Introduction
Cotton is the backbone of Pakistan’s economy, contributing a 0.8% share of GDP. However, a severe decline in its yield has been observed in the last decade, mainly due to biotic and abiotic factors. Some of these factors include improper sowing time, low nitrogen use efficiency, drought and heat stress, insect pests, and diseases [1-5]. Imbalanced fertilization is one of the major constraints to cotton production in an arid climate [6, 7].
Phosphorus (P) is an essential plant nutrient that must be present in all plant cells in optimum concentration because it helps them carry out their functions properly. It is involved in the storage and transfer of energy, photosynthesis, respiration, cell division, and expansion in plants [8]. Many plants contain P at a concentration of 0.2% of their total dry weight [9]. However, the deficiency of P leads to decreased shoot dry weight, root dry weight, and whole plant weight and an increased root-to-shoot ratio in cotton crops [10]. Previous studies have reported that optimum P availability increases chlorophyll a, b, and carotenoids and total soluble protein contents and reduces the level of malondialdehyde in plants [11]. Moreover, P use efficiency is low due to the application of lower doses than the optimum doses of P by poor and marginal farmers. This situation could become worse, which might be correlated with increased prices of P fertilizer due to the limited available resources of P worldwide [12]. In addition, soils with higher concentrations of calcium (Ca), aluminum (Al), and iron (Fe) reduce the availability of P because it forms strong bonds with Ca, Al, and Fe and thereby becomes unavailable [13]. The deficiency or lower supply leads to a minimum photosynthetic rate and metabolism that ultimately decreases the seed cotton yield of the cotton crop [12, 14].
Zinc (Zn) is an essential plant micronutrient required for cell division and expansion and helps to complete the life cycle in plants [15]. It is involved in a wide variety of metabolic processes, such as the activation of enzymes, metabolism of proteins, photosynthetic carbon, and the metabolism of indole acetic acid [16, 17].
Limited knowledge is available on the interactions between P and Zn in plants. Thus, it demands additional research, particularly in plants having high requirements of P and Zn. Cotton is one of the crops in which plants have high requirements for both P and Zn. It has been reported that when there is a balanced supply of P and Zn, the maximum biomass can be achieved, whereas an unequal supply of these nutrients results in a Zn shortage and P or Zn toxicity [18, 19]. Many studies have reported antagonistic interactions between P and Zn due to excess or low doses that lower physiological and biochemical processes [20, 21, 22]. Imbalanced P and Zn fertilization could decrease the P and Zn use efficiencies in the cotton crop [19]. Thus, the application of P and Zn in balanced quantity can increase the P and Zn use efficiency in cotton production.
This comprehensive information on the interactions between P and Zn in plants is essential for commercially valuable plants having high requirements of P and Zn. However, no extensive research work has been executed to explore the interactive effect of P and Zn fertilization on their soil availability and also uptake by the roots and physiology, growth, and yield components of the cotton crop. For this purpose, we hypothesize that the interactive effect between Zn and P may help to assess the optimum doses of P and Zn for achieving a higher yield of the cotton crop.
Materials and Methods
Experimental Site and Climate
The experiment was conducted at the selected Agronomic Research Area, MNS University of Agriculture, Multan, Pakistan (30˚14''N, 71˚43''E) during the summer season, on May 15, 2022. The climate of the experimental site was arid, being characterized as a hot and moderately humid summer (May-June) and a warm and humid rainy season (July-September). Weather data during the study period was collected from the automatic weather station of MNS-University of Agriculture, Multan, Pakistan (Fig. 1).
Experimental Design and Treatments
A split-plot design was used to conduct the proposed research trial. There were three replications, the size of the plots was adjusted to 5× 3 m, and all the blocks were equal in size. Different doses of Zn and P were applied to different experimental units to study the interactive effect of Zn and P application in cotton production. The experimental treatments were comprised of five doses of P (P1=0, P2=30, P3=60, P4=90, and P5=120 kg ha-1) and three doses of Zn (Zn1=6, Zn2=12, and Zn3=18 kg ha-1). The 90 kg ha-1 was designed as the recommended dose of P for cotton crops. Different Zn and P doses were kept in main plots and subplots, respectively.
