Effect of zinc solubilizing bacteria in enhancing growth attributes of aerobic rice in low and high zinc soil

Introduction

Rice (Oryza sativa L.) accounts for more than 20% of the world's caloric consumption and serves as the primary source of income and food security for over three billion people throughout Asia. Rice production was reported as 34.6 million tonnes (mt) during 1960-1961, which is expected to rise to 122.3 mt, during 2020-2021 (PBAS 2020; PIB 2021) [10,11] . It is estimated that more than 30% of people globally suffer from Zinc deficiency (White et al., 2009) [15] . Zinc deficiency disrupts multiple biological and physiological functions in plants, such as carbohydrate metabolism, regulation of cell turgor, and protein synthesis. At the same time, increasing water scarcity is driving a transition from flooded to aerobic rice cultivation systems, a change that can significantly alter zinc availability in soils (Hafeez et al., 2013) [7] . Zinc deficiency in rice is a major cause of yield losses worldwide. Under adequate nutrition, zinc concentrations in rice tissues generally range from about 35 to 100 ppm, while deficiency symptoms begin to appear when tissue zinc levels fall below 20 ppm (Singh and Prasad, 2014) [12] . The increasing shortage of water resources has led to the development and adoption of aerobic rice system, which saves water input and increases water productivity by reducing water use during land preparation and limiting seepage, percolation, and evaporation. Aerobic rice also reduces labour requirement and greenhouse gas emission from rice field. Chaudhary et al., 2007 [5] reported Zn deficiency as a key factor in determining the rice production in several parts of India. According to Singh (2009) [13] , 48% soils in India are affected with Zn deficiency. In rice, Zn deficiency causes multiple symptoms that usually appear 2 to 3 weeks after transplanting (WAT) rice seedlings; leaves develop brown blotches and streaks that may fuse to cover older leaves entirely, plants remain stunted and in severe cases may die also, while those which recover show substantial delay in maturity and reduction in yield. The aftermath of low Zn consumption is the impairment in growth and immune functions leading to infectious diseases such as diarrhoea and pneumonia in the developing world (Volkmar and Bremer, 1998) [14] . Brain function and development is also significantly affected by Zn deficiency, mainly in newborns (Benton, 2008) [2] . Zinc deficiency was found to be one of the major reasons for child death worldwide, and was responsible for nearly 4, 50,000 deaths in children less than 5 years of age (Black et al., 2008; Cakmak, 2010) [3,4] . According to a report published in 2002 by WHO, Zn deficiency is considered as the fifth most important risk factor responsible for illness and death in the developing world. In human body, Zn plays an important role in maintaining the structure and function of large number of macromolecules and is also found responsible for controlling over 300 enzymatic reactions. The use of chemical fertilizers and manures, to enhance soil fertility and crop productivity has often negatively affected the complex biogeochemical cycles. Continuous application of fertilizers as well as their low use efficiency has caused leaches and runoff of nutrients, especially N and P leading to environmental degradation. On the other hand, high cost associated with the application of Zn fertilizers in order to correct Zn deficiency pays considerable burden on resource poor farmers.

Materials and Methods Pot culture experiment

A pot culture experiment was conducted in the Glass house of the Department of Soils and Environment, Agricultural College and Research Institute, Madurai to study the growth of aerobic rice in low and high zinc soil as affected by Zinc Solubilizing Bacteria in comparison with control. It was conducted during Rabi 2016 with short duration Anna 4 (Parentage Pantdhan10/ IET 9911) as test crop in low and high zinc soil with different treatmental combinations in a factorial completely randomized block design replicated four times. Paddy seed (variety Anna 4) was surface sterilized and imbibed in 5 ml of Zinc solubilizing bacterial suspension (1 x 10 9 CFU ml -1 ) for 4 hours. Six seeds were placed in a pot; and irrigated immediately; four pots were used for each treatment. In case of control, seeds were imbibed in 5ml of sterile distilled water for 4 hours. The DTPA extractable Zinc, Copper, Iron and Manganese content recorded were 0.86, 0.24, 3.9, 4.6 mg kg -1 respectively. The experimental soil with a high Zn recorded DTPA extractable Zn, Cu, Fe, Mn of 1.71, 0.24, 2.7 and 3.7 mg kg -1 respectively. The treatment consists of different combinations viz., Absolute control (T 1 ), 100% RDF alone (T 2 ) and 100% RDF + ZSB (T 3 ) with recommended dose of fertilizer = 150:50:50 kg of N: P2O5: K2O ha -1 .

