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Article

Seed Priming and Foliar Application with Nitrogen and Zinc Improve Seedling Growth, Yield, and Zinc Accumulation in Rice

by
Patcharin Tuiwong
1,
Sithisavet Lordkaew
2,
Jeeraporn Veeradittakit
1,
Sansanee Jamjod
1 and
Chanakan Prom-u-thai
1,3,*
1
Agronomy Division, Department of Plant and Soil Sciences, Faculty of Agriculture, Chiang Mai University, Chiang Mai 50200, Thailand
2
Center of Agricultural Resource Systems, Faculty of Agriculture, Chiang Mai University, Chiang Mai 50200, Thailand
3
Lanna Rice Research Center, Chiang Mai University, Chiang Mai 50200, Thailand
*
Author to whom correspondence should be addressed.
Agriculture 2022, 12(2), 144; https://doi.org/10.3390/agriculture12020144
Submission received: 27 December 2021 / Revised: 17 January 2022 / Accepted: 18 January 2022 / Published: 21 January 2022
(This article belongs to the Topic Plant Nutrition Biofortification)
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Abstract

:
Improving grain yield and zinc (Zn) concentration yields a double benefit for farmers and consumers, especially when accomplished through the common practice of nitrogen (N) and Zn application. The objective of this study was to evaluate responses of a modern improved rice variety (SPT1) to Zn and N fertilizer management of seed germination, seedling growth, yield, and grain Zn accumulation. A preliminary laboratory study was conducted by priming seeds with variation of N and Zn solutions, consisting of (1) 0% urea + 0% ZnSO4 (N0Zn0), (2) 0% urea + 0.07% ZnSO4 (N0Zn+), (3) 0.05% urea + 0.07% ZnSO4 (N0.05Zn+), (4) 0.10% urea + 0.07% ZnSO4 (N0.10Zn+), (5) 0.15% urea + 0.07% ZnSO4 (N0.15Zn+), (6) 0.20% urea + 0.07% ZnSO4 (N0.20Zn+), and (7) 0.25% urea + 0.07% ZnSO4 (N0.25Zn+). Priming seeds with N0.15Zn+ led to a higher germination rate and growth performance. Seedling Zn concentration increased linearly along with the dry weights of root and coleoptile during germination. A second experiment in the field included priming the seed with (1) 0% urea + 0% ZnSO4 (N0Zn0), (2) 0.15% urea + 0% ZnSO4 (N+Zn0), (3) 0% urea + 0.07% ZnSO4 (N0Zn+), and (4) 0.15% urea + 0.07% ZnSO4 (N+Zn+); this experiment showed that simultaneous priming of seeds with 0.15% urea and 0.07% ZnSO4 (N+Zn+) resulted in the highest coleoptile length and seedling dry weight. The highest seedling Zn concentration was observed when priming seeds with N0Zn+ followed by N+Zn+, but the effect disappeared at the later growth stages. A third experiment in the field was conducted by foliar application with four different treatments of (1) 0% urea + 0% ZnSO4 (N0Zn0), (2) 1% urea + 0.5% ZnSO4 (N+Zn0), (3) 0% urea + 0.5% ZnSO4 (N0Zn+), and (4) 1% urea + 0.5% ZnSO4 (N+Zn+). The highest grain yield increases were achieved by foliar application of N+Zn0 (28.5%) and foliar application of N+Zn+ (32.5%), as compared with the control (N0Zn0). Grain Zn concentration was the highest under foliar application of N+Zn+, with a 37.9% increase compared with N0Zn0. This study confirmed that seedling growth performance can be enhanced by initially priming seeds with N and Zn solution, while grain yield and Zn concentration can be improved by foliar application of N and Zn fertilizer. The information would be useful for the appropriate combined application of Zn and N fertilizers in the practical field to improve grain yield and Zn accumulation as well as Zn nutrition among humans with rice-based diets. The result should be extended to a wider range of rice varieties under suitable management of N and Zn fertilizer.