Soil Analysis
According to the experimental plan, soil samples from each plot after sowing and at cotton harvest were collected with the help of a soil auger. These samples were collected from different soil depths (0-15, 15-30, and 30-45 cm), and one composite soil sample having soil of three depths for each treatment was made. Upper soil layers are the source of the majority of nutrients; therefore, these three soil depths (0-15, 15-30, and 30- 45 cm) were considered during the study. The sampling was performed following the methodology of Mason [23]. After analysis, soil samples were ground and sieved (2 mm mesh) to remove the dirt and other remnants of plants. Soil available P was estimated using the sodium bicarbonate method [24]. An atomic absorption spectrometer (Varian, Spectra A 220) was used to determine the Zn concentration in the soil samples. This system contains a multi-element hollow cathode lamp for Zn and is operated at 213.7 nm wavelength [25].
Cultivation of Cotton
The soil was plowed twice, followed by land leveling. The bed planter was used to make beds of 75 cm spacing, and seed beds were irrigated. After irrigation, the seeds were planted manually by maintaining a 30 cm plant-to-plant distance. The recommended dose of K (60 kg ha-1) and planned doses of P and Zn, half of the recommended dose of N (80 kg ha-1 of a total of 160 kg ha-1) were applied at sowing time, while the remaining dose of N (80 kg ha-1) was applied in two equal splits (each of 40 kg ha-1). Fertilizer sources of N, P, K, and Zn used in this study were urea, di-ammonium phosphate, murat of potash, and zinc sulfate, respectively. Irrigations were given according to the crop requirement and prevailing environmental conditions. All the intercultural operations except those under study were kept uniform during the whole growing season.
Observations and Measurements
Data on growth, phenology, physiology, morphology, and yield traits of cotton crops was recorded using standard scientific protocols and procedures. The number of days from the date of sowing to square initiation, flower initiation, boll initiation, and boll opening was recorded during the study. Physiological traits (i.e., net leaf photosynthesis rate, stomatal conductance, sub-stomatal conductance, net leaf transpiration rate, and water use efficiency) were recorded using the CIRAS-Portable Photosynthesis System. These physiological traits were recorded from 10:30 am to 3:00 pm. The plant height of randomly tagged five plants in each experimental unit from plant base to tip of the plant’s main stem was recorded using measuring tape, and the average plant height was calculated. The peak of the leaf area index was calculated at the full canopy development stage using the method of [26]. The number of monopodial, sympodial, and total bolls of the tagged five plants was counted, and their average was calculated. The boll weight of randomly selected five bolls from each experimental unit was determined, and the average boll weight was recorded. At maturity, the plants were harvested from a meter-square area and separated into leaves, stems, and reproductive parts. Then, these samples were oven-dried at 65°C-70°C until the constant dry weight of the samples was recorded and converted into total dry matter in tons ha-1. The seed cotton of each experimental unit was picked, and then it was converted into seed cotton yield in tons ha-1.
Statistical Analysis
The recorded data was subjected to the analysis of variance (ANOVA) technique to analyze the interactive effect of different P and Zn doses on the studied traits of cotton crops [27]. Furthermore, the treatment means were compared with the help of the mean separation test, Tukey’s Honestly Significant Difference (HSD), at p ≤ 0.05.
Results and Discussion
Effect of Zinc and Phosphorus on Physiological Traits of Cotton
The leaf photosynthetic rate, water use efficiency, chlorophyll content, stomatal and sub-stomatal conductance, and leaf transpiration rate varied significantly among all the treatments (Table 2). The significantly highest leaf photosynthetic rate was found in the P4Zn2, where the value of the leaf photosynthetic rate was 13.6 ±0.24 µmol CO2 m−2 s−1. However, the lowest value was observed in P1Zn1, i.e., 8.97±0.20 µmol CO2 m−2 s−1. The highest value of chlorophyll content (62.0±0.43 SPAD value) was recorded in P4Zn2, while the lowest (42.7±0.17 SPAD value) was in the P1Zn1 treatment. Similarly, the maximum sub-stomatal conductance (317±2.96 µmol H2O m−2 s−1) in P4Zn2 treatment was reduced to 221±2.75 µmol H2O m−2 s−1 in P1Zn1 treatment. Furthermore, the highest stomatal conductance (0.45±0.00 mmol H2O m−2 s−1) was recorded in the P4Zn2 treatment, while the lowest value of stomatal conductance (0.33±0.01 mmol H2O m−2 s−1) was observed in the P1Zn1. The leaf transpiration rate was recorded as the highest in P4Zn2 treatment with a value of 9.57±0.11 mmol H2O m−2 s−1, while the lowest value was in P5Zn3, i.e., 6.56±0.53 mmol H2O m−2 s−1 (Table 2).
