Effect of Seed Invigouration on Yield Enhancement in Grain Cowpea [Vigna unguiculata (L.) Walp]

Cowpea [Vigna unguiculata (L.) Walp] is an important multipurpose pulse crop which performs well as pure and inter crop in coconut garden. In Kerala it is mostly cultivated under rainfed condition in rice fallows and marginal lands. Deficiencies of micronutrients especially Zn and B have been widely reported in pulses. Deficiency of Zn causes chlorosis, sterility, reduction in leaf size and reduction in spikelet number (Cakmak, 2000). Zinc also has a significant role in plant metabolism and synthesis of auxins, carbohydrate, phosphate and nucleic acid (Latef et al., 2017). Tariq and Mott (2007) pointed out that B deficiency during flowering prevents the pollen tube growth which causes pollen sterility, flower drop and poor pod setting. It was also observed that foliar nutrition of B enhanced the flower development, pollen grain formation, pollen viability, pollen tube growth and seed development in green gram (Praveena et al., 2018). Seed priming and pelleting with micronutrients are the two simple inexpensive seed invigouration strategies for micronutrient application. Seed priming (soaking the seeds in micronutrient solutions of definite concentration for a specific period) or seed coating with micronutrients are the two easy cost-effective methods of micronutrient application. It ensures better germination, seedling development, early flowering, early fruiting, resistance to soil borne pathogen and increased yield (Kumar et al., 2008).
 
Seed invigouration treatments are environmentally safe and effective technology which can be easily adopted by resource-poor farmers and benefited the farmers in a number of ways. It may indeed consider as a valuable strategy to improve stand establishment under adverse agro-climatic conditions with enhanced yield, increased tolerance to stress situations, increased resistance against diseases and enhances cost benefit ratio.
 
With, this back ground the present study was carried out with an objective to assess the effect of seed invigouration with zinc sulphate and borax on grain cowpea and to evaluate its effect along with Trichoderma viride on field emergence, growth, yield attributes and yield of the crop.

Field experiments were conducted during rabi season of 2018-19 at Coconut Research Station, Balaramapuram, Kerala, India (latitude 8°22¢52²N, longitude,77°1¢47² E and an altitude  9 m MSL). The soil was red loam with pH 4.5, EC 0.10 d Sm^-1, organic matter 0.65 per cent, medium in available nitrogen (301.05 kg ha^-1) and phosphorous (20.52 kg ha^-1) and low in available potassium (94.02 kg ha^-1), deficient in available Zn (0.457 mg kg^-1 soil) and boron (0.018 mg kg^-1 soil). The experiment was laid out in randomized block design with three replications. The treatment comprised of eight seed invigouration treatments and a control (without seed invigouration). The treatments were seed pelleting with borax @ 50 mg kg^-1 seed (T₁),  seed pelleting with borax @100 mg kg^-1 seed (T₂), seed priming with ZnSO₄ @ 0.025 %  for 4 h (T₃), seed priming with ZnSO₄ @ 0.05% for 4h (T₄),  T₁ + Trichoderma viride seed treatment @10 g kg^-1 seed (T₅), T₂ + Trichoderma viride seed treatment 10 g kg^-1 seed (T₆), T₃ + Trichoderma viride seed treatment@ 10 g kg^-1 seed (T₇), T₄+ Trichoderma viride seed treatment @10 g kg^-1 seed (T₈) and a control. For pelleting, borax was mixed thoroughly with an adhesive (maidha 10%), seeds were individually coated with this mixture and shade dried. For priming, seeds were soaked in respective concentrations of ZnSO₄ and shade dried to initial moisture content of 8%. Trichoderma viride seed treatment was done ten minutes before sowing. Primed, pelleted and non invigoured (control) seeds of cowpea variety ‘Bhagyalakshmy’ were dibbled at a spacing 30 ´ 15 cm. Recommended fertilizer dose of 20 kg N, 30 kg P₂O₅ and 10 kg K₂O ha^-1 was uniformly applied to all treatment plots in the form of urea, single super phosphate and muriate of potash, respectively. Half N and full P and K were applied at the time of sowing. Remaining N (10 kg ha^-1) was applied at 30 DAS. Germination count was recorded on eighth day and field emergence percentage was worked out. The growth parameters viz., number of green leaves per plant were recorded at 30 DAS and 60 DAS, number of branches per plant and dry matter production were recorded at 30 DAS, 60 DAS and at harvest stage. Total chlorophyll content was determined at 30 and 60 DAS by adopting the method suggested by Yoshida et al., (1976). The yield attributes viz., number of pods per plant, pod length, pod girth, pod weight and 100 seed weight and seed yield were recorded at harvest. Days to 50% flowering was recorded by counting the number of days from sowing to 50% of the plants in each treatment plot produced first flower and was expressed in days. For dry matter production, five plants were uprooted from outside the net plot area, initially dried in sun for a day and then dried in hot air oven at 70±1°C till constant weight were obtained and expressed in g plant per plant. Economics was worked out based on the prevailing market price of inputs and market price of seed (Rs 80 kg^-1). Data were subjected to statistical analysis and treatment means were compared at 5% probability level (Panse and Sukhatme,1984).

