Nutrient Management Research Database

Potassium (K) is an important nutrient for grape because it sustains charge neutrality during organic acid transport, is a major ion involved in osmoregulation and stomatal function, and contributes to pH status in grapes and wines. We assessed the utility of a number of vegetation indexes for detection of K deficiency symptoms in a commercial vineyard of Vitis vinifera cv Pinot Noir. A hyperspectral field spectroradiometer for the 350-2500 nm spectral region was used to measure light reflectance at the leaf level. This data allowed us to screen a broad range of spectral indexes to detect absorption features affected by potassium deficiency symptoms. We found there were few indexes directly influenced by K deficiency symptoms. Absorption features in the visible region, centered at approximately 495 nm and within the chlorophyll b absorption feature at 625 nm were sensitive, and particularly after transformation to derivative spectra. Concentrations of K in leaves with visible symptoms ranging from 0% to more than 60% color change on leaf surfaces were not statistically significantly different (P = 0.05). These results suggest that observed symptoms might have been the result of heat stress and resulting. Our results indicate that non-invasive monitoring using hyperspectral reflectance may be used for spatially constraining K deficiency for use in site-specific K applications with future investigations.

• This study assessed the ability of leaf hyperspectral reflectance to detect potassium deficiency in grape vines in Napa Valley.
• In July of 2006, 40 leaf samples were taken from 25 vines; 13 stressed, 12 unstressed.
• Leaves were sorted by visual symptoms of deficiencies, (0, 5-10, 10-20, 20-30, 30-50, 50-60, and 60 or above percent of color change).
• Extractable potassium concentration was then determined, and a spectral reflectance measurements were taken with a spectrometer.

• Differences in potassium concentrations in leaves were not statistically significant.
• There were no significant distinctions between leaf spectral indexes that correlated with deficiency symptom levels.
• Few spectral indexes were directly influenced by K deficiency symptoms.