101N. Tuteja and S. Singh Gill (eds.), Plant Acclimation to Environmental Stress,DOI 10.1007/978-1-4614-5001-6_5, Springer Science+Business Media New York 2013
The role of mineral nutrition is of paramount importance for the cultivation of plants. In fact, yield of most crop plants increases linearly within limits with the amount of mineral nutrients that they absorb (Franz 1983 ; Loomis and Conner 1992 ) . Using balanced mineral nutrition, the plants maximum genetic potential can be realized successfully (Wallace and Wallace 2003 ; Khan and Mohammad 2006 ) . The scienti c cultivation may improve the yield and quality of plants including those of medicinal value. Application of optimal amounts of fertilizer may help to meet the increasing demands of medicinal plants to a great extent. This would augment the yield and quality of the medicinal herbs ensuring their steady supply in the market.
India is called the botanical garden of the world and the treasure house of plant biodiversity (Ahmedullah and Nayar 1999 ) . It also displays a rich repository of medicinal plants and, hence, is the largest producer of medicinal herbs. Around 70% of Indias medicinal plants are found in tropical areas, mostly in forests spread across the Western and Eastern Ghats, the Vindhyas, the Chota Nagpur plateau, the Aravalis, and the Himalyas. Rest of the Indian medicinal plants grow in temperate and alpine areas and on higher altitudes (Purohit and Vyas 2004 ; Seth and Sharma 2004 ) . According to WHO, the international market of herbal products is US$62 billion, which is expected to reach US$5 trillion by the year 2050. However, India has presently less than 0.5% share in the global export market of medicinal plants in spite of being one of the largest producers of medicinal plants (Purohit and Vyas 2004 ; Bhattacharjee 2004 ) .
M. Naeem (*) M. M. A. Khan Moinuddin Plant Physiology Section, Department of Botany , Aligarh Muslim University , Aligarh , 202 002 , India e-mail: firstname.lastname@example.org
M. N. Khan Department of Biology , College of Science, University of Tabuk , Tabuk 71491 , Kingdom of Saudi Arabia
Chapter 5 Adverse Effects of Abiotic Stresses on Medicinal and Aromatic Plants and Their Alleviation by Calcium
M. Naeem , M. Nasir Khan , M. Masroor A. Khan, and Moinuddin
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Because of no side effects, the interest in medicinal products of plant has increased considerably all over the world (Groenewegen et al . 1992 ; Lipp 1996 ) . Due to a high demand of medicinal plants and herbal remedies in the world, the cultivation of these plants has signi cantly increased in the recent years. Since most medicinal plants are consumed raw, proper management of crop production is needed to achieve high quality plants (Hyden 2006 ) . Among the factors responsible for achieving higher yield, adequate nutrient supply is considered one of the most effective tools (Munsi, 1992 ; Yazdani et al ., 2004 ). Therefore, optimum concentra-tions of mineral nutrients should be set up to achieve the desired yield of MAPs.
Of the essential mineral nutrients, nitrogen, phosphorus, potassium, and calcium are considered to be of prime importance as they are required by plants in large quantities. These nutrients, by virtue of their function in the generation of energy, producing the building molecules, participating in the repair of protoplasm and regulation of metabolic process, and maintaining the physical organization and function of livings cells, play several important roles in enhancing crop production and maintaining the soil fertility. Balanced fertilizer application can play a vital role in sustaining high yield of medicinal plants and maintaining the fertility status of soils on long-term basis. Of the macro nutrient elements, calcium plays structural as well as physiological roles in plants. It is important in maintaining the stability of the cell walls and cell membranes and is also required in the maintenance of cell integrity with respect to membrane-bound proteins. Plants need calcium to continue normal function of their membranes, growth of the meristematic tissues and the youngest leaves, and to send signals in response to internal and external cues (Dordas, 2009 ). In fact, calcium (Ca) has attracted much interest in plant physiology and molecular biology because of its function as a second messenger in the signal conduction between environmental factors and plant responses in terms of growth, differentiation, and development (Price et al . 1994 ; Sanders et al . 1999 ; Rudd and Franklin-Tong 2001 ; Naeem et al. 2009a, b ) . Furthermore, calcium is highly required by medicinal legumes during nitrogen xation processes.
