Photosynthesis: Environmental Impacts Of Nitrogen Fertilizers

1. Environmental Impacts of Nitrogen Fertilizers

The United Nations predict that the world population will reach 9.7 billion by 2050 as it is increasing at a rapid pace (UN, 2019). Furthermore, wheat (Triticum aestivum L.) is one of the most important cereal crops worldwide, and it is the main source of food for approximately one third of the world (Haile et al., 2012), it provides over 20% of the calories consumed by the world’s population and a similar proportion of protein for about 2.5 billion people (Driever et al., 2014). Wheat is a major arable crop that is grown on more land area than any other crop, with more than 734 million tonnes of grain harvested from 214 million hectares worldwide in 2018 (FAOSTAT, 2020). Modern wheat cultivars require high nitrogen fertilizer in-puts in order produce high grain yield or to achieve their high yield.

The concerns of agronomists about using high amounts use of fertilizer since the green revolution onwards led to movements that searched for means of less fertilizer application, with respect to crop quality and quantity. In 2019, world demand on nitrogen fertilizer was more than 107 million tons, and it is predicted to be raised to more than 111 million tons by the year 2022 (FAO, 2019). Eexcess nitrogen application is becoming a major cause for environmental pollution that has been increasing for decades (Savci, 2012). The high quantities of fossil fuels being burned in manufacturing of N fertilizers during the Haber process (Snyder et al., 2009), leaching to the soil and pollution of underground and surface water sources through nitrate (Cossey et al., 2002; Liu et al., 2018), eutrophication, increasing in the concentration of water nutrients, which encourage excessive growth of algae and depleting oxygen availability for fish and aquatic plants (Carpenter et al., 1998; Fowler et al., 2013), 10-12% of total Greenhouse gases in the globe caused by agriculture, 60% is of the nitrous oxide (Erisman et al., 2011), are the consequences of high nitrogen fertilizer inputs on the environment.

2. Nitrogen Use Efficiency

Nitrogen use efficiency (NUE) can be defined as proposed by Moll et al. (1982) as grain dry matter yield per unit of available nitrogen from the soil or/and applied fertilizers. NUE encapsulates the whole plant N uptake and the utilization to produce grain from the sum of the processes associated with the absorption, translocation, assimilation, and redistribution of nitrogen in the plant. NUE comprises two components; firstly Nitrogen-uptake efficiency (NUpE), which is the above-ground nitrogen at harvest (AGN) (kilograms) per available nitrogen from soil and fertilizer (kilograms), UPE = AGN(kg ha-1) / N (kg ha-1), and secondly Nitrogen-utilisation efficiency (NUtE), which is the grain dry mass (Y) (kilograms) per above-ground nitrogen at harvest (AGN) (kilograms), UTE = Y (kg ha-1) / AGN (kg ha-1) (Foulkes et al., 2009; Gaju et al., 2011; Pathak et al., 2012).

The efficiency of nitrogen fertilizer uptake of wheat is around 33% worldwide, and 67% of applied nitrogen fertilizer is lost through surface run-off, plant emission, soil denitrification, volatilisation and leaching (Raun and Johnson, 1999). Generally, cereal crops release nitrogen through their plant tissues, and this loss for winter wheat has been estimated to be as high as 21–41%, usually after the anthesis stage as ammonia (Raun and Johnson, 1999).

Therefore, improving nitrogen-use efficiency (NUE) is becoming a necessity as the production of 1 kg dry biomass in most of cereal crops requires 20-50 g of N absorbed by the plant roots, thus N supply potentially limits grain yield (Xu et al., 2012).

3. Phenotyping

A half century after Mendel’s experiments, in 1909 Wilhelm Johannsen who was a Danish plant scientist introduced genotype and phenotype terms for the first time after he performed experiments on self-fertilizing beans seed size (Walter et al., 2015). He observed significant variation in the beans progenies when he selected based on seed size, and concluded that this trait was influenced by genetic. However, he could not detect differences in seed size anymore within individual plants of the progenies when he selected again, he concluded that environmental effects were the sources phenotypes of the selected pure lines. Phenotyping is the assessment of complex plant traits based on measuring individual quantitative parameters; anatomical, ontogenetical, physiological and biochemical, that form the basis for complex trait assessment (Li et al., 2014; Walter et al., 2015).