Effect on field emergence
 
Seed invigouration significantly influenced the germination percentage of grain cowpea compared to control (Table 1). Due to seed invigouration, an enhancement in the final emergence of 20.54 to 27.79 percentage was observed. Though, the treatment T₄ (seeds primed in ZnSO₄ 0.05% for 4 h) recorded the highest germination percentage (97.86%), it was statistically comparable with all the seed invigouration treatments. Osmotic adjustment, metabolic process repair and build-up of metabolites occurred during seed invigouration may be the plausible reason for higher germination in seed invigouration treatments (Chen and Arora, 2013; Jisha et al., 2013). Seed invigouration shortened or reduced the lag or metabolic phase in the germination process. In normal course, metabolic or lag phase in the germination process occurs when the seeds are fully imbibed or just before the radicle emergence. During seed invigouration, the seeds have already completed the lag phase or metabolic phase and hence germination time in the field has been reduced resulting in the fast and uniform emergence of seedlings with better establishment and higher germination percentage. The result is in consonance with the observations made by Ghiyasi et al., (2008) who observed that seed invigouration improved the germination and establishment.
 

 
Effect on growth parameters
 
Seed invigouration treatments significantly influenced the growth parameters viz., number of green leaves per plant (30 DAS and 60 DAS), number of branches per plant and dry matter production (30 DAS, 60 DAS and harvest stage) (Table 1 and 2).
 

 
In general, seed invigouration treatments recorded higher number of branches per plant than control. Vaiyapuri et al., (2010) also made similar observations that seeds pelleted with Zn and B enhanced the primary branches in lentil and soybean. At 60 DAS and harvest stage, seeds primed in ZnSO₄ recorded higher number of branches than borax pelleted seeds. This might be due to the fact that Zn plays a major role in the biosynthesis of tryptophan which is the precursor of plant hormone IAA and IAA helps to trigger the growth of the plant tissues (Naruka et al., 2000). The highest number of branches were recorded in T₇ (seeds primed in ZnSO₄ 0.025% for 4 h + Trichoderma viride seed treatment 10 g kg^-1 seed) at 30 DAS, 60 DAS and at harvest, this might be due to production of plant growth hormones especially IAA by Trichoderma viride (Chagas et al., 2016).

Seed invigouration treatments significantly influenced the dry matter production (Table 2). As the crop advances its age the DMP was found to increase. Compared to control, seed invigoration treatments recorded higher DMP at all the growth stages of observation. Better DMP recorded in these treatments was due to the production of more branches and leaves which enhanced the assimilatory area and photosynthesis. The treatment T₄ (seeds primed in ZnSO₄ @ 0.5% for 4 h) recorded the highest DMP among the treatments at harvest (43.44 g). Hansen (1972) observed that net photosynthesis in crop plants is primarily influenced by leaf area index. The present results are in conformity with the observations made by Amanullah et al., (2008) reported that light interception increased with increase in leaf area, which ultimately increased the dry matter production. Increased availability and uptake of nutrients led to better expression of growth and yield attributes which also contributed to higher DMP in T₄ at harvest. Favourable influence of Zn on dry matter accumulation in pulses were also reported by several workers (Khorgamy and Farina, 2009; Valenciano et al., 2010; Usman et al., 2014).
 
Effect on yield attributes
 
Seed invigouration significantly influenced the yield attributes of grain cowpea (Table 3). Control treatment registered lesser values for the yield attributes. Among the seed invigouration treatments, T₄ (seeds primed in ZnSO₄ 0.05 % for 4h) registered higher number of pods per plant (24.13), seeds per pod (16.10), pod length (17.47 cm), pod girth (2.0 cm) and have taken lesser number of days to attain 50% flowering (41 days). This might be due to better crop growth and higher DMP achieved through the adequate supply and uptake of nutrients and also due to the increased photosynthesis and efficient translocation of photosynthates from source to sink. Siddiqui et al., (2009) who opined that during grain formation, with the sufficient supply of Zn, the uptake of N might have increased and improved the yield attributes and yield. Grzebisz et al., (2008) also revealed that the uptake of N during the grain formation stage was enhanced due to early stage Zn application. These results are in conformity with the findings of Mohsin et al., (2014) who revealed that seed priming with 2% Zn followed by foliar application of Zn (2%) increased the cob length, cob diameter and 1000 grain weight in maize.  Upadhyay and Singh (2016) also revealed that in cowpea Zn nutrition had favourable effect on pods per plant, pod length and seeds per pod.
 

