Influence of seed priming and different irrigation levels on growth parameters of cowpea [Vigna unguiculata (L.) Walp)

Cowpea [Vigna unguiculata (L.) Walp] is an important food legume and vegetable crop cultivated in tropical and sub- tropical zones of the world. Cowpea pod is a good source of protein, carbohydrate, carotene, minerals, fibre and vitamins. Cowpea is planted in an estimated area of 14.5 m ha worldwide with an annual production of 5.5 MMT of grain and productivity of 400 kg/ha (EL Niam and Jabereldar, 2010). Physiological reasons for variation in productivity may be attributed to poor source-sink relationship, poor translocation efficiency at later stages of crop growth, shedding of floral parts and low harvest index. Poor crop stand of cowpea crop is one of the major abiotic constraints encountered by resource - poor farmers in marginal areas of India (Harris et al., 1999). Seed priming is one of the pre-sowing management technique for pulses to overcome drought and stress. It entails the partial germination of seeds by soaking in water or salt solution for a specified time period and then redries before radicle emerges (Copeland and Mc Donald, 1995). Priming stimulates many of the metabolic processes involved with the early phases of germination. Nutripriming is now recently focused by using macro or micronutrient enriched seeds in rice (Hafeez Ur Rehman et al., 2015). Seed priming with KNO₃ and water improved biological and grain yield of rapeseed through enhancing rate and percentage of seedling establishment (Ali et al., 2013). Pre-sowing hardening treatment of cowpea seeds in IAA and GA₃ significantly enhanced germination and seedling growth and also withstood adverse effect of water stress (Audi and Mukhta, 2009). Seed priming with water and KNO₃ accelerates seed germination and seedling establishment in rapeseed (Alishavandi et al., 2014) under both normal and stressful environments. Keeping these views the present investigation was undertaken to study the physiological effect of seed priming on plant growth and yield parameters in cowpea.

Field experiments were conducted during summer season of 2012 and 2013 at the Indian Institute of Horticultural Research, Bangalore India (Latitude- 13°58 N, Longitude- 78°E, Altitude- 890 M). The soil was red sandy loam with pH 6.8, EC 0.15 d S/m, organic matter 0.69 %, medium available nitrogen (334.3 kg/ha) and phosphorous (42.3 kg/ha) and high available potassium (386.3 kg/ha). The experiment was laid out in split plot design and replicated thrice. Three irrigation intervals, I₁, I₂ and I₃ with 0.9, 0.7 and 0.5 Epan replenishment of evaporation were kept in main plots while the seed priming techniques were applied in subplots.
       
Two days before sowing seeds were subjected to different priming (P) treatments: T₁: GA₃, (100ppm), T₂: CaCl₂ (10^-3M), T₃: ammonium molybdate (10^-3M), T₄: KBr (10^-3M), T₅: MgNO₃ (10^-3M), T₆: ZnSO₄ (10^-3M), T₇:  Hydro-priming and T₈: Untreated control (no priming) at 15°C for 24 hours, washed three times with deionized water  and overnight shade dried to original moisture content. Primed and control seeds of cowpea variety ‘Arka Garima’ were sown in 40 cm spacing between rows in a bed of 60 cm wide and 15 cm distance between plants within row by dibbling. Recommended fertilizer dose of 25 kg N, 50 kg P₂O₅ and 25 kg K₂O /ha was applied in the form of urea, single super phosphate and muriate of potash respectively. Half of N and entire quantity of P and K were applied at the time of sowing. Remaining N (12.5 kg/ha) was applied at 20 DAS. Irrigation was provided uniformly through drip on the day of sowing for better and uniform initial germination and establishment of the cowpea crop. The drip irrigation scheduling was imposed from the seventh day after sowing (DAS) to check the germination percentage in both primed and control due to moisture stress. The daily evaporation reading was recorded by USWB class A Pan evaporimeter for calculating the amount of water to be applied to the crop based on the irrigation treatment and irrigation was given once in two days. Daily consumptive use of water was worked out based on the crop ET and at the end of the season. The seasonal consumptive use of water was calculated and expressed in millimeter. At 30 and 60 DAS and at harvest agronomic traits as well as different yield components were recorded as per standard procedure. Leaf Area Index (LAI) and Crop Growth Rate (CGR) (Watson, 1947), Relative Growth Rate (RGR) (West et al., 1920) and Net Assimilation Rate (Radford, 1967) were computed for 30-60 DAS.

Seed priming significantly enhanced the plant height with different levels of irrigation over control (Table 1). Maximum plant height was recorded in GA₃ (P₁) (84.6 cm) followed by ammonium molybdate (P₃) (84.2 cm). Increase in plant height in barley might be due to stimulation of cell elongation, cell division and enlargement (Jaliban et al., 2014). Seed priming and irrigation levels enhanced the total dry matter production (Table 1). The total dry matter accumulation in leaf increased from 30 to 60 DAS and declined thereafter due to leaf senescence. Total dry matter was highest in GA₃ followed by ammonium molybdate, CaCl₂, KBr and Mg (NO₃)₂ which suggest that seed priming with chemicals play a major role in increasing leaf dry matter, stem dry matter and  redistribution of dry matter in reproductive parts compared to hydro–priming and control. Vazirimehr et al., (2014) reported that seed priming with one percent KNO₃ shorten the time from seed emergence to harvest, improved crop stand and dry matter partitioning to grain in maize.
       
