Sept. 14, 2021
Aug. 26, 2021
The yield response of maize to excessive fertilizers has plateaued and with it the economic returns. To avoid further resource waste in smart farming, crop scientists are trying to correlate yield to how plants intercept light and allocate dry matter to grains. In two studies, a plant canopy imager measured the effect of crop density and height on maize cultivation.
Increasing the density of plants, along with the inputs of fertilizers and water, has been the common strategy to increase yield.
In maize, one of the major crops of the world, fertilizers added to nutrient-poor soils increased the number of maize ears as well as the number and weight of kernels.
However, the law of diminishing returns has resulted in seeing a decline of kernel number and weight, with very high levels of nitrogen (N) fertilizer application. Increasing density can also reduce the number of kernels due to competition between plants for resources.
Also, increased use of nitrogen fertilizers has resulted in environmental degradation and has affected human health. Therefore, the focus of crop research is shifting to increasing yield by improving N fertilizers use efficiency by plants (NUE).
In one of the approaches, a team of Chinese agricultural scientists—Shi, Li, Zhang, Liu, Zhao, and Dong—decided to delve deeper into crop physiology.
Kernel weight depends on the amount of biomass a plant produces and partitions to the maize ear. This, in turn, depends on the amount of sunlight that a plant intercepts and converts to biomass through photosynthesis. The leaf area index (LAI) of a plant and the canopy light extinction coefficient (k), or light that reaches the leaves, will determine the amount of solar radiation intercepted. An increase in LAI and k boosts dry matter accumulation due to radiation use efficiency (RUE).
The Chinese agricultural scientists had to find a way to increase RUE and NUE in maize cultivation at high density with lower levels of nitrogen fertilizers.
The agricultural scientists experimented with the maize cultivar Zhengdan 958. They planted it at three densities: 52500 (LD), 67500 (MD), and 82500 (HD) plants ha–1. All three densities were given five N rates, starting from a maximum of 320 kg ha–1, then with 15%, 30%, 45% reduction, and one set with no nitrogen. The nitrogen was split into a basal dose, with two side-dressings on the sixth leaf (V6) and 12th leaf (V12) stages. Other plant care was the same across treatments.
To estimate dry matter, five plants from each plot were sampled seven times: at V6, V12, tasselling (VT), blister (R2), milk stage (R3), dent stage (R5), and physiological maturity (R6). At R6, the N uptake by plants was also estimated, using the Kjeldahl method.
To monitor RUE, the agricultural scientists needed a field device for photosynthetic active radiation (PAR) measurement and LAI. They used the CI-110 Plant Canopy Imager, a precision field device manufactured by CID Bio-Science Inc.
The agricultural scientists measured interception of light in the PAR range (400–700 nm), between noon and 2 pm on cloudless days. They took one measurement above the canopy to get the incident photosynthetically active radiation (IPAR), and they took five readings below the lowest corn leaves in the canopy to estimate the transmission photosynthetically active radiation (TPAR). The radiation inception was calculated as a ratio of the two. Daily radiation interception was recorded by weather equipment on site, and RUE was calculated as the progressive accumulation of biomass to daily IPAR.
Figure 1: “The leaf area index (LAI) vs. days after planting for the different plant densities and N rates in 2013 and 2014 growing seasons. For the year factor: A, B, and C, 2013; D, E, and F, 2014. For the plant density factor: A and D, LD; B and E, MD; C and F, HD,” Shi et al 2017. (Image credits: https://doi.org/10.1016/S2095-3119(16)61355-2)
As expected, yield increased with the highest rates of N application and density. However, there was no significant difference in yield at high density between maximum nitrogen application and plots with 30% less N application. Also, the scientists found that increasing nitrogen in low and medium density plots did not increase yield.
The impact of fertilizers and density was seen most in kernel numbers and, to a lesser extent, in the weight of kernels.
Though most nitrogen uptake occurs in high density and maximum levels of nitrogen fertilizer, the 30% less N treatment did not produce any deficiency in plants, nor did it decrease dry matter accumulation. Maize showed the highest NUE at 30% less N treatment. Further increases in N input decreased NUE.
As N application rose in all three densities up to the tassel stage, LAI increased; see Figure 1. However, there was no significant difference between LAI of plots getting maximum N and 30% less nitrogen.
