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level. The results of using sPRI to differentiate between grape-
vine species were highly significant (F1, 59 = 39.15, p <0.01)
(Figure 11).
In addition, the results of ANOVA analysis of genotypes
within species were similar to the results found at leaf level.
We did not perform a Bonferroni adjusted t-test because the
ANOVA test did not reveal significant difference within V.
riparia genotypes for either total chlorophyll content or sPRI.
However, ANOVA results showed that there was a significant
difference between sPRI values of V. rupestris genotypes (F 4, 24
= 4.30., p <0.01). We identified four statistically separable
pairs of V. rupestris genotypes: B 38 and R-66-3 (t = 2.88, p
<0.01), R-67-2 and R-66-3 (t = 3.69, p <0.01), B 38 and R-65-44
(t = 2.58, p <0.05), and R-67-2 and R-65-44 (t = 3.94, p <0.01).
The latter two pairs were not separable at leaf level.
Discussion
The results of spectral analysis showed that V. riparia and V.
rupestris genotypes can be spectrally discriminated at the leaf
and canopy scales. Although there was no single band that
could be used to discriminate between species and geno-
type pairs within species at either leaf or canopy scales, the
results of statistical analysis used to identify such spectrally
separable bands had a confidence level of p ≤0.05. The VIS
spectral region is dominated by leaf pigment constituents, and
inconstancy in the NIR and SWIR regions is caused by differ-
ences in leaf and canopy water content that are associated
with changes in LAI (Ceccato et al., 2001; Ustin et al., 2004).
Therefore, the combination of these factors plays a crucial role
in discriminating species and genotypes with reflectance fac-
tor spectra. The VIS and NIR regions were the most useful for
species and genotype discrimination. This is supported by sta-
tistical results from previous studies using the same or other
statistical tests (Asner et al., 2008; Lacar et al., 2001; Manevski
et al., 2011; Schmidt and Skidmore, 2003). The SWIR was also
important for discriminating plant species and genotypes, and
it was almost as effective in identifying spectral differences as
the VIS and NIR regions. This finding was somewhat different
from previously reported selective significance of the SWIR
spectrum (Thenkabail et al., 2004; Van Aardt, 2000).
At leaf level, the overall spectral curve shape of the two
grapevine species was similar but with different absorption
depth; highly significant lower reflectance factor in the full
wavelength region (350-2500 nm) of V. riparia indicates that
its leaf water content and pigment constituents are higher, on
average, than the V. rupestris (p ≤0.05). This observation is
strongly supported by fresh and dry leaf weights that are the
core components of equivalent water thickness (EWT), SLA and
total chlorophyll content. Surprisingly, even though no sig-
nificant differences were found in calculated total chlorophyll
content between species and genotype pair groups, statistically
significant differences were detected in VIS region after treating
the reflectance factor data through derivative processing.
The distinguishing capability of leaf and canopy spectra
may vary because the photons reflected back to the sensor at
canopy scale is affected by absorption and multiple scatter-
ing. The spectral separability decreased markedly at canopy
level especially in the VIS and shorter NIR regions, whereas
spectral separability increased in longer NIR and SWIR regions.
Moreover, at canopy level V. riparia had higher reflectance
factor values than V. rupestris in portions of VIS, full NIR, and
SWIR spectral regions which is opposite to what we found in
leaf spectra. This may be associated with the higher canopy
pigment concentrations, LAI , and lower canopy water content
of V. riparia compared to V. rupestris. In contrast, Cho et al.
(2008) found systematically higher VIS and NIR reflectance
factor values at the leaf scale than at the canopy scale. These
different results may be caused by the methodology used in
leaf spectra collection. It is worth noting that the wavelength
region 1125-1300 nm known to be dominated foremost by
variations in canopy water content (Asner et al., 2006), did
not show significantly different bands at the canopy scale.
This is supported by LAI data that is not significant between
two species. Therefore, we agree with Cho et al. (2008) that
the change in reflectance factor values from the leaf to the
canopy scale is not only due to the LAI but also due to the
complexity of the canopy (e.g., foliage clumping and the pres-
ence of twigs, flowers, and shadow).
In two species, the spectrally separable genotypes were
found using leaf spectra analysis and these wavelength-specif-
ic bands were located only in the NIR and SWIR regions. There
were new groups of spectrally separable genotypes within
V. rupestris detected using canopy spectra. The statistically
significant differences were not only in NIR and SWIR regions,
but also in the VIS region. In addition, the genotype groups
that had separable bands in NIR and SWIR regions at leaf scale
revealed separable bands in the VIS region at canopy scale;
however the groups lost their spectral separability found at
leaf scale. Due to the similarity of spectra, the most difficult
genotype groups to discriminate were those paired with B
75 within V. riparia or with R-65-44 paired groups within V.
rupestirs. In fact, the analysis of those paired groups pro-
duced the lowest number of significantly different bands in
their spectral discrimination.
Spectral derivatives are a well-known approach to use
instead of spectral reflectance factor because of their abil-
ity to reduce variability caused by changes in illumination
and background reflectance (Curran et al., 1991; Elvidge and
Chen, 1995; Laba et al., 2005). Additionally, both the ampli-
tude of the 1st-d and 2nd-d of reflectance factor spectra and the
derivatives of the VIS spectral region can isolate the pigment
expressions in this spectral region because they are strongly
related to pigment concentrations (Boochs et al., 1990; Sims
and Gamon, 2002; Yoder and Pettigrewcrosby, 1995). Con-
cerning the performance of 1st-d and 2nd -d, the two grapevine
species displayed differences with 1st-d spectra in NIR and
SWIR regions at both leaf and canopy level. In addition, new
sets of spectrally separable bands were identified by 1st-d at
canopy scale. Nonetheless, the separability in NIR and SWIR
regions were not as great when 2nd-d spectra were tested.
Among genotype groups that had separable bands in NIR and
SWIR regions at leaf scale, the 1st-d processing identified those
separable bands, particularly in VIS region, together with
a new genotype group that we also discriminated with VIS
bands. Likewise, at canopy level, the 1st-d was able to detect
spectrally separable VIS bands in only one comparison group
and found another genotype group that was not discriminated
with original raw spectra. The 2nd-d in genotype comparison
did not perform as effectively as 1st-d did. The 2nd-d identified
the spectrally separable bands in the VIS region that overlap
with the separable bands found in 1st-d spectra. In all other
cases, the 2nd-d spectra were separable in only very few bands
or no bands throughout the full spectral region.
A change in spectral properties and a lowering of PRI val-
ues in plants exposed to various abiotic stresses has been pre-
viously reported (Ainsworth et al., 2014; Meroni et al., 2009;
Naumann et al., 2008; Richardson et al., 2001). The mid-sum-
mer heat would be the only main stress that the grapevines
would be exposed to in this study. The PRI variation between
two species may demonstrate variations in xanthophyll cycle
pigments between species with different capacities for photo-
synthetic efficiency (Nichol et al., 2006). Decreased PRI values
for V. riparia indicated a lower xanthophyll epoxidation state
and may be a reflection of the reduced photosynthetic rates
relative to the V. rupestris . This result was expected since the
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