1. Introduction

Rice is one of the most important crops, providing the world population with a sufficient nutrient source [1]. Even though rice is a staple food consumed by more than half of the world population, it contains a very low amount of micronutrients, such as Zn, which are essential in many functions in humans, e.g., protein synthesis and cell-mediated immunity, containing antioxidants and anti-inflammatory agents [2]. Therefore, improving Zn accumulation in rice is one promising way to promote Zn nutrition among the world population.
Besides being important for humans, Zn is an essential micronutrient in plant growth and development [3]. The major role of Zn in plants is to act as the cofactor for enzymes involved in N metabolism, such as alcohol dehydrogenase [4]. Zn deficiency impedes the function of alcohol dehydrogenase, reduces anaerobic root metabolism, and lessens seedlings’ capacity to cope with anaerobic soil conditions [5]. In rice, the noticeable symptoms of Zn deficiency are leaf wilting due to the loss of turgidity, basal leaf chlorosis, delay of development, leaf bronzing, and, in some cases, death of the rice seedlings [6]. Adding Zn to rice seed has been reported as a way to improve seed germination, seedling vigor, and plant development [3,7,8], as well as productivity [9,10]. The enhancement occurs during seed germination because the production of reactive oxygen species (ROS) is unavoidable, and seeds or seedlings have defense mechanisms against ROS production via the Zn-dependent superoxide dismutase enzyme [11]. Therefore, a sufficient level of Zn in seed remains highly important to supply nutrients until root uptake begins, particularly when seeds are sown in soil with low Zn availability, such as calcareous or alkaline soils [6].
Seed priming is a promising, rapid, efficient, and low-cost approach to increasing the rate of germination. For example, chickpea seeds primed with 1.0 mM Zn [12] and rice seeds primed with 2.5 mM Zn [13,14,15] had improved seed germination and early seedling growth due to the minimizing of stress-induced biochemical markers [16]. The benefit of high seed Zn in seedling growth is also indicated by the positive correlation between seed Zn concentration and the combined root and shoot dry weight [17]. The effects of priming seeds with Zn on seed germination and seedling growth are well documented, but the combined effect of Zn and N has rarely been reported, despite the fact that it may synergistically support the mechanism of seedling growth. The question has been raised as to whether priming does have a long-term effect on productivity and seed Zn accumulation or whether it only affects seed quality and seedling growth. On the other hand, foliar application with Zn and N fertilizer is suggested as an effective strategy for rapidly improving grain yield and Zn accumulation. Therefore, the foliar application of Zn and N in the later growth stage (maximum tillering and booting stages) was introduced in this study, aiming for a greater increase in yield and grain Zn concentration.
The combined application of Zn and N increases the availability of Zn by enhancing the transformation of exchangeable, loose organic, and carbonate-bound Zn from other fractions [18,19,20]. Absorption of N facilitates translocation of Zn into grain [21]. The combined application of 150 kg N/ha and 0.3% ZnSO4. H2O spray at anthesis was found to enhance the grain yield of rice [22] and wheat [23]. Information on the effect of simultaneous seed priming and continued foliar application of Zn and N on productivity and grain Zn accumulation is limited; such knowledge would be valuable for the management of Zn and N fertilizer in rice crops for improving both grain yield and quality. It was hypothesized that seed priming and foliar application of Zn and N could help to stimulate seed germination, seedling growth, productivity, and grain Zn accumulation in rice crops. The present study was carried out to evaluate the responses of a modern improved rice variety (SPT1) to Zn and N fertilizer management on seed germination, seedling growth, yield, and grain Zn accumulation.

2. Materials and Methods

2.1. Study Site, Rice Materials, and Soil Properties

This study consisted of three main experiments. The first experiment was conducted under laboratory conditions at the Agronomy Division, Department of Plant and Soil, Faculty of Agriculture, Chiang Mai University, Thailand, while the second and third experiments were conducted under field conditions at the field research station, Chiang Mai University, during June 2019 to January 2020. The field research station is located at a latitude of 18°45′51″ N and a longitude of 98°55′48″ E, with an elevation of 340 m above sea level. The average temperature during the cropping season was 29.7 ± 2.1 °C with 80.0% relative humidity and 43.4 mm rainfall [24]. The seeds of the modern improved rice variety San Pa Tong 1 (SPT1) were derived from the rice seed center at Chiang Mai, Thailand. The soil at the experimental field was a sandy loam of the Sansai series and contained 0.1% total nitrogen (N) (Kjeldahl method), 46.2 kg ha−1 available phosphorus (P) (Bray II), 53.4 mg kg−1 exchangeable potassium (K) (NH4OAc, pH 7), and 3.11% organic matter (Walkley–Black method). The pH of the soil was 5.8, and DTPA-extractable Zn in the soil was 0.8 mg kg−1 soil (DTPA) [20]. The electrical conductivity of the soil was 0.61 dS/m.

2.2. Experiment 1: Preliminary Investigation of the Effects of Seed Priming with Zn and N on Seedling Growth and Zn Accumulation in Seedlings

