All life on earth depends on the energy captured by plants. How do plants discriminate among ions to be taken up by them? How do they "exclude ions" (or in simple terms if you allow me to put a corollary) "vomit out" , i.e., sodium exclusion from membrane?
Plants have a very interesting mechanism of sodium exclusion (Schubert et al . 2009; Pitann and Muehling, 2009; Pitann et al 2009a, b, c ; Pitann et al. 2011; Schubert 2010, 2011a,b, c, d ; Hanstein et al. 2011; Wakeel et al. 2011a and b ) They have pumps which exclude Na ions toxic to plant life. Alternatively, they store sodium in their cell vacuoles. The role of PEP carboxylase in this process is under investigation ( Hartig et al. 2009; Kumar and Schubert 2010)
The supply and absorption of chemical compounds needed for growth and metabolism may be defined as nutrition and the chemical compounds required by an organism termed nutrients. The mechanisms by which nutrients are converted to cellular constituents or used for energetic purposes are metabolic processes. The term metabolism encompasses the various reactions occurring in a living cell in order to maintain life and growth. Nutrition and metabolism are thus very closely interrelated.
The essential nutrients required by higher plants are exclusively inorganic , a feature distinguishing these organisms from man, animals and many species of microorganisms which additionally need organic foodstuff to provide energy. Plants do not need organic food stuff. By contrast, plants absorb light energy from solar radiation and convert it to chemical energy in the form of organic compounds, whilst at the same time taking up mineral nutrients to provide the chemical elements also essential for growth.
Essential chemicals are mainly covered by the expression to remember “C HOPKINS CaFeMg” that transcribes to Carbon (C), Hydrogen (H), Oxygen (O), Phosphorus (P) Potassium ( K) Nitrogen (N) Sulphur (S), Calcium (Ca), Iron (Fe) , Magnesium (Mg). However according to the criteria proposed by Arnon and Stout (1939) (and accepted by Mengel and Kirkby (2001) in 5th edition of the book ) other essential elements for plant growth are : Manganese (Mn), Copper (Cu), Silicon (Si), Zinc (Zn), Molybdenum (Mo) Boron (B) , Chlorine (Cl), Nickel (Ni) and Sodium (Na).
These chemicals must meet out the criteria laid out for essential elements viz deficiency does not allow completion of life cycle, deficiency is specific to element in question, and element in question must be involved in essential metabolic process or required for action of an enzyme system. How plants adopt a technique to pick and choose and absorb a particular nutrient from the soil and leave others? When to stop uptake of a particular mineral ? Which part of the soil particle allows plants to absorb minerals best? What is the role of organic manure in this ? Do microorganisms help plants absorb minerals ? How does Mycorrhiza help plants grow better ? What strategy is adopted by Gymnosperms like Cycas and Pinus to absorb minerals from soil ? How does phosphate solubilizing bacteria help plants to obtain Phosphprous from the soil? (Steffens 2011a, b; Aziz et al. 2011) How does plant membrane allow selective permeability for ions i.e. allow specific ions in and stop others from entering the plant from the soil solution?
These are some of the questions which have been answered partly by botanists during last two centuries once it was established by Justus von Liebig that plants require mineral elements for growth besides water, carbon di oxide light and chlorophyll. Although minerals are stored in solid phase in soil , it is liquid phase in which they are transported in the soil to the plant roots and their uptake by plants can be “passive “ or “active”. How the cations become accessible to the plants and what is the role of organic fertilizers in it?
Absorption and exchange process regulate nutrient interactions between the soild and liquid phases of the soil. Colloidal soil particles both clay (particle size ≤ 2 µm ) and humus play an important role. Because of their minute size they expose a very large surface area in the soil which mostly bears negatively charged sites. Negative charges can also occur from dissociation of H+ from weak acids. Addition of FYM helps make soil slightly acidic too. This is particularly important in producing negatively charged sites on organic soil particles. The negatively charged surfaces of these various soil particles attract cations( positively charged ions) such as Calcium, Magnesium, Potassium Sodium as well as Al cation species and Magnesium ions. The cations electrostatically adsorbed to the negatively charged surface of colloidal particles are subjected to interionic and kinetic forces.
The soil property is of extreme importance in determining the physical behavior of the soil (Qayyum et al.2011) The organic carbon serves as food for microbes, fungi, and bacteria and resistant decomposable organic carbon is the precursor for the formation of humic substances. Under continental climatic conditions soils are frozen during winter so that microbial activity is inhibited and soils rich in organic matter develop. In tropical conditions with warm soil temperature all year around and with adequate amounts of water for most of the time , organic matter is quickly decomposed. This is typically the case after clearing the forests in tropical area the organic matter is quickly decomposed and regeneration of forest again becomes very difficult. This also happens because decomposition of organic carbon leads to loss of water holding capacity and the aggregate become less stable and the soils more prone to erosion by wind and water with ensuring loss of soil fertility which is extremely difficult to restore ( Dayly, 1995). To cut the story short organic fertilizer or organic farming is good for soil health and growing plants without use of pesticides is a healthy practice which needs to be promoted. However how and why a food is called as 'organic' food needs closer examination.