For many years, researches have been exploring the molecular and genetics of nitrogen use efficiency, and they sequenced the genome of wheat. However, their attempts of improving NUE was slightly significant. On the other hand, phenotyping for traits of interest appears to be a very important and useful tool for breeders. Physiological phenotyping goal is to avoid attempting quantification of emergent properties expressed at or above the whole-plant level, especially those that only become apparent at the canopy and crop level (Ghanem et al., 2015). Reynolds et al., 2020 highlighted the importance of phenotyping in crop improvement is that it might lead to significant breakthroughs in their adaptation, with appropriate investment in translational research.

4. Phenotyping Methods

4.1. Flag-leaf chlorophyll content

Leaf chlorophyll concentration is an important parameter that is frequently measured as an indicator of leaf greenness, chloroplast development, entire photosynthetic complex and potential, leaf nitrogen content or general plant health (Ling et al., 2011; Todeschini et al., 2016; Reena et al., 2017; Noulas et al., 2018).

The photosynthesis capacity increases linearly with leaf nitrogen levels and plants that have higher leaf N concentration have more chlorophyll and more light absorption by chlorophyll for photosynthesis (Foulkes et al., 2009; Makino, 2011; Garnett and Rebetzke, 2013). Furthermore, Ribulose-1,5-bisphosphate carboxylase (RuBisCo), the key enzyme in Carbon fixation for photosynthesis, makes up 12-35% of total N in leaves of C3 plants; and it is been shown that light-saturated photosynthesis rate in C3 species is closely related to the amount of Rubisco in a leaf (Zhu et al., 2010; Garnett and Rebetzke, 2013).

Chlorophyll concentration can be non-destructively and accurately measured using a hand-held battery portable optical meter quickly. The Minolta SPAD-502 chlorophyll meter is one device which measures chlorophyll concentration via based on two light-emitting diodes and a silicon photodiode receptor, that measures leaf transmittance in the red (650 nm; the measuring wavelength) and infrared (940 nm; a reference wavelength) regions of the electromagnetic spectrum and it can be adjusted compensates for differing leaf thicknesses (Uddling et al., 2007; Pask et al., 2012; Reynolds et al., 2020).

4.2. Flag-leaf chlorophyll fluorescence

Light energy absorbed by chlorophyll molecules in a leaf can undergo one of three fates :1) it can be used to drive photosynthesis, providing the chemical for CO2 fixation in the Calvin cycle, 2) it can be dissipated as heat, or 3) it can be re-emitted as light-chlorophyll fluorescence (Murchie and Lawson, 2013). These three processes occur in competition, such that any increase in the efficiency of one will result in a decrease in the yield of the other two, hence, plants under stress modifies the relative proportions of these process (Tremblay et al., 2012). Chlorophyll fluorescence is closely linked with photosystem II, which can reflect the photosynthesis efficiency in the plants. This technique has been used in the study of wheat stress response under different unfavorable conditions, such as water stress (Zhang et al., 2010). Many devices were developed for a precise and quick measurement of chlorophyll fluorescence non-destructively, and showed significant results such as Handy PEA continuous excitation plant efficiency analyzer, portable chlorophyll fluorometer (FMS 2.02), FluorPen FP- 100 (Živčák et al., 2014; Wang et al., 2016; Carmo-Silva et al., 2017).