The experiment was conducted in a completely randomized design (CRD) with three independent replications. About 20 g of each seed sample was carefully washed 3 times with DDI water. The seed samples were then primed with N and Zn by soaking in the prepared solution. The treatment of N and Zn solutions consisted of (1) 0% urea + 0% ZnSO4 (N0Zn0), (2) 0% urea + 0.07% ZnSO4 (N0Zn+), (3) 0.05% urea + 0.07% ZnSO4 (N0.05Zn+), (4) 0.10% urea + 0.07% ZnSO4 (N0.10Zn+), (5) 0.15% urea + 0.07% ZnSO4 (N0.15Zn+), (6) 0.20% urea + 0.07% ZnSO4 (N0.20Zn+), and (7) 0.25% urea + 0.07% ZnSO4 (N0.25Zn+). The priming of Zn solution was implied by a previous study [14]. The seeds were soaked in the solutions for 24 h, similar to the common practice among farmers, before being rinsed thoroughly with distilled deionized water. The primed seeds were germinated on germination paper within plastic trays, 300 seeds per tray. The moisture was kept at the same level by providing 10 mL deionized water equally into the plastic trays daily. The germination period was set as 9 days (D9). Root and coleoptile lengths of the seedlings were measured every day from day 3 (D3) to day 7 (D7), while the germination rate and the number of roots were recorded on day 9 (D9). On the final day (D9), seedlings were harvested and separated into the root, coleoptiles, and residual seed before being oven-dried at 75 °C for 72 h, before Zn concentration analysis.

2.3. Experiment 2: Effect of Seed Priming with Selected Concentrations of N and Zn Solution on Seedling Growth and Development

The experiment was arranged in a factorial design in a completely randomized design (CRD) with four independent replications. The same rice variety as in the above experiment was used in this study. Seeds were primed for 24 h, as in experiment 1, by soaking in the selected solutions of N and Zn: (1) 0% urea + 0% ZnSO4 (N0Zn0), (2) 0.15% urea + 0% ZnSO4 (N+Zn0), (3) 0% urea + 0.07% ZnSO4 (N0Zn+), and (4) 0.15% urea + 0.07% ZnSO4 (N+Zn+). The primed seeds were rinsed with DDI water and incubated in moistened fabric for 48 h. Afterward, germinated seeds were transferred into prepared commercial seedling trays for 48 h. The trays with germinated seeds were transferred into the field as in practical cultivation. A total of 300 seedlings in each tray were collected at 7, 14, and 21 days after germination. The collected seedlings were carefully washed 3 times with DDI water before being evaluated for seedling height and then separated into the youngest emerged blade (YEB) and shoot (residual seedling), before being oven-dried at 75 °C for 72 h to measure seedling dry weight. The dried samples were mechanically ground in a hammer mill for Zn concentration analysis.

2.4. Experiment 3: Effect of Foliar Application of Zn and N on Yield and Grain Zn Accumulation

This experiment was designed as a further evaluation of the second experiment. Seedlings of the SPT1 rice variety from the above experiment were transplanted into plots, 3 plants per hill, with 25 × 25 cm spacing between hills. Basal fertilizer of 100N-30P-30K kg ha−1 was equally applied at 3 weeks after transplanting and the maximum tillering stage. Foliar application was sprayed using 4 different fertilizer treatments of (1) 0% urea + 0% ZnSO4 (N0Zn0), (2) 1% urea + 0.5% ZnSO4 (N+Zn0), (3) 0% urea + 0.5% ZnSO4 (N0Zn+), and (4) 1% urea + 0.5% ZnSO4 (N+Zn+) at the rate of 1000 L per hectare. The application rate of foliar Zn fertilizer was suggested by a previous study [25]. The foliar sprays were applied two times at maximum tillering and booting stages. The youngest emerged leaf blade was marked for each plant. A week after foliar application, the new leaves were collected for Zn concentration analysis. The samples were washed with distilled water and oven-dried at 75 °C for 72 h. The dried samples were mechanically ground in a hammer mill. At maturity, grain yield (14% moisture content), straw dry weight, thousand grain weight, percentage of filled grains, spikelet per panicle, and tiller and panicle numbers per plant were evaluated. The seeds had the husk removed to yield brown rice for Zn concentration analysis.

2.5. Zn Concentration Analysis

The concentration of Zn was analyzed by the dry ashing method [26]. The samples were subjected to burning in a muffle furnace at 535 °C for 8 h. The samples were then acid-extracted before measuring the concentration of Zn by atomic absorption spectrophotometry (AAS) (Z-8230 Polarized Zeeman, Hitachi, Japan). Each batch of Zn analysis employed peach (SRM 1547) and soybean leaves as certified reference materials for Zn analysis.

2.6. Statistical Analysis

The data of Zn concentration, yield, and yield component means of four replications were evaluated using analysis of variance (ANOVA) followed by LSD comparison tests. Analysis of variance was performed using Statistix 8 (Analytical software, SXW, Tallahassee, FL, USA). Correlation analysis was used to test the significance of each association. Statistically significant differences were identified at a level of p < 0.05. Correlation and regression analyses were performed for each set of parameters.

