In order to investigate the effects of foliar application of nutrients [N, phosphorus (P), K, iron (Fe), boron (B), zinc (Zn), manganese (Mn), and copper (Cu)] on yield and yield components of rice, an experiment was performed as a randomized complete block design (RCBD) with four biological replicates.

The current study was performed during two crop years (2016 and 2017) at the Rice Research Institute of Iran. The Rice Research Institute is located in Amol City (latitude: 36° N, longitude: 52°3′ E) at 24 m above sea level. The soil at the experimental site was loamy-clay, with a pH of 7.7.

The rice variety used in this study was the Sahel variety, which was introduced to Iran in 2002 by the Rice Research Institute of Iran in Rasht.

The first plowing operation was performed in midwinter of both years in order to aerate the soil and facilitate furrow operations, and the plots were kept fallow until sowing. Midwinter plowing before rice sowing is common in this area, and allows the soil to be exposed to the air and removes weeds in the soil through drying or freezing (Melikhov & Popov,2017). After bordering and leveling the nurseries and 72 hours before planting the seedlings, the herbicides butachlor (4 L/ha) and bensulfuron-methyl (75 g/ha) were used. After germination, seeding with 50–100 g of seed per square meter was performed in early April, and nurseries were covered with transparent nylon to prevent cold damage and accelerate the growth process. In the last 10 days of April, after growth of the seeds in the nurseries (four leaves unfolded; BBCH code: 14), the plastic cover was removed, and 2 kg of urea fertilizer was applied as topdressing. Until the emergence of cotyledons, rice seedling nutrition depends on endosperm storage, but, from this stage onwards, due to the expansion of the roots and the high nutritional needs of the seedlings, the absorption of nutrients from the roots begins. Since the density of seedlings in nurseries is usually high, the soil of the nursery must be very rich in nutrients. As such, it is necessary to use different fertilizers. Fertilizer was simultaneously applied to the main field, and 100 kg/ha of pure N from urea (one half applied before planting and one half applied after transplanting), 45 kg of P (from super phosphate triple), and 80 kg of K (from potassium sulphate) was added to the soil of the main field.

Transplanting was performed by hand on May 14, 2017 and May 21, 2018, with a density of 16 hills per square meter, and a distance of 25 cm between rows, in 5 × 5-m plots as single transplants. Each block had six plots, and each plot included 20 rows of planting with 25 cm between rows. The distance between the blocks was 2 m.

Weeding was performed twice by hand, at intervals of 20 and 30 days after transplanting. Agronomical management was performed in order to control pests and diseases at different growth stages. After observation of rice stem borer larvae, 10% diazinon granules were applied at 20 kg/ha.

Experimental treatments consisted of foliar application of nutrients at different stages of plant growth. The chemical composition of the liquid fertilizer with concentrations of 1 per mille (1 per thousand) was as follows: N: 7%, P: 7%, K: 7%, Fe: 0.05%, B: 0.05%, Zn: 0.01%, Mn: 0.01%, and Cu: 0.01%. The different experimental treatments included: (A) foliar application of fertilizer in the midtillering stage (five tillers detectable; BBCH code: 25); (B) foliar application of fertilizer in the maximum tillering stage (end of tillering; BBCH code: 29); (C) foliar application of fertilizer in the panicle initiation stage (green ring stage; BBCH code: 30); (D) foliar application of fertilizer in the maximum tillering and panicle initiation stages (BBCH codes: 29 and 30); (E) foliar application of fertilizer in all three stages (BBCH codes: 25, 29, and 30); and (F) no foliar application of fertilizer, i.e., the control treatment (Lancashire et al.,1991). Liquid fertilizer was provided by the Rouyesh Now Institute, which is licensed by the Rice Research Institute of Iran. Foliar application of nutrients was performed at the end of the evening using a 20-L handy sprayer pump at a ratio of 1 per mille (1‰) in the experimental plots.