I have tried to simplify very complex phenomenon of nutrient cycle for general understanding of broader audience.
Acknowledgement: I am grateful to Professor Dr Sven Schubert for his valuable guidance, during my research stay at the Institut fur Pflanzenernaehrung der Justus Liebig Universitat and Alexander von Humboldt Foundation for providing financial support. I am also thankful to Professor Dr Konrad Mengel for his book.
Citations and some of the selected references for further reading :
Arnon, D.I. and Stout, P. R. (1939) The essentiality of certain elements in minute quanitity for plants with special reference to copper. Plant Physiol. 14: 371-375
Aziz, T., D. Steffens, Rahmatullah und S. Schubert (2011) Variation in phosphorus efficiency among Brassica cultivars II: Changes in root morphology andcarboxylate exudation. J. Plant Nutr. 34: 2127-2138Dayly, G.C. (1995) Restoring value to the world's degraded lands. Science. 269. 350-354.
Hanstein, S., X. Wang, X. Qian, P. Friedhoff, A. Fatima, Y.Shan, K. Feng und S. Schubert(2011) Changes in cytosolic Mg 2+ levels can regulate the activityof the plasma membrane H+ - ATPase in maize. Biochem. J. 435: 93-101
Hatzig, S., A. Kumar, A. Neubert und S. Schubert (2010) PEP-carboxylaseactivity: A comparison of its role in a C4 and a C3 species under salt stress.J. Agron. Crop Sci. 196, 185-192.
Kumar, A. und S. Schubert (2010) The role of genotypicdifferences in PEP carboxylase (PEPcase) activity between sensitive andresistant maize genotypes in salt resistance. Jahrestagung der Deutschen Gesellschaft für Pflanzenernährung,Hannover, 30.09.-02.10.2010.
Pitann, B. und K. H. Mühling (2009)Comparative proteomeanalysis of maize (Zea mays L.) expansins under salinity. J. Plant Nutr. SoilSci. 172: 75-77
Pitann, B., Schubert, S. und Mühling, K. H.(2009) Decline inleaf growth under salt stress is due toan inhibition of H+ pumping activity andincrease in apoplastic pH of maize (Zea mays L.)leaves. J. Plant Nutr. SoilSci. 172: 535-543
Pitann, B., Zörb, C. und Mühling, K. H.(2009) Salzstress beiKulturpflanzen: Bedeutung für dieweltweite Pflanzenproduktion. Journal fürVerbraucherschutz und Lebensmittelsicherheit: 4,202-206
Schubert, S., A. Neubert, A. Schierholt, A. Sümer und C.Zörb (2009) Development of salt-resistantmaize hybrids: The combination ofphysiological strategies using conventional breeding methods. Plant Sci. 177,196-202
Pitann, B., T. Kranz, A. Walter, U. Schurr, und K.H. Mühling(2011a) Effect of salt stress on growth of Vicia faba as affected by apoplasticpH and diurnal cycle. Environ. Exp. Bot. 74: 31-36
Qayyum, M. F., D. Steffens, H. P. Reisenauer und S. Schubert(2011) Kinetics of carbon mineralization of biochars compared with wheat strawin three soils. J. Environ. Qual. 40: 1-11
Schubert, S.(2010).Improvement of crop performance undersalinity salt stress: tolerance versus avoidance. Jahrestagung der DeutschenGesellschaft für Pflanzenernährung, Genetics of Plant Mineral Nutrition.Hannover, 30.09.-02.10.
Schubert, S.(2011a) Salt resistance of crop plants – Physiological characterization of amultigenic trait. In: The MolecularBasis of Nutrient Use Efficiency in Crops (M. Hawkesford and P. Barraclough,Hrsg.) Wiley Blackwell, Ames, USA, pp. 443-455
Schubert, S.(2011b) Salt resistance of crops – A tool to combatsoil salinity. In: Proc. Int. Conf. Soil, Plant and Food Interactions.September 06.-09.2011, Mendel University in Brno, Faculty of Agronomy. Brno, p.15
Schubert, S.(2011c) Salt resistance of crops – a tool tocombat soil salinity. International conference “Soil, Plant and FoodInteractions”, Mendel University, Brno, Tschechien, 06.09.2011.
Schubert, S.(2011d) Development and potential ofsalt-resistant maize hybrids. Key Note Address,Golden Jubilee Celebrations,University of Agriculture, Faisalabad, Pakistan, 21.11.2011.
Steffens, D (2011a) Phosphorus availability of rockphosphate and single superphosphate as related to soil structure and soilmoisture. Ann. Agr. Sci. 9: 82-87
Steffens, D (2011b) Organic soil phosphorus considerablycontributes to plant nutrition but is neglected by routine soil testing methods, IFZ-Lunch Time Seminar,16.02.2011.
Wakeel, A., A. R. Asif, B. Pitann und S. Schubert (2011a) Proteomeanalysis of sugar beet (Beta vulgaris L.) elucidates the constitutiveadaptation during the first phase of salt stress. J. Plant Physiol. 168:519-526.
Wakeel, A., M. Farooq, D. Steffens und S. Schubert (2011b)Potassium substitution by sodium in plants. Crit. Rev. Plant Sci. 30: 401-413