Agriculture Reference
In-Depth Information
the amount of red light absorbed. Combining light absorbance measures at 660 nm
and near-infrared (NIR) light transmittance at 940 nm, which in turn depend on leaf
moisture content and thickness, a good estimation of chlorophyll per unit area has
been obtained in the major crop leaves. It has been proved that the chlorophyll
meter can detect the early signs of N stress not yet detectable by visual analysis
using a leaf colour chart (Debaeke et al.
2006
).
Recently, it has been proposed a hand-held instrument (Dualex
®
), exploiting
chlorophyll as an internal sensor of photons, enables the user to contemporaneously
assess the level of both photosynthetic pigments in the mesophyll and flavonoids in
the epidermis of the leaf. Briefly, comparing the amount of chlorophyll fluores-
cence emitted under UV excitation (
λ
380
) with that emitted under visible light (
λ
660
) whether absorbed or not, the instrument is able to evaluate the level of
flavonoids absorbing in the UV range. Contemporaneously, by comparison of the
light transmittance at
λ
840,
in the
range influenced by leaf structural properties but not by chlorophyll, a reliable
evaluation of the levels of the photosynthetic active pigment is obtained. Dualex
®
thus allows measurement of a
N
itrogen
B
alance Index (NBI
®
), which indicates the
ratios of both chlorophyll and flavonoids units and as a result is related to leaf N
content (Cartelat et al.
2005
) since leaf flavonoids can be considered an indicator of
N availability. Indeed, in N-starved plants the concentration of carbon-based
secondary metabolites increase (Hamilton et al.
2001
) and in particular, due to
the enhanced expression of specific transcription factors involved in controlling
their biosynthetic pathway, those of anthocyanin and flavonols (Lea et al.
2007
).
Several experimental evaluations suggest that in the case of wheat and corn
Dualex
λ
720
, in the range of chlorophyll absorption, and at
seems to furnish more reliable information about the N status of the
plants with respect to other hand-held optical systems (Tremblay et al.
2012
).
Since leaf N status influences the quantum yield of PSII electron transport and
then the chlorophyll fluorescence parameters (Lu and Zhang
2000
), canopy fluo-
rescence quenching analyses could be considered suitable for sensing crop N status
(Tremblay et al.
2012
). In particular, the recent introduction of a hand-held fluo-
rimeter (Multiplex
®
) equipped with LEDs generating four wavelengths (
λ
375
,
λ
450
,
λ
530
,
λ
630
) and detectors monitoring fluorescence at three wavelengths (
λ
447
or
λ
590
if the excitation at
®
λ
735
) seems to be quite
promising for the in-season assessment of crop N status (Tremblay et al.
2012
and
references therein). Combining different excitation and emission bands the instru-
ment provides independent parameters related to chlorophyll, flavonoids and N
content of the plants (Tremblay et al.
2012
).
The devices described above determine optical parameters for individual leaves
or, in the case of Multiplex
λ
450
is used or not, respectively,
λ
665
and
, at a typical distance of a few centimeters thus
monitoring circular canopy surfaces of not more than 10 cm in diameter. Conse-
quently, they are not particularly suitable in evaluating the N status of a crop at field
scale. Sensors, analysing canopy reflectance properties and thus its N status and
needs are also available (Erdle et al.
2011
). They are classified as passive (Yara
N-Sensor
®
/Field Scan and FiledSpec
Portable Spectroradiometer) or active
®
®
and Crop Circle
TM
) non-contact sensors depending on the sunlight
(GreenSeeker
®
