Chlorosis is a condition in plants where the leaves have yellowish colouration due to insufficient amount of chlorophyll in their cells. Chlorosis can be caused by a number of reasons including; deficiency of nutrients (such as iron, nitrogen, manganese, zinc), disease infestation, damaged roots, high soil alkalinity and compacted roots (1). This article concentrates on chlorosis due to nitrogen deficiency in plants.
Why does nitrogen deficiency cause yellowing?
To understand why nitrogen deficiency causes yellowing in leaves we need to discuss chlorophyll. From basic knowledge of plant biology, we understand that chlorophyll is the green (green because it reflects green light) pigment that gives green plants their green colouration. It is found in the chloroplast of plant cells. Chlorophyll plays a key role in photosynthesis as it absorbs light (in many cases sunlight), and uses light energy, along with carbon dioxide and water to produce carbohydrates for plants. This means that chlorophyll is essential for crop growth and yield.
Nitrogen taken up by plants is used in the formation of amino acids which is the building block for proteins. Nitrogen is a structural component of chlorophyll (4). Research shows evidence of a close link between leaf nitrogen and chlorophyll content; increase in leaf nitrogen corresponded with increase in leaf chlorophyll content (3).
Chlorophyll – an indirect tool for nitrogen management
Because of the close relationship between chlorophyll and nitrogen, farmers and scientist can determine the nitrogen needs of a crop by measuring the chlorophyll content in leaves (chlorophyll meter), or by using crop remote sensors .
• Chlorophyll meter: The chlorophyll meter is well known by agronomist. It is a portable device which gives quick (on the field) readings of leaf chlorophyll content when the leaf is positioned in its aperture. A slight limitation to using this device is that other factors can affect leaf colouration. Nutrient deficiency or any agent causing plant stress can reduce the greenness of a plant (5). Therefore care should be taken while drawing conclusions from readings.
• Crop remote sensing: In crop remote sensing information (such as nutrient deficiency, yield or disease infestation) about the crop can be collected remotely (without coming in contact with the crop). Remote sensing in nitrogen management uses the light reflective property of chlorophyll in plants. Visible light i.e. light which the human can see falls between the ranges of 400-700 nanometres nm (8). Plants either absorb or reflect the light from the sun (or from other sources) and their chlorophyll content largely influences this action. A leaf with high chlorophyll content will give higher reflectance in the near infrared and green wavelengths than the red wavelengths, on the electromagnetic spectrum (6) (read more about the electromagnetic spectrum in Ortiz (2011)). Nitrogen deficiency in plants is diagnosed by measuring the differences in spectral reflectance using remote sensors (such as, GreenSeeker and Crop circle) (4,6).
Some ways to tell when chlorosis is caused by nitrogen deficiency
To tell if chlorosis is due to nitrogen deficiency and not other factors, one can look out for other nitrogen deficiency symptoms. Nitrogen deficient plants have stunted growth and reduced yield. In many crops nitrogen related chlorosis begins from the old lower leaves, the leaf tip and midrib turn yellow but the leaf edges stay green. As deficiency persist yellowing progresses to upper leaves and leaf necrosis (dying of leaf tissue) occurs. In maize crop, v-shaped yellowing is visible on the leaf tip and the stalk becomes spindly (7).
Feature Image: Via Wiki; https://en.wikipedia.org/wiki/File:AlbinoPopcorn.jpg
Bibliography and further reading
1. Chlorosis- https://extension.illinois.edu/focus/index.cfm?problem=chlorosis
2. Chlorophyll-https://www.chm.bris.ac.uk/motm/chlorophyll/chlorophyll_h.htm
3. Evans JR (1989) Photosynthesis and nitrogen relationships in leaves of C3 plants. Oecologia 78:9-19.
4. Daughtry et al (2000) Estimating corn leaf chlorophyll concentration from leaf and canopy reflectance. Remote Sensing of Environment. 74: 229-239.
5. The early season chlorophyll meter test for corn- https://extension.psu.edu/plants/crops/grains/corn/nutrition/early-season-chlorophyll-meter-test-for-corn
6. Ortiz (2011) Basics of crop sensing- https://sites.aces.edu/group/crops/precisionag/Publications/Basics%20of%20Crops%20Sensing%20-%20Ext%20Pub%20ANR-1398.pdf.
7. A closer look at deficiency symptoms in major crops- https://www.ipni.net/ppiweb/bcrops.nsf/$webindex/8A1BFC4E9E01AEB0852568F1005777BC/$file/97-3p08.pdf
8. Green plants and chlorophyll-https://www.webexhibits.org/causesofcolor/7A.html
Good
Nice explanation.
In the leaf side below corner, that plant has white leaves coloring. Why became white?
The reason is that microorganism cannot live in this dirt, because humans has put poison in it. And without microorganism no nitrogen.