Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/28/2004
Publication Date: 3/1/2005
Citation: Zhao, D., Reddy, R.K., Kakani, V.G., Read, J.J., Koti, S. 2005. Selection of optimum reflectance ratios for estimating leaf nitrogen and chlorophyll concentrations of field-grown cotton. Agronomy Journal. 97:89-98. Interpretive Summary: Growth of upland cotton is influenced by nitrogen (N) fertility and by applications of PIX, a plant growth regulator that controls plant height. Remote sensing studies indicate leaf reflectance measures have potential for monitoring in-season changes in leaf N concentration that affect yield. This field study determined seasonal patterns of N and pigment (chlorophyll) concentrations in uppermost, fully-expanded leaves and their relationship to different wavelengths of reflected light. Four fertilizer N rates were compared, and four spray rates of PIX were compared in field-grown plants. Leaf N concentration varied across sampling dates, treatments, and experimental plots. Leaf N concentrations decreased as fertilizer N rate decreased and as plants matured, and as expected were associated with decreased leaf chlorophyll. Strong relationships were found between leaf N and a simple ratio of leaf reflectance at two wavelengths of 517 and 413 nanometers. Leaf chlorophyll concentration also could be estimated using selected wavelength ratios. Results showed these reflectance ratios in cotton are sensitive to changes in leaf N status due to decreasing soil N availability and PIX-induced reductions in plant height. These measures can be used in remote sensing of cotton N status, which may help producers make timely crop management decisions.
Technical Abstract: Knowledge of critical leaf nitrogen (N) levels should enhance yield in cotton (Gossypium hirsutum L.), and lessen environmental impacts of excess fertilizer use. Because leaf reflectance is largely determined by chlorophyll (Chl) concentration, it can provide a rapid, non-destructive estimate of plant N status. Field studies were conducted to determine the effects of fertilizer N and a plant growth regulator, mepiquat chloride (MC), on hyperspectral reflectance (400-2500 nm), and Chl and N concentrations in leaves. The N study comprised four treatments of 0, 56, 112, and 168 (control) kg N ha-1, split applied at the 2nd true-leaf and first square stages. The MC study comprised four treatments of 0 (control), 0.59, 1.17, and 2.34 L MC ha-1, split-applied foliarly at the first square and first flower stages. Uppermost, fully expanded mainstem leaves were measured weekly or biweekly. Leaf reflectance at 556 and 710 nm increased as N fertilizer rate decreased; whereas, changes in reflectance at high MC rates were complex. Leaf Chl was associated with leaf N (r2 = 0.67-0.80). In both studies, leaf N was closely associated with a simple ratio of reflectance at 517 and 413 nm (R517/R413) (r2 = 0.65-0.78). A linear correlation was obtained between Chl and either R708/R915 or R551/R915 (r2 = 0.67-0.76). Results of strong correlation between simple reflectance ratios and cotton leaf N or Chl suggest these measures may be used in remote sensing assessments of cotton N status, which may help producers make timely N management decisions.