, Ca2+, and Mg2+ are adhered to the surface. Plant roots will secrete H+ in the soil to displace those ions.

Minerals are attracted to soil particles by electrical attraction On the other hand, negatively charged minerals like nitrate (NO3-), phosphate (H2PO4-), and sulfate (SO42-) are usually not bound tightly to soil particles and thus tend to leach away more quickly.

Soil conservation is one step toward sustainable agriculture Because it may take centuries for a soil to become fertile through the breakdown of rock and the accumulation of organic material, it is important to manage the soil wisely. Agriculture is unnatural. Fertilizers, Irrigation, Erosion, and Phytoremediation are all important for soil conservation.

Fertilizers – chemical fertilizers Chemical fertilizers are minerals mined or prepared by industrial processes. They are usually enriched in nitrogen, phosphate and potassium, the three mineral elements most commonly deficient in farm and garden soils. A fertilizer marks “10-12-8” is 10% nitrogen, 12% phosphorus, and 8% potassium.

Fertilizers – organic fertilizers Organic fertilizers are of biological origin and contain organic material that is in the process of decomposing. Manure, fishmeal, and compost are organic fertilizers. They release minerals gradually. On the other hand, chemical fertilizers are available immediately but may not be retained by the soil for long.

How to use fertilizers wisely Monitoring soil pH can help farmers to use fertilizers wisely. Soil pH not only affects cation exchange but also influences the chemical form of all minerals. Adding sulfate to lower the pH or using calcium carbonate or calcium hydroxide (liming) to raise the pH are the usual practices for farmers.

Irrigation Irrigation in arid region can gradually make the soil so salty that it becomes completely infertile (salinization). Irrigation in arid region will also dry the river. Drip irrigation instead of flooding fields will reduce the risks of running out of water or losing farmland to salinization. Crops require less water are also being bred.

Erosion Water and wind erosion make thousands of acres of topsoil lost every year. Windbreaks (rows of trees dividing fields) and contour tillage (planting crops around the hills) will prevent the loss of topsoil by wind or rain.

Phytoremediation Plants like Thlaspi caerulescens can accumulate zinc in its shoots at concentrations that are 300 times the level that most plants can tolerate, so it can be used to remove zinc in an area polluted with zinc.

The special case of nitrogen as a plant nutrient The metabolism of soil bacteria makes nitrogen available to plants Improving the protein yield of crops is a major goal of agricultural research

The metabolism of soil bacteria makes nitrogen available to plants Although the atmosphere is nearly 80% nitrogen, plants cannot use it until it is converted to ammonium (NH4+) or nitrate (NO3-). Nitrate (NO3-) is the form plant acquire mostly.

Nitrogenase reaction: N2+8e-+8H++16ATP  2NH3+H2+16ATP+16Pi

Improving the protein yield of crops is a major goal of agricultural research Many plants have a low proteins content, and the proteins that are present may be deficient in one or more of the amino acids that humans need from their diet. Breed new variety of crops that are enriched in protein or improve the productivity of symbiotic nitrogen fixation will help to improve the protein yield of crops.

Nutritional adaptations: symbiosis of plants and soil microbes Symbiotic nitrogen fixation results from intricate interactions between roots and bacteria Mycorrhizae are symbiotic association of roots and fungi that enhance plant nutrition

Symbiotic nitrogen fixation Members of the legume family have swellings called nodules composed of plant cells that contain nitrogen-fixing bacteria of the genus Rhizobium (root living), which will take the form of bacteroids inside the nodules.

Symbiotic nitrogen fixation The symbiotic relationship between a legume and nitrogen-fixing bacteria is mutualistic. Plant provides bacteria with carbohydrates and other organic compounds and bacteria supplies plant with fixed nitrogen.

Symbiotic nitrogen fixation Leghemoglobin makes the nodules pink.

Molecular biology of root nodule formation

Mycorrhizae Mycorrhizae also forms mutualistic relationship with plant. Mycorrhizae acquires sugar from the host plant and plant extends its areas for water and other mineral (for example, phosphate) uptake. There are two types of mycorrhizae: ectomycorrhizae and endomycorrhizae.

Ectomycorrhizae The mycelium (mass of branching hyphae) of ectomycorrhizae forms a dense sheath over the surface of the root. They do not penetrate the root cells. It is especially common in woody plants.

Endomycorrhizae Endomycorrhizae do not form a dense mantle around the root. It penetrates root cell walls, grows into a tube formed by invagination of the root cells’s membrane.

Agricultural importance of mycorrhizae Maize seedlings grow less well without mycorrhizae.

Mycorrhizae and root nodules may have an evolutionary relationship Because mutations in these early nodulin genes block development of both root nodules and mycorrhizae in legumes that form both structures, researchers suspected mycorrhizae and root nodules are somehow related in evolution.

Nutritional adaptations: parasitism and predation by plants Parasitic plants extract nutrients from other plants Carnivorous plants supplement their mineral nutrition by digesting animals.

mistletoe

Carnivorous plants Carnivorous plants usually live in acid habitats where soil is poor in nitrogen, so they have to obtain some nitrogen and minerals by killing and digesting insects and other small animals.