4.1. Weather conditions
Weather situations are accredited to have an immense impact on the performance of crops (Fig. 1). Consequently, it becomes utmost important to take into consideration the conclusion from the experimental results. Across the 2 years of experimentation, weather conditions showed wide variations. These variations in the rainfall resulted in more irrigation to cotton during 2014 than 2013. Crop yield was also affected due to rainfall and it was less during 2014 than 2013.
4.2. Productivity
Statistically similar seed cotton, lint and seed yields were recorded under methods of crop establishment in both the years (Table 2). Zinc levels significantly influenced seed cotton, lint and seed yields and application of 5.0 and 7.5 kg Znha− 1being at par recorded significantly higher seed cotton, lint and seed yield over 2.5 kg Znha− 1and control. Foliar application of 0.5% zinc recorded seed cotton, lint and seed yield on par with 2.5 kg Znha− 1and significantly inferior to 5.0 and 7.5 kg Znha− 1. On an average, application of 5.0 kg Znha− 1recorded9.2, 14.5 and 8.3% increase in yield over control. Increase in seed cotton yield and stalk of cotton due to application of Zn was largely a function of improved growth, translocation of more photosynthates towards sink and consequent development of yield. Application of Zn significantly increased seed cotton, seed and lint yield ha− 1 in both seasons ascompared with the control (Sawanet al., 2008). A possible explanation of such results might be the improvement of yield components due to the application of Zn. Zinc could have a favorable effect on photosynthetic activity of leaves (Welch, 1995), which improves mobilization of photosynthates and directly influences boll weight. Further, Zn is required in the synthesis of tryptophan, a precursor of indole-3- acetic acid (Oosterhuiset al., 1991), which is the major hormone that inhibits abscission of squares and bolls. These results were in agreement with those obtained by Rathinavel et al. (1999, 2000) when ZnSO4 was applied to the soil at 50 kg ha− 1; Sawanet al. (2001) when Zn was applied as foliar application at 48 g ha− 1; Alikhanova and Tursunov (1988) by application of Zn at 2.5–7.5 kg ha− 1, by Sawan et al. (1989) when cotton was sprayed with Zn at 12.5 ppm and by Gomaa (1991) when cotton was sprayed with 0.952 kg ZnSO4 ha− 1. Results are confirmed by those of Zeng (1996) and Ibrahim et al. (2009), Prasad and Prasad (1998),Ratinavelet al. (1999),Nikolov (2002),Ahmed et al. (2010), Ali et al. (2011),Abidet al. (2013) andLi et al. (2022)are in agreement with the present investigation.Zn application did not impact ginning outturn of cotton grown in an Indian sandy loam soil (Srivastava and Singh, 1988).
4.3. Zinc content and uptake
Zinc content and uptake by Bt cotton, was influenced significantly due to cotton establishment methods and zinc treatments (Table 3and4). It was evident that the treatments that received Zn had a higher concentration and content of both the years than the plants not receiving Zn. The increased CEC of roots which helped in increasing absorption of Zn from the soil. This was true for both the concentration and total uptake of Zn were equally good measures of response. Further, the beneficial role of Zn in chlorophyll formation, regulating auxin concentration and its stimulatory effect on most of the physiological and metabolic process of plant, might have also helped to plants in absorption of greater amount of zinc from the soil and finely translocation and assimilation into the stalk, cotton seed and lint. Similar results have been reported by Reddy et al. (2012); Fritschi et al. (2004); Mullins and Burmester (1990); Dineshkar and Babhulkar (1998); Ahmed (2009),Ahmed, et al. (2011) and Nehra, (2013). The increased Zn content attributed to greater absorption of Zn by the crop owing to higher availability in soil due to direct addition of zinc, otherwise the soil was deficient in available Zn. The increase in content of zinc was also reported by Meenaet al. (2005) and Rochester (2007). Since, uptake of the nutrient is the function of nutrient content and biomass production, the significant increase in content of nutrients coupled with increased grain and stover yield under zinc applied treatments enhanced the total uptake of these nutrients (Jain and Dahama, 2005; Jakharet al., 2006; Gupta and Sahu, 2012; Jyothiet al., 2013 and Shivayet al., 2014). Hormones might play a role in increasing total Zn concentration. For example, IAA levels are elevated in orange with the symbiosis (Ek et al., 1983). Tryptophan synthetase requires Zn as an activator and is part of the biochemical pathway for the synthesis of IAA (Nason et al., 1951). With higher levels of IAA there might be a feedback mechanism which drives plants to uptake Zn. The practical implication for this work was that Zn nutrition improve Zn uptake from soil which is low in Zn and, therefore, suggests the possibility that the Zn deficiency might be managed through Zn nutrition.