We measured and analyzed the following rice plant traits: height (cm), number of tillers, number of filled grains per panicle, and 1,000-grain weight (g). These traits were measured using the standard evaluation system (SES) of rice (International Rice Research Institute,2002). Plant height was delineated by measuring the distance from the soil surface to the tip of the highest panicle of a hill. In order to measure leaf area index (cm²) and total dry weight (g), sampling was performed every 15 days from 30 days after planting for a total of six times, while plants at the edges of the plots were excluded. The leaf area index was determined using a leaf area meter (LI-31000, LI-COR; Lincoln, NE, U.S.) after leaves were detached from the plants. The amount of dry matter accumulation (g/plant) was measured after separating different parts of the plants and placing them in an electrical oven at 70 °C for 48 hours. In order to determine chlorophyll concentrations, a chlorophyll meter (SPAD-502; Minolta Camera, Japan) was used, and chlorophyll concentrations were measured at three points of perfectly developed leaves, with 10 plants measured in each experimental plot and measurements conducted at six different growth stages. Equation 1 and Equation 2 were applied in order to measure leaf area and the growth rates of rice plants, respectively (Hunt,1990); where LAI is leaf area index; LA₁ and LA₂ are leaf areas; GA is growing area per plant (m²); CGR is crop growth rate; W₁ and W₂ indicate dry matter weights per plant on investigation dates T₁ and T₂.

At the completely mature stage, an area of 2 m² in each plot was harvested (rows and plants near the plot margins were not harvested), and rough rice was separated from the grain. After reducing the humidity of the rough rice to 11%, the final yield of each plot was computed and recorded separately.

Statistical analysis was conducted using SAS software. To analyze the data, the results from each crop year were first analyzed separately, and subsequently the homogeneity of the variance of the mean of both crop years was evaluated using Bartlett’s test. Mean comparison of traits was performed by least significant difference (LSD) tests at the p < 0.05 probability level. Graphs were created using Sigma-plot software, version 11.

Foliar application of nutrients had a significant effect on the final height of the rice plants in both crop years (Table 1). Foliar application of nutrients in treatment E (foliar application of fertilizer at BBCH stages 25, 29, and 30) resulted in the greatest plant heights. The second greatest plant heights were achieved with Treatment D (foliar application of fertilizer at BBCH stages 29 and 30). The mean final height of plants in Treatments E and D were 7% and 5% greater than the mean final height of plants in the control treatment, respectively. Mean final heights of rice plants (117.25 cm) in Treatment A (foliar application of fertilizer at midtillering or BBCH 25) did not significantly differ from mean final heights of rice plants in the control treatment (113.55 cm) (Figure 1).

Foliar application of nutrients significantly affected the mean number of plant tillers during the 2 crop years, with the exception of 30 days after transplanting (Figure 2). Results indicate that the highest number of tillers 60 days after transplanting was recorded in Treatment E (foliar application of fertilizer at BBCH stages 25, 29, and 30). Plant tiller numbers in Treatment C (foliar application of fertilizer at panicle initiation or BBCH 30) were statistically significantly different from plant tiller numbers in Treatment F (control). Treatments D and E led to 19% and 21% more plant tillers in comparison with the control treatment (Figure 2). Ninety days after transplanting, foliar application of nutrients also had significant effects on the number of tillers per plant. Results indicate that in this stage, Treatments D and E resulted in the highest numbers of tillers per plant. Furthermore, Treatments C and F (the control treatment) resulted in the lowest plant tiller numbers, with means of 14.50 and 13.25, respectively. This trend continued 90 and 105 days after planting.

Foliar application of nutrients had significant effects on the mean chlorophyll concentrations in rice plant leaves in both crop years (Figure 3). Thirty days after transplanting, rice plants in Treatment A had the highest chlorophyll concentrations, and rice plants in the control treatment had the lowest chlorophyll concentrations. Treatments A and F significantly affected the chlorophyll concentrations of rice plants, a pattern which remained 45 days after transplanting. Forty-five days after transplanting, rice plants in Treatment A had the highest chlorophyll concentrations, and chlorophyll concentrations in plants in Treatments B and D were not significantly different from those of rice plants in the control treatment (Treatment F). Sixty days after transplanting, the highest chlorophyll concentrations were found in Treatments D and E (Figure 3), and there were no significant differences between these two treatments. However, treatments which involved foliar application of nutrients resulted in higher chlorophyll concentrations than the control treatment, and the differences were clearly statistically significant. At this stage, the mean chlorophyll concentrations in rice plants in Treatments D and E were 13% and 9% higher than in rice plants in the control treatment, respectively. Ninety days after transplanting, foliar application of nutrients also had significant effects on the leaf chlorophyll concentrations. Rice plants in all nutrient application treatments had higher leaf chlorophyll concentrations than rice plants in the control treatment, and the highest chlorophyll concentrations were found in Treatments D and E. Finally, 105 days after transplanting, the leaf chlorophyll concentrations differed significantly between all treatments (Figure 3). Results indicate that, at this stage, rice plants in Treatments D, B, and E had the highest leaf chlorophyll concentrations. Average leaf chlorophyll concentrations in Treatments D, B, and E were approximately 18% higher than in the control treatment. Generally, chlorophyll concentrations increased significantly during the early growth stage (30 days after planting), in Treatments A, E, and B in both crop years. Chlorophyll concentrations in rice plants in Treatments A, E, and B were 8%, 5%, and 4% higher than in rice plants in the control treatment, respectively. As the time since transplanting increased and plants reached the final stage of their growth, this trend remained.