Calcium is known to protect the integrity of cell membranes, reduce membrane permeability, and prevent ion leakage caused by environmental stresses. It has emerged as a key secondary messenger and signal transducer of various environ-mental stress stimuli. The effect of calcium and the changes in its signature depend on the particular environmental stress, the rate of stress development, previous exposure to stress conditions, and the tissue type (Bin 1995 ) . The effects of Ca on shoot elongation, senescence process, and photosynthetic activity are dependent on its cytosolic concentration, which is governed by the activity of Ca channels in the plasma membrane (Knight 2000 ) . When taken up into the plant system, Ca is involved in the regulation of plant responses to various abiotic stresses by contributing either directly or indirectly in plant defense mechanisms. Exogenously applied Ca alleviates the stresses caused by salt, heat, and drought by regulation of antioxidant activities such as GR, superoxide dismutase (SOD), AP, and CAT, decreasing the membrane lipid peroxidation (LPO), and helping the plant cells to survive during stress conditions ( Jiang and Huang 2001a, b ; Nayyar and Kaushal 2002 ; Fu and Huang 2003 ; Khan et al . 2010 ) .
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The literature regarding importance of calcium nutrient in crop improvement (medicinal legumes and other MAPs) under normal and abiotic stress conditions has been reviewed in this article.
2 Functions of Calcium in Plants
A wide variety of diverse biochemical functions are initiated by the changes in cel-lular Ca. Calcium is particularly important for structural stability and functional integrity of biological membranes and tissues. An adequate supply of Ca maintains membrane integrity and contributes to ion selectivity of membranes (Marschner 2002 ; Grattan and Grieve 1999 ; Mengel and Kirkby 2001 ; White and Broadley 2003 ) . Calcium binds as pectate in middle lamella to strengthen the cell walls and plant tissues. The concentration of Ca in the cytosol is extremely low, ranging from 0.1 to 1.2 m M as free Ca. However, this low calcium concentrations is much essential for the functioning of certain key enzymes, including ATPases at the plasma mem-brane of roots and NADPH oxidases bound to plasma membrane (Marschner 2002 ) . Calcium also regulates various responses of plants to biotic stresses caused by micro-bial or pathogen attacks (Yang and Poovaiah 2002 ; Grant and Loake 2000 ; Sagi and Fluhr 2001 ) . During pathogen attacks in plants, a membrane-bound enzyme, resem-bling the neutrophil NADPH oxidase, was identi ed to contribute to the pathogen-induced oxidative burst of O 2 in plants that indirectly leads to the generation of other reactive oxygen species (ROS). NADPH oxidases are the major source of ROS produced during the oxidative stress in plants (Lamb and Dixon 1997 ; Sagi and Fluhr 2001 ) . Coordination between Ca and ROS production during pathogen attack is mediated mostly through NADPH oxidases. The activity of NADPH oxidase and the production of both O 2 and H 2 O 2 is enhanced in the presence of Ca either directly through the Ca binding domains (EF hands) located on the NADPH subunit (gp91phox) or indirectly by activating NAD kinase (Sagi and Fluhr 2001 ) .
There are several climatic factors that either in uence the availability of Ca in soil or restrict the transport of Ca inside the plant cells. The most common factor that in uences the availability of Ca in soils is soil acidity (Goenaga and Smith 2002 ) . Acid soils usually contain very high concentration of toxic Al, which limits availability as well as uptake of Ca and enhances precipitation of Ca in soils, caus-ing occurrence of Ca de ciency in plants (Marschner, 1995 ; Mengel and Kirkby 2001 ; Goenaga and Smith 2002 ) . Interestingly, under certain conditions, plants may suffer from Ca de ciency stress despite huge amounts of Ca in soils and high Ca concentration in plants. In such cases, the absorbance of Ca by roots or the translocation of Ca to the sink organs is prevented either by high humidity or due to the high salt concentration in the growth medium (Choi et al . 1997 ) . For the produc-tion of fruits and vegetables with high quality, an adequate Ca supply is particularly important. If the xylem sap is low in Ca or the rate of transpiration of the fruits is poor, as occurs under humid conditions, inadequate levels of Ca might move into the fruits resulting in formation of Ca de ciency symptoms. In tomato, Ca de ciency
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disease is known as blossom-end rot and is characterized by a cellular breakdown of tissue at the distal end of the fruit (Saure 2001 ; Schmitz-Eiberger et al . 2002 ) . In apple, Ca de ciency problem is called bitter pit that is characterized by the occur-rence of small brown necrotic pitted spots (Mengel and Kirkby 2001 ) . In vegetables, Ca de ciency causes the tipburn disorder in leaves (Saure 1998 ) .