4.4. Zinc use indices
Agronomic efficiency, physiological efficiency, apparent recovery and zinc harvest index of Zn in Bt cotton was decreased with the increase in levels of zinc fortification, however, recovery efficiency and Zn harvest index were increased with increasing levels of zinc fortification (Table 5). The reduction in zinc use efficiencies with successive increase in levels of zinc might be due to the increase in levels of zinc did not bring corresponding increase in grain yield (Sammauria and Yadav, 2010). The other reason of reduction in use efficiencies of applied Zn at higher application rates because of its rapid adsorption over soil organic matter and clay minerals and its subsequent slow desorption (Shivay et al., 2010).Differences in Zn use efficiency have been reported in various crops such as wheat, rye, barley and oats (Cakmak et al., 1998) and cotton (Shukla and Raj, 1987).
4.5. Fibre quality parameters
Crop establishment method did not significantly affect the fibre quality of cotton viz., 2.5% span length, 50% span length, uniformity ratio, fibre strength, fibre elongation and micronaire (Table 6). The values were slightly increased in direct sown cotton than transplanted cotton.This could be attributed to the fact that fibre quality is genetic character of cotton and it does not change easily by cultivation process (Blaise et al., 2005; Mukherjee and Verma, 1994;Chellamuthu and Purushothaman, 1980; Robinson, 1973; Li et al., 2022). Zinc levels significantly affect all the fibre quality parameters of cotton viz., 2.5% span length, 50% span length, uniformity ratio, fibre strength and fibre elongation except uniformity ratio in 2013 and fibre fineness. Among different Zinc levels, 7.5 kgha− 1recorded higher values compared to 2.5 kgha− 1, 0.5% Zinc foliar and control and it was at par with 5 kgha− 1. This was in line with the findings of Sawan et al. (1997); Chhabra et al. (2004); Suresh and Kumar (2005), Ahmed et al. (2010) and Abid et al. (2013). Fibre quality is primarily determined by variety but is also influenced by temperature, water, and nutrient stresses (USDA 1993). Furthermore, fibre length is affected by daytime and nighttime temperature (Gipson, 1968, DeLanghe, 1986). Fibermicronaire exhibited a 3.8 to 4.1 range in variability during both the growing years. Lower micronaire values are associated with immature fibers (Munro, 1987).Earlier researchers reported that lint quality is largely controlled by plant genotype and weather conditions(Hearn, 1981; Grimes and El-Zik, 1990) andZn application did not impact staple length of cotton grown in an Indian sandy loam soil (Srivastava And Singh 1988).
4.6. Correlation analysis
The correlation coefficient between seed cotton yield and different Zn use indices was positive and significant (P ≤ 0.05) except zinc harvest index (0.02) (Table 7). Seed cotton yield is having maximum correlation with AE of zinc (0.73) followed by recovery efficiency and physiological zinc use efficiency (0.64). The R2 of the fitted models varied from 0.41 to 0.53 depending upon the explanatory variable which is maximum for AE of zinc followed by recovery efficiency and physiological zinc use efficiency.Therefore, variability in zinc use was an important factor influencing variability in seed cotton yield under the conditions of this study.
The correlation coefficient between Zn uptake and different cotton quality parameters was positive and significant (P ≤ 0.05) (Table 8 and Fig. 3) with the all the cotton quality parameters such as uniformity ratio, 2.5% span length, 50% span length, fibre strength and fibre elongation affirming that improvements in Zn uptake andfibre quality were the consequence of better Zn nutrition of plants. The R2 of the fitted models varied from 0.11 to 0.44 depending upon the dependent variable which is maximum for fibre elongation and minimum for uniformity ratio.
4.7. Optimum fertilizer dose (zinc)
Correlation and regression analysis between seed cotton yield and zinc dose revealed that the optimum fertilizer dose was calculated 5.6 kg Znha− 1during 2013 and 5.4 kg Znha− 1during 2014. Therefore, on an average, 5.5 kg Znha− 1is sufficient to produce higher seed cotton yield while other resources are being in sufficient amount. Therefore, variability in zinc dose was an important factor influencing variability in seed cotton yield under the conditions of this study.