Citrus is grouped under the salt sensitive crops. Mycorrhizal fungi, a symbiotic relationship between plant roots and beneficial fungi, are supposed to impart the stress tolerance in the host plants. The stress tolerance improved due to Arbuscular Mycorrhizal fungi (AM fungi) colonization can be attributed to enhanced mineral nutrition. In the present study the efforts are made to evaluate the effectiveness of AM fungi with two citrus genotypes under salt stress. Three-month-old seedlings of Karna Khatta (Citrus Karna) and Troyer Citrange (Poncirus trifoliata × Citrus sinensis) were inoculated with the indigenous soil based AM inocula (mixed strains). The salinity gradient was developed by frequent irrigation with NaCl (0, 50, 100, 150 mM w/v). The results indicated that all the physical parameters were affected with increasing salinity. The proline accumulation increased while the chlorophyll, calcium and magnesium contents decreased significantly with increasing salinity. In general, the decreased AM colonization did not show any significant effects under salt stress.
Keywords: Citrus karna; Poncirus trifoliata × Citrus sinensis;
71treatment. The available phosphorus content in the substrate was 63.2 ppm. The soil based mycorrhizal inocula (Glomus sp., Gigaspora sp., spores) procured from the Division of Microbiology, I.A.R.I., New Delhi, were applied (ca. 300 spores/100 g soil) in the root zone (V1 -non-mycorrhized control, V2 -mycorrhized) while transplanting the seedlings. The salinity gradients were developed by regular irrigation with salt water with NaCl (S1 -control, S2 -50, S3 -100 and S4 -150mM w/v).The physical parameters viz. number of leaves, stem diameter and height of seedling were recorded 90 days after planting (90 DAP). The mean values of three replicates were given. The root colonization by Arbuscular Mycorrhizal fungi (AM fungi) was observed at the end of experiment i.e. 90 DAP. The percentage of root colonization was recorded using the method defined by Phillips and Hayman (1970).All the biochemical estimations were done at the end of the experiment, i.e. 90 DAP. The proline estimation was done using the shoot tip portions (stem and leaves) of fresh plant samples according to Bates et al. (1973), while the chlorophyll estimation was done by using only tender leaves according to Barnes et al. (1992). The total sugars and reducing sugars estimations were done by the Nelson-Somogyi method using oven dried samples as described by Thimmaiah (2004).The macro-and micronutrient analyses were done by using the oven dried whole plant samples. The seedlings were uprooted from pots and washed with dilute acid (HCl N / 10 N) and then rinsed thrice with distilled water. The excess water was wiped with tissue paper and then the seedlings were oven dried at 60°C for three days. The brittle and well-dried plant samples were crushed to a powder and 500 mg of ground sample was digested with diacid solution (HNO 3 and HClO 4 , 9:4). This extract was used for the estimation of phosphorus, potassium, ...
Citrus, mainly mandarin (Citrus reticulata Blanco) is an economically important fruit crop in Bhutan. Despite having favorable agro-climatic conditions for citrus cultivation, the early decline of fruit-bearing orchards coupled with low crop productivity is a major concern among the citrus growers. During the recent survey, an association of ‘Candidatus Liberibacter asiaticus’ (citrus greening) and citrus tristeza virus (CTV), either singly or as mixed infection in declined citrus trees was recorded in all four major citrus-growing districts viz. Tsirang, Dagana, Zhemgang and Sarpang. Using PCR-based diagnosis, a higher incidence of citrus greening (27.45 %) and tristeza (70.58 %) was observed in symptomatic field samples. Detection and characterization of ‘Ca. L. asiaticus’ (CLas) was performed based on 16S rDNA, Prophage gene, 50S ribosomal rplA-rplJ gene, and tandem repeats of CLIBASIA_01645 locus. Similarly, coat protein, p23, and p18 genes were used as genetic markers for the detection and characterization of Bhutanese CTV. The CLas isolates from Bhutan segregated into Class-II and III based on CLIBASIA_01645 locus, analogous to Indian isolates from the north-east region and Term-A based on CLIBASIA_05610 locus. CTV isolates of Bhutan were observed as closely related to VT strain, which is considered as most devastating. To the best of our knowledge, this is the first study on molecular characterization of CLas and CTV isolates and their association with citrus decline in Bhutan.
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