Seed quality parameters
Seed priming of one-year-old pigeon pea seeds with growth regulator (100 ppm GA
3), in-organic salt (0.2% KNO
3) and tap water separately, significantly enhanced the germination over un-primed control (Table 1a,b,c,d,e,f). Among treatments, seed priming with GA
3 displayed maximum seed germination (90.17%) which was at par with KNO
3 (85.83%) followed by tap water (80.83%) and differed significantly with each other. The varieties of pigeon pea evaluated responded well to priming treatment and germination per cent was significantly higher in Bahar (90.16%) followed by Malviya -13 (77.50%). Interaction between varieties and treatments were non-significant. Maximum germination (95.00%) was recorded in variety Bahar with GA
3. The magnitudes of improvement in germination were 2.97, 9.34 and 14.67% with tap water, KNO
3 and GA
3 respectively over control. Similarly, other seed quality parameters including root length, shoot length, Seedling dry weight, Vigour index I and Vigour-II also showed the significant improvement through seed priming treatment in both the varieties evaluated. Among the priming agents, GA
3@100ppm performed better than KNO
3(0.2%) and tap water in respect of all the seed quality parameters studied. The percent improvement over control in root and shoot length, seedling dry weight, vigour IandII was recorded higher in GA
3 priming followed by KNO
3 and tap water. Variety Bahar displayed significantly higher values of all the seed quality parameters over Malviya-13 (Table 1a to f).
Our results were also confirmed by the findings of
Bose and Mishra, (1992) and
Bose, 1997 who reported that osmo- priming with different chemicals to seeds enhances the rate of germination and encourages the fast emergence of seedling in field and this might lead in enhancement in subsequent phases of plant growth and finally to better performance of a crop
. Results obtained were also supported with the findings of
Bose and Mishra (1999 and 2001) who opened that during soaking of seed in Mg (NO
3)
2 or KNO
3 solution the cations Mg++ or K
+ and anions NO
3- in fluxed in the seeds and showed their carry over effects during vegetative growth period and consequently the plant/crop stand was improved and it was further supported by
Bose and Pandey (2003) and
Tiwari et al., (2018).
Biochemical attributes
Seed priming with GA
3@100 ppm, KNO
3(0.2%) and tap water significantly improved the chlorophyll a and b contents in the leaves of pigeon pea varieties over unprimed control, whereas the proline accumulation in leaves was decreased with the priming treatments. The reduction in proline content over unprimed control was maximum with GA
3 followed by KNO
3 and tap water priming might be due to socking of water which minimizes the level of water stress in primed seed as compare to unprimed seed (Table 2 a,b and c).
Nitrate assimilatory enzymes were more influenced by KNO
3 priming as compared to GA
3 and tap water over un primed control. This indicates the role of priming in the enhancement of activities of nitrate assimilatory enzymes and particularly of KNO
3 which showed the highest activity of both the enzymes might be due to No
3 salt which is the source of nitrogen and act as substrate for enzyme activity (Table 3 a, b). Nitrate is considered the primary source of nitrogen from the soil and the main function of enzyme nitrate reductase (NR) is to reduce nitrate to nitrite (
Beevers and Hageman, 1969). Positive effect of auxin hormone and Ca (NO
3)
2 on NRA was due to its possible role in the activation of the inactive nitrate reductase protein and prevention of enzyme degradation by proteolysis. This might also be involved in the enhancement of enzyme synthesis or its maintenance in the active form and thus, has a protective role on nitrate reductase activity (
Richard and Stanely, 1981). The stimulated NR activity in seed priming treatments compared to control plants might be due to enhanced nitrogen uptake by plants
Muthuchelian et al., (1994). Inclusion of nitrate and in-organic ions like Cu
2+, K
+, Mn
2+, Mg
2+ and Zn
2+ in assay medium has also been reported to increase the nitrate reductase activity in roots and leaves of maize under salinity (100 mM NaCl) (
Khan and Srivastava, 2000). Beneficial effects of priming have been associated with various biochemical, cellular and molecular events including synthesis of DNA and proteins
(Srivastava et al., 2010). Metabolic repair processes and build-up of germination metabolites or osmotic adjustments during priming are due of germination enhancement techniques
(Mukhtar et al., 2013). These are the possible reasons for the observed enhancement in seedlings growth, improvement in composition of chlorophyll and enhanced activity of nitrate assimilatory enzymes. Seed priming with beta amino butyric acid (BABA) in rice seeds in varied abiotic stresses (NaCl/PEG-6000) leads to increased seedling growth, photosynthetic pigment chlorophyll, photosystem activities and mitochondrial activities in rice seedlings. Moreover, BABA priming significantly reduced malondialdehyde content in the seedlings and also resulted in accumulation of proline in NaCl tolerant variety. It also enhanced the activities of nitrate reductase and antioxidant enzymes and thus improved the drought and salinity stress in rice varieties (
Jisha and Puthur, 2016).