The interaction between foliar application of nutrients and crop year had a significant effect on the mean plant dry weight (with the exception of the stage of 30 days after transplanting) (Figure 4). Forty-five days after transplanting, plant dry weight was highest in Treatments D and E, and plant dry weights were 17% and 11% higher in Treatments D and E than in the control treatment, respectively. At this stage, plant dry weights in Treatments A, B, and C were not statistically significantly different from those in the control treatment. Sixty days after transplanting, the highest plant dry weights were recorded in Treatments E, D, and B, and the lowest dry weight was recorded in the control treatment (Treatment F). The dry weight of plants in Treatment E was 31% higher than that of plants in the control treatment, and this trend remained 75 days after transplanting. Seventy-five days after transplanting, plants in Treatments D and E had the highest dry weights, and plants in the control treatment and Treatment A had the lowest dry weights. There were no significant differences between the other treatments and the control treatment. Ninety days after transplanting, the highest dry weights were recorded in Treatments D and E, and the lowest dry weights were recorded in the control treatment and Treatment C. Finally, 105 days after transplanting, the highest dry weight was recorded in Treatments D, B, and E. Dry weights in Treatments D, B, and E were 14.5%, 14.2%, and 13.0% higher than in the control treatment, respectively (Figure 4).

Foliar application of nutrients had significant effects on the mean leaf area index in both crop years 75 and 90 days after transplanting (Figure 5). Seventy-five days after transplanting, the highest and lowest leaf area indices were recorded in Treatment B and the control treatment, respectively. The highest and lowest leaf area indices 90 days after planting were recorded in Treatment D and the control treatment, respectively.

Foliar application of nutrients had significant effects on the mean rice growth rate in both crop years during the later growth stages (Figure 6). Between 75 and 90 days after transplanting, the highest crop growth rate was recorded in Treatments E and D, and the lowest crop growth rate was recorded in the control treatment. Crop growth rates in Treatments E and D were 23% and 22% higher than in the control treatment, respectively. Subsequently, foliar application of nutrients (mean: 19.22 g/m²/day) was conducted at the maximum tillering stage (B). Furthermore, treatments were applied during panicle initiation (C) and midtillering (A). However, our results indicate that 75–90 days after transplanting, the crop growth rate decreased when treatments were applied during maximum tillering + panicle initiation (D) and during all three stages (Treatment E) (Figure 6). The highest crop growth rate at this stage was recorded in Treatments A, B, and C, although there were no statistically significant differences among treatments of foliar application of nutrients at different vegetative growth stages. Differences in growth rates at maturity stages between treatments applied during midtillering and maximum tillering indicate that foliar application of nutrients during the vegetative stage resulted in less than 1% difference in the measured plant traits between the treatments, but when treatments were applied after panicle initiation, differences in plant traits between the different treatments exceeded 10%.

Foliar application of nutrients had significant effects on the mean number of filled grains per panicle (Table 1). Foliar application of nutrients resulted in significantly higher numbers of filled grains per panicle. The highest and lowest numbers of filled grains per panicle were recorded in Treatments D and the control treatment, respectively. Foliar nutrient application resulted in filled grain numbers per particle that were 22.5% (Treatment D), 21.3% (Treatment B), and 21.2% (Treatment E) higher than in the control treatment (Table 2).

Foliar application of nutrients did not significantly affect the mean 1,000-grain weight. This result was found for both 2017 and 2018, and for the mean 1,000-grain weight of the two years combined (Table 1).

Foliar application of nutrients had significant effects on mean grain yields in both crop years (Table 1). The highest and lowest grain yield were recorded for Treatments D and the control treatment, respectively. Treatments E and B resulted in the second and third highest mean grain yields.