Figure 2: “Relationship between total N uptake and radiation use efficiency (RUE) (A) and the relationship between RUE and grain yield (B) under LD, MD, and HD for all N supply levels,” Shi et al 2017. (Image credits: https://doi.org/10.1016/S2095-3119(16)61355-2)
Similarly, light interception increased with density and N application, as shown in Figure 2, and was reflected by increases in dry matter accumulation and yield. RUE increased from 2.8g at no nitrogen to 3.6g MJ–1 PAR for maximum nitrogen levels, but there was no significant difference between the RUE of plants getting maximum N and 30% less nitrogen. The light interception was 0.95 at high densities, showing that increasing densities beyond those in the experiment would not be useful.
It should be possible to get high yields by reducing fertilizers to 30% of current rates, as maximum LAI, light interception, and nitrogen use efficiency are reached for this level of N input.
Another team of agricultural scientists checked the effect of combining increasing density with height on light interception and yield in maize cultivation.
Higher plants could have a better distribution of light to improve biomass accumulation; however, higher plants are prone to lodging. High yields have been associated with both shorter and taller varieties. Hence, the team comprised of Ren, Li, Dong, Liu, Zhao, and Zhang wanted to study the effect of the interaction of density and height on photosynthesis in maize.
In this case, the scientists used three hybrids of different heights: DH661 (low-height), CS3 (medium-height), and XY335 (high-height). They were grown at three densities of 45000, 67500, and 90000 plants ha–1. All were given the same levels of fertilizers, irrigation, and plant care.
Fifteen plants were sampled at sixth-leaf (V6), tasselling (VT), VT+20 d, VT+40 d, and physiological maturity (R6) stages, measuring leaf length and width to estimate leaf area and LAI. Chlorophyll content was estimated at VT with an ultraviolet spectrophotometer. Net photosynthetic rates were estimated by a gas analyzer on V12, VT, VT+10 d, VT+20 d, and (R3).
For PAR measurement, to find the light transmittance in the top, middle, and bottom canopy layers, the scientists used the CI-110 Plant Canopy Imager. At the end of the season, the grain yield was also determined along with ear characteristics such as length, width, weight, number of kernel rows, and grains per row.
The scientists could not find any advantage of using higher maize plants on the yield of corn. It was maize with the lowest height that gave the highest yield. Moreover, it was the short maize crop that was able to increase yield as density increased.
The same advantage of increasing density was not seen in medium and tall maize cultivars due to crowding stress. The low maize variety suffered the least reduction in kernel numbers and weight due to crowding stress.
The effect of crowding was not evident in the early stages but became prominent in the middle and later stages in medium and tall varieties. As density increased, it accelerated leaf senescence, reducing net photosynthesis of the plant due to a fall in LAI. Though there is no difference in LAI between plants of different heights grown in varying densities, the shorter plants maintain their LAI in later life stages compared to medium and tall maize varieties. Similarly, the low maize variety had the highest chlorophyll content at all densities compared to the medium and tall maize cultivars.
There was also no difference in the photosynthetic rates among leaves found in any of the canopy layers in short maize. This is because, in the short variety, there is little difference in the light transmittance between the top and middle layer.
Moreover, shorter plants can allocate more dry matter to kernel development than taller varieties. This trade off was one of the first physiological features that crop scientists changed in all grains, not just maize. This study shows this trade off remains relevant to increase grain yield. Since shorter maize suffers less from crowding and shading as density increases, shorter varieties are better.
Under high nitrogen application, only 5-15% of the fertilizers are used. The rest leaches into the soil, killing beneficial microbes, or is released as greenhouse gas emissions. Or, the excess of fertilizers is washed off as part of the runoff, which causes eutrophication of inland water bodies and coastal regions killing aquatic plant and animal life. Since increasing height is not an advantage, increasing the density of shorter maize with 30% less use of fertilizers should still give high yields. This achieves the aim of making maize cultivation less pollutant, while increasing resource use efficiency and sustainability in the industry.
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture
Ren, B., Li, L., Dong, S., Liu, P., Zhao, B., & Zhang, J. (2017). Photosynthetic characteristics of summer maize hybrids with different plant heights. Agronomy Journal, 109(4), 1454-1462. doi:10.2134/agronj2016.12.0693
Shi, D., Li, Y., Zhang, J., Liu, P., Zhao, B., & Dong, S. (2016). Increased plant density and reduced N rate lead to more grain yield and higher resource utilization in summer maize. Journal of Integrative Agriculture, 15(11): 2515–2528. https://doi.org/10.1016/S2095-3119(16)61355-2
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