Nutripriming with zinc sulphate and borax for early growth and seedling vigour in grain cowpea [Vigna unguiculata (L.) Walp]

Pulses are the most important food crops after cereals, being the cheapest and quickest means to augment protein production in developing countries. Pulse production in Kerala is not sufficient to meet the dietary requirement of our people. The major constraint in pulse production is the low productivity which emphasis the need for the adoption of improved management techniques. Cowpea [Vigna unguiculata (L.) Walp] is an important multipurpose grain legume crop which performs well as pure and as inter crop. Its cultivation in Kerala is mostly adopted under rainfed condition in rice fallows and marginal lands.
        
Germination and establishment are the two critical stages in the crop life cycle. In crop production, stand establishment determines the plant density, uniformity and management options. Poor crop germination and establish ment result in significant reduction in crop yield. Seed priming is a pre-sowing strategy for influencing the seedling development by modulating pre-germination activity prior to emergence of the radicle (Mc Donald, 2000). Farooq et al., (2007) reported that the beneficial effects of seed priming includes increased germination rate, higher uniformity in germination, better allometric attributes, faster emergence of seedlings, early germination, better stand establishment and increased yield.
        
Deficiencies of micronutrients especially Zn and B have been widely reported in pulses. Both the micronutrients play a vital role in plant growth and development. Zinc have an important role in protein and nucleic acid synthesis and in the utilization of N and P in seed formation and development (Shinde et al., 2017), while B plays a key role in the translocation of metabolites from source to sink, flower retention, pollen tube development, seed formation and setting (Tanaka and Fuiwar, 2008).
        
The most common methods of micronutrient application are soil application, foliar application and seed treatment. But there are limitations in the adoption of soil application due to its difficulty to spread uniformly over the soil and foliar nutrition of micronutrients due to high labour cost involved in spraying. However, nutripriming is a simple priming technique in which seeds are soaked in aerated solution of nutrients (Farooq et al., 2012). Since micronutrients are required in smaller amounts, priming in solutions with higher concentrations may cause seed damage and poor germination.  Therefore, concentration of nutrient solution is to be optimized before field testing.
        
With, this background the present study is formulated with an objective to determine the optimum concentration of zinc sulphate and or borax for seed priming in cowpea, to study the effect of nutripriming on the early growth and seedling vigour and to determine the Zn and B content in cowpea seedlings.

The experiment was conducted at Coconut Research Station, Balaramapuram, Kerala, India located at 8° 22’ 52” North latitude and 77° 1’ 47” East longitude and at an altitude of 9 m above mean sea level. The experiment was conducted in completely randomized block design with six nutripriming treatments and a control (no priming) in three replications during May 2018. The variety used for the study was Bhagyalakshmi, a bush cowpea variety collected from Department of Vegetable Sciences, College of Agriculture, Vellayani. Seeds were soaked in solutions of ZnSO₄ 0.025 per cent (T₁), ZnSO₄ 0.05 per cent (T₂), borax 0.01 percent (T₃), borax 0.02 percent (T₄), ZnSO₄ 0.025 percent + borax 0.01 percent (T₅), ZnSO₄ 0.05 percent + borax 0.02 percent (T₆) and a control. For nutripriming, seeds will be soaked in respective concentrations of ZnSO₄ and borax solutions for 4 h keeping seed to solution ratio 1:5 (w/v). After priming for a period of 4 h, seeds were taken from the respective nutrient solutions rinsed thoroughly in water and shade dried near to original moisture before sowing in the pots. Primed seeds will be sown @ 25 seeds per pot filled with pure sand. The crop will be maintained in the pots for 8 days. The observation viz., days to germinate, number of seeds germinated on each day were recorded up to eight days. The root length (cm), shoot length (cm), fresh root weight (g), fresh shoot weight (g) were recorded on the eighth day.  Dry weight of root (g) and dry weight of shoot (g) were recorded after the samples were dried in hot air oven at 60°C to constant weight.  Based on the above observations the germination parameters viz., speed of germination (Barlett, 1973), mean daily germination (Czabator, 1962), germination index (Bench et al., 1991), germination percentage, co-efficient of rate of germination (Jones and Sanders, 1987), germination rate index (Esechi,1994), mean germination time (Ellis and Robert, 1980) and time to reach 50 percent germination (Coolbear et al., 1980) were calculated. The seedling vigour index 1 and seedling vigour index 11 were also worked by adopting the formula suggested by Abdul-baki and Anderson, 1973). The Zn and B content in the seedling were also determined by the method suggested by Lindsay and Norvell (1978) and Wolf (1971), respectively. The experiment was repeated for confirmation and data were statistically analyzed and the treatment means were compared at 5 percent probability.