4.3. Normalized difference vegetation index – NDVI

Recently, researchers have got more interest in multispectral radiometry in as an important and efficient tool for evaluating photosynthetic traits. These devices shown their practicality in measuring nitrogen use efficiency in crops. The Normalized Difference Vegetation Index (NDVI) is one of the most widely used vegetation indices as an indicator of canopy green area and it is associated with grain yield as well (Reynolds et al., 2007). It is based on the difference between the maximum absorption of radiation in the red spectral band and the maximum reflection of radiation in the near-infrared spectral bands (Ghorbani et al., 2012). Canopy NDVI values are between –1.0 and +1.0 and for soil and vegetation are usually positive values. The healthy green plant absorbs most of the incident red light and they reflect a large rate of the near infrared radiation (high NDVI value) while in non-green plants the absorption of red light is less (low NDVI value) (Moriondo et al., 2007).

Many types of devices have been developed for estimating NDVI, for example GreenSeeker HandHeld, CROPSCAN-Multispectral Radiometers, ASD Field Spec Pro spectrometer. The first one is cheap, easy to use and quick. However, the other devices can measure more indices but their price is higher, and not easy to use as the Greenseeker (Cabrera-Bosquet et al., 2011; Yao et al., 2013; Sultana et al., 2014; Ratanoo et al., 2018; Vian et al., 2018; Ali et al., 2019; Gianquinto, et al., 2019).

4.4. Canopy temperature

Canopy temperature is an indicator any type of stresses whether temperature, moisture, nutrient in crops. Plants with low canopy temperature reveals higher genotypes performance in a given environment in traits related to NUE; capacity for transpiration, stomatal aperture, root depth, increased CO2 uptake associated with larger stomatal conductance (Reynolds et al., 2000; Cormier et al., 2016; Kaur et al., 2018). Canopy temperature can be measured very fast, a few seconds, using an infrared thermometer IR that measures the surface temperature of the plant canopy. Beside the speed of measurement, this method is simple, the device is not expensive, the variation among the plants is low as the device integrate the whole canopy due to scoring many plants at once (Reynolds et al., 2012).

4.5. Flag-leaf net photosynthesis rate

The amount of nitrogen uptake by the plant has a positive effect on the photosynthesis capacity of the plant, hence the biomass. In cereal crops, mesophyll cells countians up to 75% of the nitrogen captured by the leaves. Plants with higher amount of leaf nitrogen are more likely have greater photosynthesis capacity (Gaju et al., 2016). Rubisco (Ribulose-1,5-bisphosphate) catalysis the photosynthetic assimilation of CO2 into organic compounds. Rubisco is the enzyme that supports the CO2 assimilation in photosynthesis, hence the enhancement of plant productivity and resource use efficiency (Carmo‐Silva et al., 2015). This measurement can be taken using a portable Infra-Red Gas Analysis (IRGA) system such as (LI-COR 6400-XT Licor) (Cabrera-Bosquet et al., 2009; Gaju et al., 2016).

4.6. Flag-leaf senescence

The most significant event when the leaf starts to senesce is the change that happen in the leaf chloroplast where photosynthesis occurs. Decline in photosynthetic activity is associated with reduction in dark reactions of the Calvin cycle which is mainly due to the degradation of Rubisco disassembly of the photosynthetic apparatus (Lu et al., 2002). There is evidence for significant variation in duration of flag-leaf senescence during the grain-filling associated with NUE in wheat genotypes (Verma et al., 2004; Bogard et al., 2011). Flag-leaf senescence can be assessed visually by recording the percentage green 5area senesced using a standard diagnostic key based on a scale of 0–10 (100 % senesced) (Gaju et al., 2011).

5. Conclusion

Improvement in wheat nitrogen-use efficiency is not that much, even though its genome was sequenced. On the other hand, phenotyping became more valuable for NUE screening. Compared to genetic and molecular methods, the cost of phenotyping is less. Reynolds et al., 2020 highlighted the importance of phenotyping in crop improvement is that it might lead to significant breakthroughs in their adaptation, with appropriate investment in translational research. Understanding the physiological processes related to NUE traits and their phenotyping methods might lead to significant results in discovering genotypes among the existing cultivars or among the landraces with sufficient investment in translational research. The different phenotyping contexts will help agronomists to understand better the complexity of the trait and the major Genotype – Environment interaction, offers the opportunity to make real progress in improving NUE.