3. Results

3.1. Experiment 1: Preliminary Investigation of the Effects of Seed Priming with Zn and N on Seedling Growth and Zn Accumulation

Priming seeds with N and Zn had different impacts on seedling growth and Zn concentration in germinating seeds. The priming had clear effects on seedling growth as measured by the lengths of roots and coleoptiles (p < 0.05; Figure 1). Priming seeds with N and Zn promoted root and coleoptile growth during the first few days of germination. Seedlings from N0.10Zn+ primed seeds resulted in the highest root length at D3, D4, D7, and D9, while N0.15Zn+ seedlings had the highest root length at D6 and D7 (Figure 1A). The most effective N and Zn priming rates for coleoptile length were N0.10Zn+, N0.15Zn+, and N0.20Zn+, up to D9 (Figure 1B). Priming seeds with the various N and Zn concentrations significantly affected seed germination rate, root number, and dry weight of seedlings during the 9 days of germination (p < 0.001; Figure 2). Priming with N0.15Zn+ yielded the maximum germination rate of 76.0% at 9 days after germination, a 12.7% increase compared with nil N and Zn (Figure 2A). Priming with N0.15Zn+ and N0.20Zn+ produced more roots than the other treatments (Figure 2B). Seedling dry weight was increased 15.1% by priming with N and Zn at N0.05Zn+ to N0.25Zn+, averaging 7.3 mg per seedling, compared with priming with nil N and Zn, while priming with Zn alone yielded an increase of 7.5% (Figure 2C). The root and coleoptile Zn concentrations of seedlings were affected by N and Zn priming (Figure 3). Seedlings from seeds primed with N0.10Zn+ through to N0.25Zn+ had the highest root Zn concentration with an increase of 89% compared with nil N and Zn (Figure 3A). The highest Zn concentration in the coleoptile was found when seeds were primed with N0.20Zn+, 58% higher than priming with nil N and Zn (Figure 3B). There was a positive correlation between Zn concentration and dry weight of roots (r = 0.80, p < 0.001) and coleoptiles (r = 0.60, p < 0.01) during the germination period (Figure 3C).

3.2. Experiment 2: Effect of Seed Priming with Selected Concentrations of N and Zn Solution on Seedling Growth and Development

Seed priming with various N and Zn concentrations significantly affected coleoptile length at 7, 14, and 21 days after germination (p < 0.001; Figure 4). The effect of the priming treatments on coleoptile length was in the following order: at 7 days, N+Zn+ > N+Zn0 > N0Zn+ > N0Zn0, at 14 days, N0Zn+ > N+Zn+ > N+Zn0 > N0Zn0, while the order for increasing coleoptile length at 21 days was N+Zn+ > N0Zn+ > N0Zn0 > N+Zn0. Seedling dry weight was significantly higher in seeds primed with N+Zn+ solution at both 7 and 21 days, and N0Zn+ at 14 days (p < 0.001; Figure 5A). Seed priming significantly improved Zn concentration in YEB of the rice seedlings (p < 0.001; Figure 5B). The highest seedling Zn concentration was obtained from the plots with seeds primed with N0Zn+ solution followed by N+Zn+ solution at 7, 14, and 21 days.

3.3. Experiment 3: Effect of Foliar Application of Zn and N on Yield and Grain Zn Accumulation

Foliar application with N and Zn significantly affected yield and yield components in rice (Table 1). Plant height, panicle number per plant, and percentage of filled grains were significantly higher under both foliar application of N+Zn0 and N+Zn+ solutions, while tillers per plant and spikelets per panicle were the highest with N+Zn+. Foliar application of N+Zn0 and N+Zn+ solutions produced an average plant height of 89.6 cm, greater than with N0Zn0 solution by 5.3%. In SPT1, foliar application of N+Zn0 and N+Zn+ produced higher numbers of panicles per plant with respective averages of 16.5 and 16.3 panicles per plant with an increase of 1.6 panicles per plant, compared with N0Zn0 application. Foliar application of N+Zn0 and N+Zn+ produced averages of 16.0 and 16.8 tillers per plant, an average increase of 1.4 tillers per plant compared with N0Zn0 and N0Zn+. The average number of spikelets were 214.0 spikelets per panicle under foliar application of N+Zn+, higher than in the N0Zn0, N0Zn+, and N+Zn0 treatments by 12.5, 10.5, and 6.0 spikelets per plant, respectively. Foliar application of N0Zn+ resulted in the lowest filled grain percentage at 70.5%, while N+Zn0 and N+Zn+ application increased the percentages of filled grains by 14.7% compared with N0Zn0. In SPT1, foliar application of N0Zn+ and N+Zn+ produced the highest thousand grain weight, averaging 28.0 g. The foliar application also affected straw and grain yields; the highest straw yield was found with N+Zn0 and N+Zn+ at 0.98 kg m−2, and grain yield was increased by 28.5% and 32.5% under N+Zn0 and N+Zn+, respectively, compared with N0Zn0.
Foliar N and Zn affected Zn concentration differently in different plant parts of SPT 1 (p < 0.001) (Table 2). Straw Zn concentration was increased up to 19.5%, 8.6%, and 16.7% under N0Zn+, N+Zn0, and N+Zn+, respectively, compared with N0Zn0, while leaf Zn concentration was increased by 23.2%, 15.9%, and 30.1%, respectively. For unhusked grain, Zn concentration was increased by 14.2% and 24.0% under N0Zn+ and N+Zn+, respectively. In brown rice, the highest Zn concentration was associated with N+Zn+, yielding an increase of 37.9% compared with the N0Zn0 application. There was strong positive correlation between leaf Zn concentration and grain yield (r = 0.78, r2 = 0.61, p < 0.01) along with straw yield (r = 0.71, r2 = 0.50, p < 0.01).

