Abstract

Media Summary

Keywords

Vegetative fronds, sporulation, acidic soil, germination frequency, survival of young seedlings.

Introduction

The majority of tropical paddy soils of Asia are deficient in nitrogen and organic matter (Kawaguchi and Kyuma 1977). Role of organic matter in relation to increased rice production and soil fertility is well recognized (Tanaka 1978). The Azolla–Anabaena consortium, a promising biological system, is beneficial in contributing nitrogen and organic matter for lowland rice. As regards the biomass production, and quantity of nitrogen fixation and nutrient recycled, Azolla is highly efficient, cost effective and ecologically sound bio-fertiliser (Singh et al 1990; Watanabe and Liu 1992; Wagner 1997; Pabby et al. 2003). To produce Azolla inoculum in paddy fields, its vegetative fronds in large scale are required but there are several physical constraints in Azolla production and utilization. The thick wall of megasporocarp can withstand high temperature, drought condition and pest attack (Nayak and Singh 1988). Proper utilization of biological nitrogen fixation by the system depends on various factors like soil pH, temperature, light, etc. Hence, the objective of this study was to evaluate the sporocarp production at local condition and to determine the effect of soil pH on percent germination of megasporocarps and on survival of young seedlings using paddy soils of different pH collected from various states of India.

Materials and methods

Seven species Azolla, A. caroliniana, A. filiculoides, A. mexicana, A. rubra, A. microphylla , A. nilotica, and A. pinnata, collected from Central Rice Research Institute, Cuttack, Orissa having accession codes, were cultivated in P. N. College, Khurda, Orissa (84⁰20′ West to 85^0.42′ East Long. and 19⁰12′ to 20⁰10′ North Lat.) from 2000 to 2002. Seven numbers for each species were multiplied in an RBD (4m× 2m size multiplication plots) with 30 cm water tight bunds and interception of 50 cm water channel in a paddy field (soil haplaquept clay loam, pH, 6.4, organic C, 0.79%, total N, 0.09% and available P, 8.9 mg/kg). One hundred grams of vegetative fronds of each species were used for multiplication of Azolla in field condition, after sterilization with 0.01% mercuric chloride. Single super phosphate @ 3.5 kg/ha and furadon (Carbofuran, 3G) @ 2.5 kg/ha were applied. A standing water of 6 cm depth was maintained. Then multiplication plots with Azolla were allowed for mat formation and for sporulation without administering P fertilizer and pesticide for 75 days. Collection, separation, up-gradation, cleaning, drying and storage of megasporocarps were carried out. Undesirable materials like grass culms, empty husks and weed seeds, etc. were separated with test sieves. Unripe, chaffy, shrivelled, or unusually small-sized megasporocarps were discarded by winnowing to select the full-sized megasporocarps (Fig. 2 a,b). Stages of germination of megasporocarps were recorded (Fig. 2c,d,e and 3a,b). Under Khurda condition, average percent of sporocarp bearing fronds and average number of fronds giving 100 megasporocarps were recorded in mat forming condition at 30^th, 45^th, 60^th and 75^th days of inoculation. Megasporocarps were kept in field soils up to 3 years. The mature and full sized megasporocarps of A. caroliniana, A. mexicana, A. microphylla and A. pinnata were placed in earthen pots having different soil samples collected from the Division of Agricultural Soil Chemistry, Central Rice Research Institute, Cuttack. The soil belonged to different places/states of India, such as Thottapallikari , Kerala ( pH, 4.3 ) Khopadan, Kerala (pH, 5.2), Burdwan , West Bengal (pH, 6.1), Baragarh, Orissa ( pH , 6.3), Khurda , Orissa ( pH , 6.4 ), CRRI, Cuttack , Orissa ( pH, 6.5), Berhampur , Orissa ( pH 7.1 ), Hyderabad , Andhra Pradesh (pH, 7.4 ) and Karnal, Haryana (pH, 8.3). At the cotyledonary leaf stage, young seedlings were placed in N-free sterile IRRI-nutrient media.