Higher total dry matter production and its partition in case of seed priming and irrigation levels can be attributed to better environment with respect to soil moisture, nutrition and availability of light to the plants and increase photosynthetic area and efficiency. Increase in total dry matter towards maturity may be due to growth pattern, higher rate of CO₂ fixation and RuBP carboxylase activity during crop growth. Thus, total dry matter accumulation is an important parameter in boosting the source sink relationship and yield potential. Bandana Bose and Pandey (2003) and Muhammad Arif et al., (2010) reported that priming treatment significantly increased the TDM accumulation in mustard and soyabean respectively. Increase in biological yield might be due to better early and synchronized seedling growth and plant nutrition (Zhang et al., 1998). Seed priming with ammonium molybdate recorded higher TDM accumulation and LAI during harvest boosting the source–sink relationship and yield potential in green gram (Sarkar and Pal, 2006). Halo priming may cause high partition of photo assimilates to reproductive parts in chickpea compared to control (Ebadi and Sajed 2009).
       
Seed priming with chemicals recorded higher LAI compared to hydro–priming and control at all the stages. Increase in LAI of cowpea plants might be due to established root system, improved emergence and seedling growth of primed seeds. Watson (1952) indicated that LAI and CGR are useful growth parameters for estimating production efficiency of crop. Seed priming with GA₃ and inorganic salts significantly increased CGR (30 – 60 DAS) over hydro–priming and control. Application of GA₃, CaCl₂ and ammonium molybdate significantly increased the CGR at all stages. Pre–sowing treatment improved both LAI and CGR in cowpea to 22 and 25 percent respectively over control. The present data also revealed that the seed priming application increased CGR and is due to increased TDM and partitioning in reproductive parts. LAI and CGR increased significantly with seed priming at 0.5 percent ZnSO₄ in maize (Sher Afzal et al., 2013). CGR is influenced by LAI, photosynthetic rate, leaf angle and amount of radiation energy intercepted. Khan and Khalil (2010) opined that seed priming with distilled water and PEG solution significantly increased the AGR and CGR at 60 DAS in mung bean seed. Basra et al., (2003) observed higher CGR for osmoprimed in rapeseed seed over unprimed seed.
 
RGR represents increase in dry weight per unit of dry weight already present per unit time. It is found in the present study that RGR declined with advancement in crop growth with regard to irrigation levels and seed priming treatments. Decline in RGR with advancement of crop growth could reduce TDM production. RGR was highest in seed priming with GA₃, ammonium molybdate and MgNO₃ at 30–60 DAS over control. Increase in RGR with seed priming chemicals increase in photosynthetic efficiency by increasing the leaf thickness and retaining more chlorophyll and ultimately enhance TDM accumulation. Seed priming with GA₃ significantly increased RGR and leaf size (Zhao et al., 2007) and PEG increased PGR (Muhammad Arif et al., 2010) over respective controls. Thus it indicates that the architecture of plant has been modified so as to intensify the process which is evident from the higher mean value of CGR and RGR.
       
NAR also called “unit leaf rate” is the rate of increase in dry weight at any instant and it represents the size of the assimilatory (Gregory, 1926). In general, among the seed priming treatments, growth hormones and inorganic salts recorded significantly higher NAR compared to hydro–priming and control at all the stages during both the years. Seeds primed with GA3 and ammonium molybdate exerted favourable effect on NAR throughout the crop growth due to early emergence, better leaf development and increase in TDM accumulation. Seed priming improved the CGR and NAR under moisture stress condition in wheat (Iqbal et al., 2013), NAR with 1.5 percent ZnSO₄ over control in maize (Sher Afzal et al., 2013) and GA₃ increased photosynthesis, food translocation, leaf expansion, TDM production and NAR in chickpea (Mohammad Mazid, 2014) over non-primed seed.
       
Yield is the summation of all metabolic processes and growth events during life cycle of crop plants and any abiotic stress which influence the potential productivity of crop yield (Table 2). Both seed priming techniques and irrigation regimes exerted a significant effect on biological and grain yields during both crop growth seasons (Plate 1). Data showed that both biological and grain yields were significantly increased with each increment in irrigation level. Results indicated that a decrease in irrigation supply at 0.5 E_pan induced drought stress, which depressed the biological as well as grain yields of cowpea by more than 20 percent. Among different seed priming techniques, GA₃ (P₁) significantly produced maximum grain yields (1282.2 Kg/ha) followed by ammonium molybdate (1262.6 Kg/ha) (P₃) and least in control (888.7 Kg /ha). Higher grain and biomass yield in pre-germinated seeds could be attributed to early germination and vigorous growth, consequently good crop establishment. Poor translocation of metabolites to the reproductive stage may be one of the reasons for lower yield in control or unprimed crop. Similar results were also reported in wheat (Ali et al., 2013) and wheat (Kalpana et al., 2015).



Biological yield is an indirect index of photosynthetic machinery that along with different yield components determines the final grain yield in a given set of environmental conditions (Table 2). The maximum biological yield was obtained in 0.9 Epan replenishment (4204.5 kg/ha) which was superior to 0.7 Epan replenishment (3940.3 kg/ha) and 0.5 Epan replenishment (3399.7 kg/ha), respectively. Among the seed priming treatments, ammonium molybdate (4178 Kg/ha) recorded highest biological yield followed by GA₃ (4112.2 kg/ha) and KBr (3990.8 Kg/ha) and minimum biological yield was recorded (3160.8 kg/ha) in control. Arshad et al., (2013) depicted that seed priming with different chemicals increased the biological yield in spinach over control.
 
Thus based on the findings, seed priming increased the plant height, number of trifoliate leaves, total dry matter accumulation, LAI, RGR, CGR, NAR, grain yield and biological yield over hydro-priming and control. Seed priming with GA₃ and ammonium molybdate at low concentration and temperature (15°C) for 24 hours can be best employed for boosting productivity of cowpea under rainfed conditions.