4. Discussion

Rice cultivation requires a long period of crop growth and development before achieving the desirable grain yield and quality. This study has shown that simultaneously priming rice seeds with N and Zn can promote seed germination and seedling performance depending on the priming ratio. The appropriate priming combination of N and Zn for rice seeds improved seedling growth and development during the seedling stage at 7–14 days under practical field conditions. Additionally, grain yield and Zn concentration in brown rice were increased by simultaneous foliar application of N and Zn. This information should be very useful to farmers and rice consumers, particularly among the health-conscious population.
The better seedling growth and development resulted from simultaneous priming with N and Zn compared with control treatments without priming. In this study, priming seeds with N and Zn significantly enhanced germination rate, root and coleoptile length, root number, and dry weight of seedlings. This could be the effect of higher Zn concentration in root and coleoptile compared with the controls. A previous study indicated the interaction effect mechanisms between N and Zn that Zn plays a major role in plants by acting as the cofactor for enzymes involved in N metabolism, such as alcohol dehydrogenase [4]. The deficiency of Zn was found to be involved in the function of alcohol dehydrogenase, reducing anaerobic root metabolism, and lessening seedlings’ capacity to cope with anaerobic soil conditions [5]. The improvement of seedling growth parameters might be due to positive interactions between N and Zn within the plant body and the increased activity of several enzymes that facilitated vegetative growth and photosynthesis. A previous study reported that enzyme activities of esterase, glycerol 3-phosphate dehydrogenase, and alpha-amylase increased in primed seeds, leading to increased metabolism of seed storage materials, such as carbohydrates, lipids, and proteins [27]. The increased germination rates of corn, rice, chickpeas, and bread wheat by priming seeds with N or/and Zn fertilizers have been reported [12,28,29]. In general, N and Zn are required for seed germination, as nitrogen-containing compounds stimulate germination [30,31], while Zn priming induces metabolic activities of germination, including metabolism of sugars that can be used in protein synthesis during germination, and this can improve germination rate and uniform growth of the plants [32]. Aboutalebian [27] showed that seed priming with Zn increased the speed and percentage emergence of wheat by 35.7% and 24.6%, respectively. However, the appropriate concentrations used in combination should be applied in the priming process. This study has shown that priming seeds with N0.15Zn+ produced a higher percentage of germination compared with other priming treatments and the control N0Zn0. Additionally, priming with N0.15Zn+ also had the highest growth performance and yield attributed traits during early seedling growth. The effect of N and Zn in combination may be due to addition of Zn with N maintaining a favorable balance between N and Zn in the rice plants for optimum growth. Since a wide range of soils are deficient in available N and Zn [33,34], seed priming combined with N and Zn can be effective for the treatment of N and Zn deficiency in the soil. Thus, the optimal N and Zn in plant tissues are agronomic characteristics that could benefit plant growth under low N and Zn supply in the soil, especially the aspects of active protein synthesis and/or other related functions during root and coleoptile development.
The response of rice seedlings to priming seeds with Zn solution has been well documented [16,35], but interaction effects between N and Zn application on yield and quality of rice seedling are less well studied. The present study found that seedlings of rice variety SPT1 at 21 days from seeds primed with 0.15% urea and 0.07% ZnSO4 (N+Zn+) had the highest coleoptile length and seedling dry weight compared with the control treatment. A previous study reported that the coleoptile growth is highly dependent upon elongation of a large number of cells pre-formed in the embryo [36], especially under waterlogged conditions that are a common practice for seedling preparation in rice cultivation, and successful seedling establishment requires rapid coleoptile growth to ensure access to oxygen near the water surface [37]. Seedling dry weight is another character used to indicate seedling vigor, and it has recently been found that faster seedling growth results in better plant development [38]. The simultaneous priming of seeds with N and Zn is an innovative technique for enhancing rice seedling vigor during transplanting in the field. Generally, the application of N fertilizer under waterlogging conditions can cause N fertilizer loss through ammonia volatilization and runoff, thereby reducing the utilization efficiency of N fertilizer [39,40]. The innovation not only minimizes fertilizer costs but also improves the quality of rice seedlings by alleviating seedling injury during transplanting and maintaining the efficiency of root activity, e.g., nutrient absorption, and consequently, maintaining vigorous initial growth. However, the effect may not last until maturity stage—the criterion for improving grain yield and Zn accumulation for health benefits to rice consumers.