Table 2.
Effect of zinc and phosphorus on physiological traits of cotton crop.
Effect of Zinc and Phosphorus on Vegetative Growth of Cotton Plants
The main growth and morphological features of cotton, such as leaf area index, monopodial branches, plant height, and sympodial branches, varied significantly among all the treatments (Table 3). The highest value of leaf area index, i.e., 3.15±0.02, was observed in the P4Zn2 treatment, and the lowest value of 2.17±0.02 was observed in the P1Zn1 treatment. The maximum monopodial branches (2.00±0.00 branches plant-1) were observed in P4Zn2, and the minimum value (1.00±0.00 branches plant-1) was recorded in P1Zn2 treatment. The highest value of plant height (95.0±0.68 cm) was observed in the P4Zn2 treatment, while the lowest (64.3±0.55 cm) was in the P1Zn1 treatment. The sympodial branches (22.7±0.96 branches plant-1) were also recorded as the highest in P4Zn2, while the lowest value (13.7±0.39 branches plant-1) in P1Zn1 treatment (Table 3).
Effect of Zinc and Phosphorus on Phenology and Reproductive Growth of Cotton
The number of days to flower and boll formation and boll opening, total bolls per plant, and boll weight varied significantly among all the studied treatments (Table 4). The cotton crop took significantly the highest days to flower formation (57.67±0.72) in the P4Zn2 treatment, while it took the lowest (49.0±0.47) in the P1Zn3 treatment. Similarly, the maximum days to boll formation (48.0±0.47) were observed in the P4Zn3 treatment and the minimum value (43.7±0.54) in the P5Zn3 treatment. On the other hand, the highest days to boll opening (78.7±0.98) were recorded in the P4Zn2 treatment and the lowest value (66.3 ±3.03) in the P3Zn3 treatment. The total boll per plant was observed to be substantially the highest (34.0±0.12) in P4Zn2, and the lowest value (23.7±0.05) was recorded in P1Zn2 treatment. The highest value of boll weight (9.59 ±0.15) was observed in P4Zn1 and the lowest value of boll weight (5.92±0.30) in the P1Zn1 treatment (Table 4).
Table 3.
Effect of zinc and phosphorus on vegetative growth parameters of cotton.
Table 4.
Effect of zinc and phosphorus on phenology and reproductive growth parameters of cotton crop.
Effect of Zinc and Phosphorous on Seed Cotton Yield, Total Dry Matter, and Phosphorus and Zinc Availability in the Soil
Seed cotton yield and total dry matter also varied significantly among all the studied treatments (Fig. 2). The highest seed cotton yield (1.278 ±0.067 ton ha-1) was found in the P4Zn2 treatment. However, the lowest value of the seed cotton yield (0.828±0.038 ton ha-1) was observed in the P1Zn1 treatment (Fig. 2). Similarly, the highest total dry matter (10.492 ±0.870 ton ha- 1) was observed in P4Zn2 treatment, while the lowest total dry matter contents (7.271±0.573 ton ha-1) were recorded in P1Zn1 treatment (Fig. 2). The maximum availability of the P (16.0±0.240 mg kg-1) was observed in the P4Zn2 treatment, and the minimum value of available P (13.4±0.201 mg kg-1) was recorded in the P5Zn3 treatment. Similarly, the highest availability of Zn (9.00±0.335 mg kg-1) was recorded in P4Zn2 treatment, and the lowest value of Zn (4.49±0.280 mg kg-1) was recorded in P1Zn1 treatment (Fig. 2).
Discussion
Growth and morphological traits (plant height, peak of leaf area index, and sympodial branches per plant) were affected significantly due to the application of different rates of Zn and P. The cotton crop produced the highest growth and the values of morphological traits in the object with the combined application of 12 kg Zn ha-1 and 90 kg P ha-1 in comparison to other treatment combinations. Based on the results, there was a gradual increase in studied growth and morphological traits of cotton crops with the increasing Zn and P doses. In contrast, cotton showed a decreasing trend in values of studied growth and morphological traits at 18 kg Zn ha-1 and 120 kg P ha-1. Phosphorus is directly involved in energy transfer in the processes of photosynthesis and respiration and is a structural component of nucleic acids [28] that led to the maximum studied growth and morphological traits of cotton in the current study. Similarly, higher growth and morphological traits of cotton crops were recorded with the application of the optimum dose of Zn with P application [29]. In addition, the highest growth and morphological traits of the cotton crop were recorded due to the imperative role of Zn in the activation of enzymes, metabolism of proteins, photosynthetic carbon, and the metabolism of indole acetic acid [17, 30]. Thus, the application of 12 kg Zn ha-1 and 90 kg P ha-1 are the optimum doses of Zn and P, respectively, to achieve the maximum growth and morphological traits of the cotton crop.