Observations recorded

Growth attributes viz., root length, root hairs and shoot length were in low and high zinc soils of aerobic rice and expressed in cm, numbers and cm respectively.

Results and Discussion

Root length

The effect of seed treatment with zinc solubilising bacteria along with combination of inorganic fertilizers on root length of aerobic rice at different stages revealed a significant increase over control. The values ranged from 5.20 to 14.20, 8.30 to 20.50 and 12.10 to 25.20 cm during tillering, panicle initiation and at post-harvest stages, respectively in low zinc soil. With respect to high zinc soil the root length ranged from 6.22 to 11.20 cm, during tillering stage, 8.10 to 22.15 cm during panicle initiation and 10.50 to 23.40 cm during post-harvest stage. Application of RDF with ZSB recorded the maximum root length of 25.20 cm in low zinc soil and 23.40 cm in high zinc soil at postharvest stage, the maximum number of root length was marked by the application of zinc solubilizing bacteria in both the soils as depicted in Table 1. This might be due to the bacterial isolate responsible for production of phytohormone which is responsible for increasing the length of the root hairs which could promote better absorption of nutrients from the soil. According to Volkmar and Bremer, (1998) [14] rapid establishment of roots including elongation of primary roots or proliferation of lateral/ adventitious roots is advantageous for young seedlings as it increases their ability to explore the soil to easily obtain water and nutrients.

Shoot length

In both the soils experimental soils, viz., low zinc and high zinc soils, the length of shoot was longer in the treatment of RDF with ZSB and has recorded least shoot length in absolute control. In low zinc soil, the shoot length varied from 37.20 to 52.10 cm, 48.30 to 65.10 cm and 58.24 to 72.45 cm during tillering, panicle initiation and at postharvest stage. The longer shoot length of 52.10, 65.10 and 72.45cm was recorded by the application of 100% RDF (150:50:50) and ZSB at tillering, panicle initiation and postharvest stages respectively as depicted in Table 2. There was a significant increase in shoot length over absolute control in high zinc soil at different growth stages. The treatmental mean values extended from 38.20 to 51.20 cm, 46 to 66.20 cm and 59.10 to 69.60 cm tillering, panicle initiation and post-harvest stages respectively. Nitrogen application thus results in growth of shoot system, which ultimately helps to increase the straw yield. Nayak et al. (1986) [9] reported that ZSB inoculation can increase height and number of tillers in rice plants. They mentioned that PGPR treatment increases chlorophyll content in plants and thereby enhances the rate of photosynthesis leading to improved overall plant health. Similar observations were reported by Mohite, (2013) [8] .

Root hairs

Increasing trend of root hairs was observed during the growth stages in both the experimental soil. The mean values irrespective of growth stages ranged from 15 to 32, 24 to 52, 36 to 63 in low zinc soils and 18 to 29, 27 to 50, 40 to 58 in high zinc soils (Table 3). The maximum number of root hairs registered in low zinc and high zinc soils during the harvest stage is 63 and 58 numbers respectively, by treating the seeds with zinc solubilizing bacterial culture and 100% RDF. One of the most commonly reported plant growth promotion mechanism by bacteria is the challenging of morphological and physiological changes in root system (Dalla et al., 2004) [6] . An increase in number of lateral roots and root hairs cause addition of root surface available for nutrients and water uptake. Higher water and nutrient uptake by inoculated roots caused an improved water status of plant, which in turn could be the main factor enhancing plant growth (Bai et al., 2003 and Wu et al., 2005) [1,16] . The maximum number of root hairs was recorded by application of zinc solubilizing bacteria in both the soils.

Conclusion

The present investigation revealed that the application of 100% RDF (150:50:50) + ZSB influenced the root of both the experimental soils at all growth stages. Other growth attributes namely viz., root length and number of root hairs were also significantly influenced by the combined application of 100% RDF + ZSB. This study represents a preliminary effort to improve the growth and yield of aerobic rice through the use of microbial cultures in zincdeficient soils. The results suggest that the application of zinc-solubilizing bacteria has the potential to serve as an effective microbial inoculant for enhancing the growth and productivity of aerobic rice. Nevertheless, further research is required to evaluate different treatment combinations involving zinc-solubilizing bacteria and to verify their effectiveness under field-level conditions.

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