Results

Among the 7 species of Azolla grown in multiplication plots (Fig. 1a,b,c) under Khurda condition, A. filiculoides and A. rubra did not form sporocarps whereas a good number of sporocarps were observed in A. caroliniana, A. mexicana, A. microphylla, A. nilotica and A. pinnata in prolonged mats (Fig. 1d,e,f) at 60^th DAI during January to March (Table 1). Similarly, lower average numbers of fronds contributing 100 megasporocarps were observed in A. microphylla and A. pinnata. The average percentage of emergence of plantlets from megasporocarps was higher (70-80%) in Khurda (pH, 6.4) and CRRI, Cuttack (pH, 6.5) soils. In Bargarh (pH, 6.3) and Berhampur soils (pH, 7.1), the germination rates were above 60% (Table 2). In Hyderabad soil (pH, 7.6) germination rates were stagnant at 24%. In very acidic soil of Thottapallykari (pH, 4.3) and alkaline soil of Karnal (pH, 8.3) germination was nil. In Khopadan soil (pH, 5.2), germination was meagre and the young seedlings that were emerged did not survive. Nitrogen-free IRRI nutrient medium was suitable for nourishing young seedlings at the cotyledonary leaf stage (Fig. 2 d,e and 3c ).

Table 1 :Record of sporulation in Azolla sps. (A–average percent of sporocarp bearing fronds; B- average no. of fronds giving 100 megasporocarps).

Table 2: Effect of soil pH on average percent of germination of megasporocarps (G) and average percent of survival of young seedlings of Azolla sps (S)

Figure 1 Growth and sporocarp formation of Azolla sps. in lowland paddy field.

Figure 2 Megasporocarps and process of germination.

Figure 3 Germination process and young seedlings of Azolla (Camera –lucida drawings).

Discussion

Sporocarp survey indicated that species like, A. microphylla, A. nilotica, A. caroliniana and A. pinnata were efficient sporocarp-producing species under Khurda condition. Prolonged mats in winter for 60 to 75 days resulted in higher yield of megasporocarps. Due to slow initial growth of young seedlings, mega sporocarps were not directly broadcast into a rice field (Singh et al. 2001). However, for the exceptional physical characters like thick wall and texture, megasporocarps were ideal for germplasm preservations and plantlets could be raised as primary inoculum in nursery-bed in adverse agro-ecological climates. Both the highly-acidic soil (pH, 4.3) and the alkaline soil (pH, 8.3) were observed to be deleterious for germination of megasporocarps. Soil pH of 6.0 to 7.6 belonging to coastal states of India, such as Orissa, West Bengal, and Andhra Pradesh were observed to be favourable for germination of megasporocarps and establishment of young seedlings.

Conclusion

From this experiment, it can be concluded that medium acidic paddy soils of coastal India are suitable for germination of megasporocarps of Azolla. The local species, A. pinnata, is also ideal for sporocarp yield and germination under local rice field condition.

References

Nayak SK and Singh PK (1988). Observations on sporocarps in Azolla sp. In: ‘Biofertilizers: Potentialities & Problems’ (Eds. SP Sen and P Palit), pp. 139-144.

Pabby A, Prasana R and Singh PK. (2003). Azolla-Anabaena symbiosis- from traditional agriculture to biotechnology, Indian Journal of Biotechnology 2, 26-37.

Singh DP, Mishra S and Kar PP (2001). Prospects of use of Azolla sporocarps as inoculum in rice fields. In: ‘Recent advances in the exploitation of BGA and Azolla’, (Eds PK Singh) pp.109 –127.

Singh PK, Bisoyi RN and Singh RP (1990) Collection & germination of sporocarps of Azolla coroliniana. Annals of Botany 66, 51-56.

Tanaka A (1978). Role of organic matter. In ‘Soil and Rice’ IRRI, Los Banos, Philipines, 605-620.

Wagner GM (1997). Azolla: a review of its biology and utilization in Botanica. Review 63 (1), 1-25.

Watanabe I and Liu CC (1992). Improving nitrogen fixing systems and integrating them into sustainable rice farming. Plant and Soil 141, 57-67.