Foliar application of a combination of N and Zn has been introduced during flowering and grain developmental stages for boosting productivity and grain Zn accumulation. This study has demonstrated a 44.0% increase in grain yield by foliar application of N+Zn0 and N+Zn+ compared with the control without N or Zn resulting from increased yield components (plant height, panicle number per plant, and percentage of filled grains). A previous study reported that N applied at the early vegetative stage could promote tillering and increase panicle number per plant, while N applied at the panicle initiation stage is necessary to enhance spikelet number per panicle [41]. This study confirmed that foliar application of 1% urea solution with or without Zn fertilizer was sufficient to produce the highest grain yield. The foliar application of N raised grain yield by enhancing the assimilate source through increasing N concentration and by enhancing sinks through the reduction in spikelet degeneration [42]. However, foliar Zn application increased thousand grain weight, with the greatest increases under N0Zn+ and N+Zn+ solutions. The foliar Zn application at early stages will ensure better crop nutrition at tillering and booting stages that in turn may result in increased grain weight. These results are in agreement with those of Singh [43], who found considerable enhancement in thousand grain weight of wheat crops by foliar Zn spraying. In addition, the positive interaction between Zn with N provided a basis for higher crop yields due to the increased N uptake that boosted chlorophyll content, as N is a structural component of many enzymes associated with chlorophyll synthesis [44].
Additionally, foliar application of Zn to crops is an effective way to increase the grain concentration of Zn. However, the development of more efficient foliar Zn fertilizers is limited by a lack of knowledge regarding the distribution, mobility, and speciation of Zn in leaves once it is taken up by the plant. Foliar Zn application may more effectively enhance grain Zn accumulation when it is applied concurrently with foliar N. Another aspect that remains poorly investigated under rice field conditions is the impact of foliar Zn application when combined with N. Therefore, there is an urgent need to understand the complex interactions between foliar-applied Zn and N solutions to ensure the efficient use of Zn-containing N solutions. A previous study by Li [45] reported that foliar-supplied Zn and N are synergistically effective for Zn availability. The results of this study showed that the Zn concentrations in leaves, unhusked grains, and brown rice were considerably increased by foliar N+Zn+ application. Wang [46] reported that foliar N application resulted in an 8.6% increase in grain Zn concentration when foliar Zn was applied concurrently. In wheat crops, foliar N+Zn+ enhanced the protein concentration in grains [45]. This is important because the amount and composition of endosperm storage proteins have a marked influence on Zn accumulation. Additionally, the effect could be due to foliar N+Zn+ increasing the vegetative growth of the plant, and thus it takes a number of days to reach the maturity stage, resulting in longer Zn accumulation in plant tissues. Gul [47] reported that the maximum day to the flowering stage was recorded in wheat crops sprayed with foliar N solution. However, Zn increases in plants are related to several physiological mechanisms operating during plant growth, including the ability of Zn uptake from root to shoot, translocation, and partitioning into different plant parts [25]. Therefore, the increase in grain Zn concentration following foliar N+Zn+ application resulted from the significant positive interaction between foliar N and Zn spraying. However, an increase in Zn concentration in leaves, resulting from the foliar N and Zn applications, resulted in an increase in straw and grain yield. Leaf Zn concentrations had strong positive correlations with grain and straw yield. Foliar Zn applied at early stages of maximum tillering is transported only by xylem to the leaves (sink tissues), where it remains until leaf senescence begins, and a small amount of such applied Zn is expected to flow into the developing grains [48]. Additional research needs to be done to confirm this hypothesis (foliar N combined with Zn application) in other growth stages of rice. Moreover, the increase in yield in response to foliar N and Zn application in this study affecting the Zn accumulation in grain and plant tissues also requires further investigation.

5. Conclusions

The application of N and Zn in rice offers a practical and useful approach in boosting both yield and grain Zn accumulation. The current study revealed that priming seeds with N and Zn solutions are recommended as an excellent protocol to promote seed Zn accumulation and seedling growth performance, while foliar spraying increases production by enhancing each yield component. This study has demonstrated that priming seeds with 0.15% urea + 0.07% ZnSO4 solution has proved to be suitable for effectively improving germination rate and seedling growth in early stages in both the modern and traditional improved rice varieties. Grain yield was enhanced by foliar application of combined N and Zn fertilizer, while it was slightly reduced under foliar application of N alone. The grain yield was increased by 28.5% and 32.5% under N+Zn0 and N+Zn+, respectively, compared with N0Zn0. Additionally, foliar application of combined N and Zn also resulted in the highest increases of 37.9% grain Zn concentration in brown rice compared with the N0Zn0 application, but the concentration was decreased under foliar application of Zn alone. These results indicated that N synergistically acted together with Zn fertilizer in rice cultivation to promote seed germination, seedling vigor, grain yield, and grain Zn concentration. The future study could be strengthened by increasing a wider range of rice varieties under the suitable management of N and Zn fertilizer, which would be very useful information for increasing grain yield and Zn accumulation.