However, monopodial branches per plant of cotton were not affected due to the application of Zn and P, which might be due to the genetic character of the studied cotton cultivar. The highest physiological traits of the cotton crop were recorded when the cotton crop was fertilized with 12 kg Zn ha-1 and 90 kg P ha-1. Higher physiological traits of cotton crops were closely associated with the important role of P in the synthesis of chlorophyll content [31], which led to increased studied physiological traits. Similar results have been reported in sheep grass [32], wild barley [33], tea [34], and oats [35]. In contrast, one study showed that cotton fertilization with 70 kg P ha-1 resulted in higher leaf gas exchange and was regarded as an optimum dose for the cotton crop [36]. It has been reported that high growth and yield components of cotton crops could be achieved with 10-15 kg Zn ha-1 and 80-100 kg P ha-1 application under conditions of arid climate [37]. In this context, 12 kg Zn ha-1 and 90 kg P ha-1 are the optimum doses of Zn and P, respectively, to achieve the highest physiological traits of the cotton crop.
Fig. 2.
Phosphorus and zinc available in the soil and seed cotton yield and total dry matter in dependence on particular P-Zn treatments.
All the yield-contributing traits (total bolls per plant, mean boll weight, seed cotton yield, and biological yield) were also affected significantly due to the application of Zn and P. Results showed a linear increase in studied yield traits of the cotton crop with the gradual increase in Zn and P doses, and the maximum yield traits of the cotton crop were recorded at 12 kg Zn ha-1 and 90 kg P ha-1. However, our research findings showed that there were no significant differences in yield traits of cotton with P application up to 120 kg P ha-1. The higher studied yield traits of the cotton crop were due to the vital role of Zn in assimilates translocation to plant parts of the cotton crop and P in energy regulation in photosynthesis and respiration [8]. In the present context, our findings suggest 12 kg Zn ha-1 and 90 kg P ha-1 as the optimum levels of Zn and P for improving yield traits of cotton crops and also support our hypothesis. In contrast, previous studies have shown higher growth and yield components of the cotton crop with P application at a dose of 100 kg ha-1 under an arid climate [38-40]. In the present context, the application of 12 kg Zn ha-1 and 90 kg P ha-1 are the optimum doses of Zn and P, respectively, to achieve the highest yield traits of cotton crops.
Our research findings showed that there was an increasing trend in the availability of P and Zn in the soil, growth, morphological, physiological, and yield traits of the cotton crop with the increasing Zn and P doses up to the maximum at a dose of 12 kg Zn ha-1, and 90 kg P ha-1 and then they started to decrease, which might be due to the antagonistic effect of Zn with P. Overall, the performance of the crop was poor due to excessive rainfall during the growing season (Fig. 1), which might cause submerged conditions and ultimately suffocation of the plants. Moreover, there was no significant improvement when the cotton crop was fertilized with 18 kg Zn ha-1, and 120 kg P ha-1. In crux, increasing Zn and P doses significantly increased the studied physiological, growth, and yield traits of the cotton crop and showed the maximum growth, physiological, and yield traits of the cotton crop were recorded with the combined application of 12 kg Zn ha-1 and 90 kg P ha-1.
Conclusions
Growth, morphological, physiological, and yield traits of cotton crop and phosphorus and zinc availability in the soil were significantly affected due to the interactive effect of Zn and P. Based on the findings of the present study, increasing Zn and P doses increased the growth, morphological, physiological, and yield traits of the cotton crop. The growth, morphological, physiological, and yield traits of the cotton crop were recorded with the application of 12 kg Zn ha-1 and 90 kg P ha-1. Thus, research findings suggest that the combined application of 12 kg Zn ha-1 and 90 kg P ha-1 should be recommended to the farmers to achieve a higher seed cotton yield of the cotton crop.