 
Effect on yield
 
Seed yield per plant was significantly influenced by seed invigouration treatments (Table 4). Compared to control, all seed invigouration treatments which registered higher seed yield. Seed invigouration with Zn and B along with recommended dose of organic manures and nutrients increased the seed yield from 842 to 1446 kg ha^-1 in grain cowpea. Fast and uniform germination, high germination percentage and seedling vigour resulting from seed invigouration gave a vigorous start for the crop to continue its growth. This would be resulted in the production of higher number of branches  per plant, pods per plant, pods with more length and girth and seeds per pod. Among the treatments, T₄ (seeds primed in ZnSO₄ @ 0.05 % for 4 h) registered the highest seed yield (1446 kg ha^-1), which was statistically on par with T₃ (seeds primed in ZnSO₄ @ 0.025% for 4 h), T₁ and T₂ (seeds pelleted with borax 50 and 100 mg kg^-1 seed) and T₆ (T₂ + Trichoderma viride seed treatment 10 g kg^-1 seed). Higher seed yield observed in these treatments might be due to the better expression of yield attributes especially pods per plant resulting from better uptake of nutrients. Higher chlorophyll content (Table 2) observed in these treatments significantly improved the photosynthetic activity leading to the production of more amount of carbohydrates and translocation of assimilates from source to sink which finally contribute to higher seed yield. Masuthi et al., (2009) observed that Zn and B play a major role in pollen germination, fruit setting, seed development and translocation of starch from source to sink. Similar observation was also made by Peda-Babu et al., (2007) who observed that Zn fertilization enhanced the carbohydrate synthesis and their transport from source to sink. Role of B in N utilization, protein synthesis and carbohydrate translocation were also reported by Masuthi et al., (2009). Significant increase in the seed yield of green gram due to seed treatment with ZnSO₄ @ 4 g kg^-1 seed was reported by Usman et al., (2014). Arun et al., (2017) observed that in cowpea, seeds primed in ZnSO⁴ 10^-3 M solution registered significantly higher seed yield (1100.5 kg ha^-1). The increase in haulm yield due to Zn fertilization in pulses was also revealed by Shanti et al., (2008).  Higher seed yield due to seed pelleting with B in soybean was reported by Srimathi et al., (2001).
 

 
Seed invigouration with Zn or B followed by Trichoderma viride seed treatment @10 g kg^-1 registered lesser seed yield than the treatments without Trichoderma viride seed treatment. This might due to the lesser production of lesser number of pods per plant. Several researchers (Howell, 2003; Verma et al., 2007; Segarra et al., 2010) revealed that Trichoderma have the ability to release fungal siderophores and the fungal siderophores increase the Fe availability and uptake by crop plants. Dimpka et al., (2009) and Santiago et al., (2011) pointed out that the increased availability of Fe decreased the availability of other micronutrients due to antagonistic effect.
 
Effect on economics
 
Economic evaluation of treatments is of great relevance for its acceptance in the farmers field. Seed invigouration treatments significantly influenced the net income and B:C ratio (Table 3). Control treatment recorded the lowest net returns and B:C ratio. This was due to the lowest seed yield recorded in the treatment. Seed invigouration treatments resulted in an increase in net income of Rs 17, 310 ha^-1 to Rs 48,001 ha^-1 compared to control (Table 3). This was owing to the fact that fast and uniform emergence of seedlings with vigourous growth resulting in the better expression of yield attributing characters particularly pods per plant which in turn lead to higher seed yield. Better utilization of nutrients due to early emergence of seedlings resulted in higher grain yield in cereal crops was reported by Badiri et al., (2014). Among the seed invigouration treatments, higher net income was recorded in T₄ (seeds primed in ZnSO₄ @ 0.05 % for 4 h), this might be due to higher grain yield recorded in the treatment. Harris et al., (2007) observed that monetary benefits are high in seed priming with one percent solution of Zn compared to soil application of 2.75 kg Zn ha^-1 in maize. Benefit cost ratio was also significantly influenced by seed invigouration treatments. It followed the similar trend as that of net income and the highest B:C ratio was registered in T₄. These results are in conformity with the observations of Harris et al., (2007) who observed that ZnSO₄ priming enhanced the grain yield of maize by two times and is a low-cost technology which can be easily adopted by resource poor farmers.

Seed invigouration significantly enhanced the growth and yield attributes over control. Among the seed invigouration treatments, seed priming with ZnSO⁴ @ 0.05 % for 4h recorded the highest dry matter production at harvest, pods per plant, pod length, pod girth and seed yield ha^-1. Hence it can be concluded that seed priming with ZnSO₄ @ 0.05 % for 4h) along with recommended dose of FYM (20 t ha^-1), lime (250 kg ha^-1) and NPK (20:30:10 kg ha^-1) could be recommended for boosting the productivity of grain cowpea.