Author Contributions

Conceptualization, C.P.-u.-t.; methodology, S.L., J.V. and P.T.; validation and analysis, C.P.-u.-t., S.J. and P.T.; draft preparation, P.T.; writing—review and editing, C.P.-u.-t. All authors have read and agreed to the published version of the manuscript.

Funding

We are grateful to the Thailand Research Fund under Royal Golden Jubilee PhD Program (grant no. PHD/0168/2560). This research was also partially supported by Chiang Mai University.

Institutional Review Board Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Agriculture 12 00144 g001 550
Figure 1. Root (A) and coleoptile (B) length of seedlings in rice variety SPT1 when seeds were primed with simultaneous Zn and N during 3–9 days of germination. Each data point is a mean of three independent replications. LSD (0.05)—least significant difference at p < 0.05.
Figure 1. Root (A) and coleoptile (B) length of seedlings in rice variety SPT1 when seeds were primed with simultaneous Zn and N during 3–9 days of germination. Each data point is a mean of three independent replications. LSD (0.05)—least significant difference at p < 0.05.
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Figure 2. Percent germination (A), number of roots per seedling (B), and seedling dry weight (C) in rice variety SPT1 when seeds were primed with simultaneous Zn and N at 9 days of germination. Each data point is a mean of three independent replications. *** indicates significantly different at p < 0.001. Different lowercase letters above the bars indicate significant differences between seed priming treatments. LSD (0.05)—least significant difference at p < 0.05.
Figure 2. Percent germination (A), number of roots per seedling (B), and seedling dry weight (C) in rice variety SPT1 when seeds were primed with simultaneous Zn and N at 9 days of germination. Each data point is a mean of three independent replications. *** indicates significantly different at p < 0.001. Different lowercase letters above the bars indicate significant differences between seed priming treatments. LSD (0.05)—least significant difference at p < 0.05.
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Agriculture 12 00144 g003 550
Figure 3. Root (A) and coleoptile (B) Zn concentrations of seedlings in rice variety SPT1 when seeds were primed with simultaneous Zn and N at 9 days of germination. Each data point is a mean of three independent replications. The relationship between seedling dry weight and Zn concentration in root and coleoptile (C). **, *** indicate significantly different at p < 0.01 and p < 0.001, respectively. Different lowercase letters above the bars indicate significant differences between seed priming treatments. Least significant difference at p < 0.05 (n = 21).
Figure 3. Root (A) and coleoptile (B) Zn concentrations of seedlings in rice variety SPT1 when seeds were primed with simultaneous Zn and N at 9 days of germination. Each data point is a mean of three independent replications. The relationship between seedling dry weight and Zn concentration in root and coleoptile (C). **, *** indicate significantly different at p < 0.01 and p < 0.001, respectively. Different lowercase letters above the bars indicate significant differences between seed priming treatments. Least significant difference at p < 0.05 (n = 21).
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Figure 4. Coleoptile length (A) of seedlings of SPT1 rice variety after simultaneous priming with different N and Zn concentrations at 7, 14, and 21 days in a seedling culture tray. Each data point is the mean of four independent replications. Different lowercase letters above the bars indicate significant differences between seed priming treatments. *** indicates significantly different at p < 0.001. Least significant difference at p < 0.05. The appearance of seedlings in different seed priming treatments at 14 days after germination (B).
Figure 4. Coleoptile length (A) of seedlings of SPT1 rice variety after simultaneous priming with different N and Zn concentrations at 7, 14, and 21 days in a seedling culture tray. Each data point is the mean of four independent replications. Different lowercase letters above the bars indicate significant differences between seed priming treatments. *** indicates significantly different at p < 0.001. Least significant difference at p < 0.05. The appearance of seedlings in different seed priming treatments at 14 days after germination (B).