3 Importance of Calcium in Medicinal Legumes
Majority of the medicinal plants belong to angiospermic families, of which legume family (Fabaceae) is the third largest one, distributed in approximately 650 genera and 20,000 species. A handsome number of medicinal legumes are potential sources of glycosides (e.g. aloe-emodin, chrysophenol, emodin, and rhein, etc.), antibiotics, avonoids, alkaloids, and phytochemicals (Fig. 5.1 ), which are used in drug manu-facturing by various pharmaceutical industries (Tyler et al . 1976 ; Morris 1996, 1997, 1999, 2003 ) . Legumes produce primary and secondary metabolites and the phytochemicals such as nutraceuticals, pharmaceuticals, pesticides, and industrial products. The natural products of leguminous plant have been and will continue to be important sources of forage, gums, insecticides, phytochemicals, and other industrial, medicinal, and agricultural raw materials. Many legumes have been used as folk medicines too (Duke 1992 ) .
Generally legumes require high amount of phosphorus and calcium for their growth, nodule formation, and N 2 - xation. Hence, intensive research efforts should
Fig. 5.1 Structural formulae of various active constituents present in the medicinal leguminous plants
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be expanded for maximizing the yield of potentially useful leguminous medicinal plants particularly through optimal supply of the required fertilizers (phosphorus and calcium). For the cultivation of these medicinal legumes, the role of mineral nutrition is of paramount importance. Using optimal quantity of fertilizer applica-tion, the yield and quality of these medicinally important leguminous plants could be improved in order to meet their increasing demands.
3.1 Cassia tora L. ( Cassia obtusifolia L.)
Naeem and Khan ( 2006 ) studied the in uence of calcium application (0 (control), 40, 80, 120, and 160 mg Ca per kg soil) on growth, yield, and quality attributes of Cassia tora L. The results showed that calcium signi cantly enhanced most of the attributes studied. Calcium, applied at 120 mg Ca per kg soil, gave the highest values of these attributes (plant fresh and dry weights, number of leaves, leaf area, 100-seed weight, yield of the pods, and seed quality as well as leaf-nitrogen, - phosphorus, -potassium, and -calcium contents). Chlorophyll content and nitrate reductase activity were also increased over their respective controls. This treatment enhanced the seed yield, seed-yield merit, and seed-protein content by 31.6, 33.1, and 10.3%, respectively, over the control. The soil-applied Ca also stimulated the production of anthraquinone and sennoside content, increasing the therapeutic property of the plant.
3.2 Cluster Bean ( Cyamopsis tetragonoloba L.) Taub
Garg et al . ( 1997 ) studied the effect of sodium chloride (NaCl) on cluster bean ( Cyamopsis tetragonoloba L.). They observed that increasing NaCl concentrations (0, 50, 100, and 150 mM) decreased the growth and seed yield of the crop progres-sively, which was associated with decreased concentration of potassium and calcium and increased concentration of sodium in the shoots. Exogenous application of cal-cium (0, 2.5 and 5.0 mM) signi cantly ameliorated the adverse effects of NaCl due to enhanced uptake of Ca and K and reduced uptake of sodium. Calcium also alleviated the negative effects of NaCl on enzyme activities of nitrogen metabolism and contents of soluble protein and free amino acids.
3.3 Coffee Senna ( Senna occidentalis L.)
Naeem et al . ( 2010 ) explored that plant biological yield was comparatively low in calcium-de cient soil of Aligarh, western Uttar Pradesh, India. Ca de ciency poses a serious yield and quality limitation for several crops, including medicinal herbs,
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in this region of India. Calcium application through soil enhanced crop productivity, photosynthetic ef ciency, enzymatic activities, and contents of nutraceuticals in coffee senna, a medicinal legume. Plants were grown in pots containing soil sup-plied with ve levels of calcium, viz . 0, 40, 80, 120 and 160 mg Ca per kg soil applied as calcium chloride (CaCl 2 ). Calcium application (120 mg Ca per kg soil) effectively increased various growth, physiological, biochemical, yield, and quality attributes studied at three growth stages (120, 270, and 300 DAS). Further, it increased the seed-yield and seed-protein content by 27.6 and 10.6%, respectively, compared to the control.
3.4 French Bean ( Phaseolus vulgaris L.)
Ssali ( 1981 ) studied the effect of various levels of CaCO 3, inoculation and lime-pelleting on nodulation, dry matter yield, and nitrogen content of bean plant ( Phaseolus vulgaris ) in green house conditions, using ve acid soils. The range of soil pH was 3.895.1, with exchangeable aluminium ranging from 0.0 to 4.0 mg per 100 g, exchangeable man...