Endophytic fungi increase the plant ability to stand and continue its life cycle in harsh environments
Guava (Psidium guajava L.) is an important horticultural fruit crop. It belongs to family Myrtacea, which have more than 80 genera and 300 species are dispersed throughout the world among different environments, mostly Australia, America and Asia (Nakasone and Paul, 1988).The family of guava is further sub divided into two sub families, one is Leptospermoidea (in which fruit is of capsular shape) and other is Myrttoideae (in which fruit is barriers or drupe) (Cronquist 1981; Wagner et al., 1990). Pulp of guava contains two types of cell-wall tissues; stone cells and parenchyma cells. Stone cells are high lignified woody type materials which give a sandy or gritty sense when they are consumed. These characteristics of guava fruit make it resistant to enzymatic degradation. Mesocarp tissues account 74% of guava fruit, while parenchyma cells are present in endocarp. The presence of parenchyma cells makes fruit soft in texture (Marcelin et al., 1993). Color of upper skin of guava fruit is light green to yellow. The pulp of guava fruit varies from variety to variety. It varies from white, pink, yellow or light red. Immature fruits are hard textured, astringent and acidic nature. They are astringent due to high phenol content and low sugar content. When the fruit is ripened, its skin becomes soft, sweet and non-acidic (Malo and Campbell, 2004; Mitra, 1997). There are many cultivars of guava but they are broadly classified as white and pink guava. In many countries, seedless varieties are also available, which have potential to become popular (Yadava, 1996).
Guava is a good source of many important antioxidants and phytochemicals which include ascorbic acid, a good amount of carotenoids, antioxidant dietary fiber content, and polyphenolics. It is considered that guava has second highest amount of ascorbic acid, which ranges from 60-1000 mg/100g, after acerola cherries (Mitra, 1997). Carotenoids have many beneficial effects on human health relating to their antioxidant properties. These are pigments which are red, yellow and orange in color (Wilberg and Reodriguez-Amaya, 1995).Pink color in guava flesh is due to lycopene, a major guava carotenoid (Mercadante et al., 1999). Guavas are considered as frost-free. It is because they can tolerate a wide range of climates and soil conditions (Menzel, 1995). Mostly guava is propagated through seeds. Honey bee is considered as a major source for pollination of guava trees (Yeshitela and Woldetsadik, 2003). The range of cross pollination is usually considered to be 25.7 to 41.3%. For attaining high yield and good quality fruit production, the choice of superior seedlings is considered to be essential (Morton, 1987). Guava fruits have short shelf life ranging from 3-8 days. Short shelf life is due to high ripening rate. Shelf life depends upon climatic conditions, harvest time and variety (Reyes and Paul, 1995; Basseto et al., 2005). Guava reaches its peak between day 4 and 5(mature green harvested fruits) and then decreases (Akamine and Goo, 1979; Bashir and Abu-Goukh, 2002).
When, guava fruit become ripe, total sugar contents and total soluble contents increases both in pulp and peel. During ripening, sugar contents changes in guava as sucrose and starch are breakdown into glucose (Bashir and AbuGoukh, 2002). In tropical climates, there is moisture loss in guava which results in 35% weight loss (Mitra, 1997). At mature green stage, guava fruits have high amount of ascorbic acid and decreases as the fruits become ripen both in pink and white guava fruits (Bashir 3 and Abu-Goukh, 2002). During ripening, lycopene production increases in pink guava. When lycopene accumulates, respiration rate decreases (Thimann, 1980). During ripening, fiber contents also decreases 2-12g/100g (El-Zoghbi, 1994). In terms of fruit production, guava is placed at third position in Pakistan after citrus and mango. The area of production of guava in Pakistan is 56,800ha, mango is cultivated on area of 90,900 ha, and citrus is cultivated on area of 194,700 ha. The guava is a hardiest fruit and that’s why it is cultivated on large area. Guava can tolerate a pH range from 4.5 to 8.5 (Singh, 1990). The yield of guava is low, 8.1 tones/ha (Anka, 2003).