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Figure 5. Seedling dry weight (A) and Zn concentration in YEB (B) of the SPT1 rice variety after simultaneous priming with different N and Zn concentrations at 7, 14, and 21 days in the seedling culture tray. Each data is a mean of four independent replications. *** indicates significantly different at p < 0.001. Different lowercase letters above the bars indicate significant differences between seed priming treatments. LSD (0.05)—least significant difference at p < 0.05.
Figure 5. Seedling dry weight (A) and Zn concentration in YEB (B) of the SPT1 rice variety after simultaneous priming with different N and Zn concentrations at 7, 14, and 21 days in the seedling culture tray. Each data is a mean of four independent replications. *** indicates significantly different at p < 0.001. Different lowercase letters above the bars indicate significant differences between seed priming treatments. LSD (0.05)—least significant difference at p < 0.05.
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Table
Table 1. Yield and yield components of the SPT1 rice variety grown using four different N and Zn foliar application rates. Foliar application comprised treatments of (1) 0% urea + 0% ZnSO4 (N0Zn0), (2) 0% urea + 0.5% ZnSO4 (N0Zn+), (3) 1% urea + 0.5% ZnSO4 (N+Zn0), and (4) 1% urea + 0.5% ZnSO4 (N+Zn+). The samples were evaluated at maturity.
Table 1. Yield and yield components of the SPT1 rice variety grown using four different N and Zn foliar application rates. Foliar application comprised treatments of (1) 0% urea + 0% ZnSO4 (N0Zn0), (2) 0% urea + 0.5% ZnSO4 (N0Zn+), (3) 1% urea + 0.5% ZnSO4 (N+Zn0), and (4) 1% urea + 0.5% ZnSO4 (N+Zn+). The samples were evaluated at maturity.
Foliar ApplicationsPlant Height (cm)Panicle Number per PlantTillers per PlantSpikelet per Panicle Filled Grains (%)Thousand Grains Weight (g)Straw Yield (kg m−2)Grain Yield (kg m−2)
N0Zn085.0b14.8b15.0c201.5d72.5b27.0c0.83b0.83c
N0Zn+85.3b14.8b15.0c203.5c70.5c28.0a0.80b0.85c
N+Zn089.3a16.5a16.0b208.0b84.5a27.5b0.98a1.16b
N+Zn+89.8a16.3a16.8a214.0a85.5a28.0a0.98a1.23a
F-test************************
LSD 0.052.50.90.41.81.30.50.060.06
The lowercase letters indicate a significant difference between the treatments by LSD0.05, least significant difference at p < 0.05. Data were statistically analyzed by F-test (*** indicates significantly different at p < 0.001).
Table
Table 2. The concentration of Zn in straw, leaves, and unhusked and brown rice of the SPT1 variety grown using four different N and Zn foliar application rates. Foliar application treatments were (1) 0% urea + 0% ZnSO4 (N0Zn0), (2) 0% urea + 0.5% ZnSO4 (N0Zn+), (3) 1% urea + 0.5% ZnSO4 (N+Zn0), and (4) 1% urea + 0.5% ZnSO4 (N+Zn+).
Table 2. The concentration of Zn in straw, leaves, and unhusked and brown rice of the SPT1 variety grown using four different N and Zn foliar application rates. Foliar application treatments were (1) 0% urea + 0% ZnSO4 (N0Zn0), (2) 0% urea + 0.5% ZnSO4 (N0Zn+), (3) 1% urea + 0.5% ZnSO4 (N+Zn0), and (4) 1% urea + 0.5% ZnSO4 (N+Zn+).
Foliar ApplicationZn Concentration (mg kg−1)
StrawLeavesUnhusked GrainBrown Rice
N0Zn032.0d35.5d21.2c27.2c
N0Zn+39.8a43.9b24.7b40.1b
N+Zn035.0c42.2c20.5c25.1d
N+Zn+38.5b46.2a27.9a43.8a
F-test************
LSD 0.051.30.81.61.6
The lowercase letters indicate a significant difference between the treatments by LSD0.05, least significant difference at p < 0.05. Data were statistically analyzed by F-test (*** indicates significantly different at p < 0.001).
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Tuiwong, P.; Lordkaew, S.; Veeradittakit, J.; Jamjod, S.; Prom-u-thai, C. Seed Priming and Foliar Application with Nitrogen and Zinc Improve Seedling Growth, Yield, and Zinc Accumulation in Rice. Agriculture 2022, 12, 144. https://doi.org/10.3390/agriculture12020144

AMA Style

Tuiwong P, Lordkaew S, Veeradittakit J, Jamjod S, Prom-u-thai C. Seed Priming and Foliar Application with Nitrogen and Zinc Improve Seedling Growth, Yield, and Zinc Accumulation in Rice. Agriculture. 2022; 12(2):144. https://doi.org/10.3390/agriculture12020144

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Tuiwong, Patcharin, Sithisavet Lordkaew, Jeeraporn Veeradittakit, Sansanee Jamjod, and Chanakan Prom-u-thai. 2022. "Seed Priming and Foliar Application with Nitrogen and Zinc Improve Seedling Growth, Yield, and Zinc Accumulation in Rice" Agriculture 12, no. 2: 144. https://doi.org/10.3390/agriculture12020144

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