Effect on germination parameters
 
Nutripriming treatments had significant effect on mean daily germination, germination percentage, germination index and speed of germination (Fig 1). Compared to unprimed seeds (control) except treatment T₅ (nutripriming with ZnSO₄ 0.025 percent + borax 0.01 percent), the other treatments recorded higher germination percentage, germination index, speed of germination and mean daily germination. The result is in conformity with the findings of Farooq et al., (2007) and Sajjan et al., (2017) who observed that the seed priming enhanced the germination rate, better allometric attributes, faster emergence of seedlings as well as early germination. Faster emergence of primed seeds might be due to metabolic repair during imbibition (Bray et al., 1989) and build of germination enhancing metabolites (Basra et al., 1995), while higher and synchronized emergence was the consequence of reduced physiological non-uniformity in the seeds due to priming (Juraimi et al., 2012). Nutripriming with ZnSO₄ either at 0.025 or 0.05 percentage recorded higher germination percentage and registered 19.68 and 18.51 higher germination percentage than control. Germination index were 1.4 times higher in seeds primed with ZnSO₄ @ 0.025 and 0.05 percent. Nutripriming with ZnSO₄ @ 0.025 percent for 4 h recorded the highest mean daily germination and it was closely followed by ZnSO₄ @ 0.05 percent for 4 h and nutripriming with borax @ 0.02 percent for 4 h. Compared to control, nutripriming with ZnSO₄ @ 0.025 or 0.05 percent for 4 h recorded 25.02 and 36.22 percent higher mean daily germination than control. The speed of germination was also the highest in nutripriming with ZnSO₄ @ 0.05 percent for 4 h and it was closely followed by nutripriming with ZnSO₄ @ 0.025 percent for 4 h. This might be due to the increased seed imbibition process and enhanced seed metabolism (Rowse, 1995).
 

        
Germination parameters, co-efficient of rate of germination, mean germination time and germination rate index was significantly influenced by nutripriming with ZnSO₄ and borax. Co-efficient of rate of germination gives an indication of the rapidity of germination (Fig 2). The highest co-efficient of rate of germination and germination rate index was recorded by nutripriming with ZnSO₄ @ 0.05 percent and it was closely followed by ZnSO₄ @ 0.025 percent for 4 h. Time to 50 percent germination and mean germination time are the indicators of seedling vigour. Nutripriming with ZnSO₄ either @ 0.025 or 0.05 percent recorded the lowest mean germination time and time to reach 50 percent germination, though the time to 50 percent germination was not significantly influenced by the treatments. Lesser time required for 50 percent germination and mean germination time might be due to the fact that seed priming stimulates the activities of enzyme like amylase which increases the breakdown of food reserves and supply of energy to growing embryo and due to the triggering of physiological, biochemical and molecular changes inside the seed (Farooq et al., 2006; Chen et al., 2012). Arun et al., (2017) observed that  priming with zinc sulphate (10^-3M) reduced the time for 50 percent germination and mean germination time compared to control in cowpea.
 

 
Effect on seedling growth and vigour index
 
Data on seedling shoot and root length revealed that nutripriming treatments recorded higher shoot and root length compared to unprimed seeds (control) (Table 1). Seeds treated with ZnSO₄ @ 0.025 percent for 4 h recorded the highest shoot length and it was statistically on par with ZnSO₄ @ 0.025 percent for 4 h and ZnSO₄ @ 0.025 percent + borax 0.01 percent for 4 h. In the case of root length, nutripriming with ZnSO₄ @ 0.05 percent + borax @ 0.02 percent for 4 h recorded the highest root length and it was on par with nutripriming with @ borax @ 0.01 percent for 4 h, nutripriming with ZnSO₄ either @ 0.025 or 0.05 percent.   Seedling dry weight was significantly influenced by nutripriming with ZnSO₄ and borax. Nutripriming with ZnSO₄ @ 0.025 percent for 4 h recorded the highest dry weight and it was on par with nutripriming ZnSO₄ @ 0.05 percent for 4 h.  Compared to control (unprimed seeds), the seedling dry weight was enhanced by 56.67 percent and 49.32 percent due to nutripriming with ZnSO₄ 0.025 and 0.05 percent, respectively. The finding is in accordance with the observations of Ajouri et al., (2004) who observed that seed priming with Zn improved the seed germination and seedling development in barley. Vigour index was also significantly influenced by nutripriming with ZnSO₄ and borax. Higher vigour index 1 and 11 were recorded by nutripriming with ZnSO₄ @ 0.025 percent for 4 h which was statistically on par with priming with ZnSO₄ @ 0.05 percent for 4 h. Higher vigour index recorded by these treatments might be due to better crop growth as evident from data on shoot length, root length and dry weight. Ozturk et al., (2006) reported that Zn is involved in the vital physiological process in the early stage of radicle and coleoptile during seed germination.
 

 
Effect on Zn and B content of the seedlings
 
Zinc and boron content of the seedlings were significantly influenced by nutripriming with ZnSO₄ and borax. (Table 2) The nutripriming treatments recorded almost double the Zn content than that of control. The B content of the seedlings in the nutripriming treatments was also found to be more compared to unprimed seeds (control). The result is in conformity with the findings of Muhammed et al., (2015) who observed that the primed nutrients were effectively translocated to the growing shoot and could maintain the supply for at least three weeks of the culture period. The high content of Zn and B content in the seedling might have enhanced the root and shoot growth due to the role of Zn and B in cell elongation, protein synthesis and membrane function (Cakmak, 2000; Tanaka and Fujiwar, 2008).
 

It can be concluded from the results that nutripriming with ZnSO₄ either at 0.025 or 0.05 per cent recorded higher values for the germination parameters viz., germination per centage, mean daily germination, mean germination time, co-efficient of rate of germination, germination rate index, germination index, speed of germination and time to reach 50 percent germination. The treatments also recorded higher growth and vigour index and higher Zn and B content. Hence it can be concluded that nutripriming with ZnSO₄ either at 0.025 or 0.05 percent can be recommended for early growth and seedling vigour in grain cowpea.