Anti- Oxident enzymes
The anti-oxidant enzymes including catalase, peroxidase and super oxide dismutase were assayed using the respective standard methods mentioned in the text in growing seedlings. The observations recorded indicated that seed priming with GA
3@100 ppm, KNo
3 (0.2%) and tap water significantly enhanced the enzymes activity of catalase, peroxidase and super oxide dismutase over unprimed control. Among the priming agents used, KNO
3 enhanced relatively more activity of catalase, peroxidase and super oxide dismutase followed by GA
3@100ppm and tap water (Table 4 a, b and c). In Varieties, Bahar displayed significantly higher values of enzymes activities including catalase, peroxidase and super oxide dismutase. Enhancement in anti-oxidant enzymes activity through osmo- priming with PEG socked for 24 h in okra seeds has been reported by
Chawla et al., (2014) and priming of rice seeds with distilled water, Mg (NO
3) 2 salt, Kinetin and salicylic acid were also found to enhance the activity of SOD, NR, Ascorbate peroxidase, proline and protein content over non primed control under timely and late sown conditions
(Kumar et al., 2018). These findings confirm our results in case of pigeon pea where the priming agents used are also of same nature. Further, Priming has been used to improve the performance of germination at the field and potassium nitrate (KNO
3) is a promising compound for the purpose. The nitrate (NO
3) could be absorbed, being used in the metabolism of the embryo, through the enzyme nitrate reductase (NR). Besides, the priming could also activate the response of the antioxidant system, becoming the primed seeds more prepared for possible stresses. The results obtained showed an increase in the NR activity, as well as in the antioxidant enzymes and seems to be the beneficial effects of priming with KNO
3 in pigeon pea seeds
(Lara et al., 2014). GA
3 priming of alfalfa ‘Bami’ seeds resulted in increased germination percentage and seedlings growth, decreased lipid peroxidation and enhanced antioxidant defences and may be an efficient method to overcome seed germination problems and to improve seedlings growth in the field (
Younesi and Moradi, 2014). Osmo-priming strengthens the antioxidant system and increases seed germination potential, resulting in an increased stress tolerance in germinating seeds. Osmo-priming-mediated promotive effect on stress tolerance, however, may diminish in relatively older (
e.g. ~5-week) seedlings (
Chen and Arora, 2011). In contrary to above findings,
Bhattacharya et al., (2019) reported the deteriorative changes in enzyme activity of dehydrogenase, Amylase. Catalase and peroxidase in invigorated soybean seeds.
Seed priming with beta amino butyric acid(BABA) in rice seeds in varied abiotic stresses (NaCl/PEG-6000) also enhanced the activities of nitrate reductase and antioxidant enzymes and thus improved the drought and salinity stress in rice varieties (
Jisha and Puthur, 2016). Priming potential of AGE (aqueous garlic extracts), SA and MeJA to enhance seed germination and early seedling growth in eggplant and the effects were obvious in various morphological and physiological traits. Seed priming significantly altered the antioxidant enzymes activities such as superoxide dismutase (SOD) and peroxidase (POD) with alteration in the reactive oxygen species (ROS). Interestingly, priming duration also affected the bioactivity of these chemicals because seed priming with 300 µg mL
-1 AGE for 4 h had a positive influence, however, prolonged exposure to the same concentration inhibited the seed germination process and induced oxidative stress on the seedlings with elevated levels of malondialdehyde (MDA) content. AGE (aqueous garlic extracts) seed priming as a bio-stimulant to enhance seed germination and early seedling growth in eggplant and the results hence lay the foundation for the preparation of garlic-based compounds to improve vegetables production under plastic tunnels and greenhouse production units
(Ali et al., 2019).
In the light of above findings, it may be concluded that the observed improvement in present study with respect to germination, seedling growth, biochemical attributes and enzymatic activities are the results of seed priming with growth regulator GA
3 and osmo-priming with KNO
3 and might be considered as an effective and cheapest tool for the improvement of pigeon pea germination and crop stand establishment.