Assessing field spectroscopic methods for grapevine chlorophyll content estimation
Date
2016-05-05
Authors
Parton, Diana
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Abstract
Vancouver Island, British Columbia, is at the northern extent of natural
climate zones conducive for grape growing, making vineyards susceptible to any
changing weather patterns and temperature extremes. Grapevine monitoring is an
important aspect of the viticulture industry, and remote sensing technologies are a
powerful aid in reporting vegetation information for better vineyard management
practices. However, the understanding of vine spectral responses as viewed by
optical sensors has to be developed further, and was undertaken in this study.
Chlorophyll pigments drive photosynthesis, a biochemical process in plants,
which contributes to physiological performance and productivity, making it an
appropriate leaf characteristic for detailed examination. This study aimed to
develop a thorough understanding of the relationship between (i) leaf-level spectral
reflectance and transmittance properties and (ii) pigment concentrations, via
ground-based sampling. This was achieved through the examination of two ground
campaign tools, as well as current spectral data processing techniques and
workflow methods. A spectrometer and SPAD chlorophyll meter collected nondestructive
measurements during leaf senescence and grape harvest, and wet
chemical extraction methods determined chlorophyll content (expressed in terms of
unit leaf area and leaf fresh weight).
Reflectance indices,first order derivative indices, and a continuum removal
approach were used to generate eighteen reflectance-based attributes. This study
performed a series of chlorophyll estimation models through iterative ordinary least
square regression, followed by two methods of model validation. Performance
metrics indicated strong models with high explanatory power; the continuum
removed depth normalized total area metric was presented as the optimal nondestructive
attribute for accurate chlorophyll estimation for leaf level field
campaigns (R2 = 0.93). Chlorophyll expressed in units of fresh weight yielded
more consistent models than in units of leaf area. The chlorophyll meter also
presented compelling results (R2 ≥ 0.78), and both sensors were determined to be
appropriate for field validation campaigns for this vineyard study.
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Keywords
chlorophyll, grapevine, senescence, spectroscopy, hyperspectral, SPAD, modeling