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Soybean Leaf Greenness and Nitrogen

All plant leaves exhibit some shade of greenness during their growing season, and this green color is attributed to chlorophyll. Leaves of soybean plants that are supplied with adequate nutrients and water are dark green in color during the majority of the growing season.

Chlorophyll is the green pigment found in the chloroplasts of plants. It absorbs light mostly in the blue and red portions of the spectrum, but is a poor absorber of green portions of the light spectrum; thus, chlorophyll-containing tissues appear green. Chlorophyll is vital for photosynthesis in plants because it absorbs light energy which is then transferred to other parts of the photosystem in the leaves.

Nitrogen [N] is a key component of chlorophyll. Thus, it is theorized that the degree of greenness in the leaves of a plant species can be used as an indicator of N sufficiency or deficiency in plants of that species. The trick, of course, is measuring that degree of greenness and comparing it to known levels of greenness in the leaves of that species with known N concentrations.

In a journal article titled “Evaluation of Soybean Greenness from Ground and Aerial Platforms and the Association with Leaf Nitrogen Concentration in Response to Drought”, authors Bai and Purcell report results from research that used the Dark Green Color Index [DGCI] determined from both ground and aerial photography to estimate DGCI values in soybean in relation to drought. Pertinent points from that article follow.

•    Nitrogen is a major component of soybean seed. During seedfill, N is moved from soybean leaves to seeds, which in effect results in decreased chlorophyll and N in the leaves, and subsequent leaf yellowing. Thus, it is surmised that leaf color can be used as a measure of the N status of plants.

•    When soybeans are subjected to drought conditions, N2 fixation is decreased and does not recover. This results in less movement of N to the seeds, which results in a shortened seedfill period and lowered yield.

•    DGCI is a type of vegetation index that expresses the intensity of leaf greenness. DGCI values range from 0 to 1, which corresponds to light yellow and dark green, respectively. DGCI values have been closely associated with N concentration in leaves.

•    The above-linked article reports results from a study that was conducted for 3 years at Fayetteville, Arkansas [36°05' N lat.] on a site with silt loam soil that included side-by-side well-irrigated [IRR] and drought [DR] experiments. Varieties from MG’s 2 through 5 were evaluated.

•    In the study, measurements using both ground and aerial photography were evaluated to determine the DGCI response of soybean to drought during seedfill. Only aerial measurements that were taken at 50-75 meter heights above the canopy will be discussed since aerial imaging had relatively greater sensitivity in detecting the effect of drought compared to ground measurements.

•    Leaf samples for N analysis were collected on the same day ground photographic images were taken.

•    Leaf N concentration decreased with increasing days after R5 in all years, as did DGCI. The decrease in DGCI in the late reproductive stages of soybean was similar to the response of leaf N concentration vs. days after R5.

•    Yellowing that resulted from this decrease in DGCI occurred earlier in the DR treatment. Thus, within a variety, the DR treatment senesced prior to the IRR treatment.

•    Results from this study indicate that aerial DGCI measurements can be used as a relative indicator of soybean canopy N concentration.

•    The authors conclude that the decreasing levels of DGCI during soybean seedfill as measured by aerial imaging may be used to 1) determine maturity progress, 2) distinguish differences among genotypes in response to drought stress in water-limited conditions, and 3) lay the foundation for identifying genetic differences in canopy greenness that may be related to rate of senescence of soybean varieties with similar maturity when grown in water-limited environments.

•    The increasing sophistication and subsequent use of unmanned aerial systems [e.g. drones] for low-altitude aerial photography greatly simplifies collecting aerial images. This should enhance use of the DGCI as a tool for assessing N deficiency/sufficiency and/or mobilization in soybean plants, as well as how water-limited growing conditions may affect soybean varieties differently.

Composed by Larry G. Heatherly, Nov. 2022,