Guava fruits twice in a year that’s why it is available in market throughout the year. Commercially grown varieties in Pakistan are white Allahabadi, red Allahabadi, shaikhupori and local/Desi amrood. Potential constraint for production of guava is less post-harvest techniques. It is normally considered that about 18% of production of guava is wasted or thrown away. Instead of gaining, the production is lost (Khan, 2015). When a guava plant does not acquire iron from the soil, iron chlorosis appears on guava plants. Ferrous sulphate is used as major treatment for iron chlorosis. It is also available to farmer at very low prices (Morikawa et al., 2006). Endophytic fungi are group of fungi that causes infections like asymptomatic infections on aerial tissues of different group of plants. The majority of fungal endophytes are ascomycetes and their anamorphs (Petrini, 1986).Group of these fungi do not affect the plants communities too much. They are not harmful to plants. They provide resistance to plant against insects (Azevedo et al., 2000), fungal pathogens (Arnold et al., 2003). Endophytic fungi increase the plant ability to stand and continue its life cycle in harsh environments (Redman et al., 2002). There have been a lot of work has been done on endophytic fungi and their relation with plants (Bills, 1996) and many tropical plants have been observed for their relation with fungal endophytes association (Arnold et al., 2001). Fungal endophytes causes many damages to their host such as stress enhancement, disease and insect resistance (Elzik 1985; Bush et al., 1997; Clay and Holah 1999; Shimizu, 2000). In pea plants, nickel is major part of the hydrogenase enzyme. Hydrogenase enzyme increases its activity in plants as we increase nickel concentration in plants. A very small amount of nickel is added to hydrogenase enzyme after the application of nickel to pea plants. Nickel is accumulated in soluble proteins of the plant. Nickel which is accumulated is not exchangeable with medium in which nickel is present (Brito et al., 1994).The colonization of endophytes, their propagation and secondary metabolites which are present inside the plants may be important for above effects. These opinions show that endophytic fungi may be used as biological agents for the control of insects and many diseases (Miller, 1995). Nickel is one of the important micronutrient required by plants. It is required by plants in very minute quantity for optimum growth of a plant (Brown et al., 1987; Eskew et al., 1983). Higher concentrations of nickel applied to plants may cause serious damage to plants. If it is applied near to stem or, when applied to plants, if it touches the stem of the plants or leaves may cause yellowing of leaves or branches (Bingham et al., 1986; Farago and Cole, 1986; Foy et al., 1978). High concentration of nickel may affect many important physiological and biological processes like making leaf chlorophyll content of plants and necrosis of leaves (Pandolfini et al., 1992; Piccini and Malavolta, 1992). Nickel is mobile part of enzyme urease, which increases its essentiality among micronutrients. Urease is an enzyme which hydrolyzes urea in tissues of plants. Mostly in plants, two different types of urease enzymes are present. One is present in seed which hydrolyze urea in seeds to get food. Urease present in seeds is highly active. Second urease enzyme is present in vegetative tissues of plants. Activity of urease enzyme present in vegetative tissues is usually low, despite the fact that playing an important role in nitrogen cycle (Fabiano et al., 2015).
Different plants have different nickel requirement depending upon the root to shoot transport and plant uptake. Minute concentrations of nickel are required by plants. Nickel aids in plants metabolism and required in small amount, so nickel availability to plants is not itself is a big concern. For leguminous crops, nickel is an essential micronutrient as well as for higher plants. It is reported that necrotic spots appear on soybean if grown without nickel. Delay in nodulation and reduction in growth are also appeared due to deficiency of nickel (Shafeeq et al., 2012). Nickel also affects the chemicals that aid in photosynthesis and also affect the transpiration rate of a plant (Carlson, 1975; Jones and Hutchinson, 1988a; Morgutti et al., 1984; Rauser and Dunbroff, 1981). The symptoms of nickel toxicity include chlorosis of leaves which is followed by mottling. Plant leaves also show sign of necrosis and stunted growth of plant stem and other parts (Misha and Kar, 1974; Khalid and Tinsley, 1980;Hutchinson, 1981;Yang et al., 1996). These symptoms in plants often noticed when there is imbalance of mineral nutrients or disturbance in nutritional status of soils. Nickel symptoms in plants are often considered to be Cu symptoms when plants are grown in contaminated soils or in soils that have high concentration level of nickel (Craig, 1978; Dang et al., 1990 ; Farago and Cole, 1986). Nickel toxicity affects the uptake of other nutrients like Zn, Fe, Cu and Co in tomatoes (Lycopersicon esculentum L.).
Nickel is considered as an important micronutrient for plants because it is activator of the enzyme urease. Recent studies have shown that Nickel can also activate enzyme glyoxalase (Fabiano et al., 2015). Addition of Nickel in soil takes place either by two ways. It might be added in soil by anthropogenic way or either by natural sources present in environment. There is no doubt in this aspect that nickel is an important micronutrient for plant and plant need it for its functions but toxicity of nickel is also very important as it causes many disorders in plants. Nickel present in plant tissues range from 0.05-10 μg/g of dry matter. The concentration of Ni varies in different plant species. It is due to difference in uptake and root to shoot transport mechanism 100 μg/g is usually considered as threshold level of Ni (Shafeeq et al., 2012). Ni is an heavy metal that can stimulate plants at very low concentrations and also act as an substitution for molybdenum (Gopal et al., 2002).
Nickel is a silvery white metal that takes on a high polish. It is a transition metal, hard and ductile. Nickel is primarily found combined with oxygen or sulphur as oxides or sulphides that occur naturally in the earth’s crust. As for most metals, the toxicity of nickel is dependent on the route of exposure and the solubility of the nickel compounds. The purpose of this review is to provide a detailed overview of current state of knowledge of the nickel toxicity and in the formation of reactive oxygen and nitrogen species and tissue damage and role of some antioxidant supplementation (Da et al., 2008). Nickel toxicity has become a worldwide issue. Nickel toxicity is causing big problems to sustainable agriculture. The critical nickel toxicity level is more than 10.7 mg-1 dry weight in some sensitive species. Different physiological parameters of plants are affected by nickel toxicity depending upon the growth stage, type of plant species, nickel concentration, time of exposure and conditions during cultivation. Toxicity of nickel causes damages like mitotic activity inhibition, relation of plant with water, plant growth reduction, rate of photosynthesis inhibition of nitrogen metabolism and enzymatic inhibition of plant (Ahmad et al., 2009).