Agriculture Reference
In-Depth Information
mechanisms of toxicity have not been elucidated completely,
NH
4
+
accumulation in leaves has been
found to depress photosynthesis and leaf growth (Raab and Terry, 1994). Also,
NO
3
−
is an important
osmoticum that intervenes in the expansion of plant cells, and a reduction of its absorption may
depress growth (McIntyre, 1997). The reduction of the absorption of cations such as K
+
, Ca
2+
, and
Mg
2+
by
NH
4
+
may also limit plant growth (Salsac et al., 1987).
Numerous studies have demonstrated that the presence of
NH
4
+
in the growth medium reduces
NO
3
−
uptake into roots of crop species and into microorganisms within minutes of exposure (Lee
and Drew, 1989; Glass, 2003). The mechanism responsible for this effect is still unclear. Based
upon their demonstration that
NH
4
+
reduced
NO
3
−
uptake within 3 min in barley roots, Lee and Drew
(1989) and Ayling (1993) proposed that the effect might be based upon the direct effects of
NH
4
+
on
membrane potential, since
NH
4
+
typically depolarizes the membrane electrical potential difference.
Because
NO
3
−
traverses the plasma membrane as a cation (through its association with at least two
H
+
), membrane depolarization would reduce the proton motive force driving the uphill transport of
NO
3
−
and concomitantly reduce
NO
3
−
uptake (Glass, 2003).
Uptake of N (
NO
3
−
and
NH
4
+
) is regulated by genes in higher plants (Glass, 2003). Glass (2003)
reported that there are many genes involved fin
NO
3
−
and
NH
4
+
absorption and transport in the higher
plants. Higher plants may possess genes encoding as many as 11 nitrate transporters and at least
6 high-affinity ammonium transporters, and the systems for regulating N fluxes are complex and
highly integrated (Glass et al., 2001, 2002). Nevertheless, there are clear indications that only a
limited number of the
NO
3
−
transporter genes are responsible for nitrate absorption from soils; the
remainder probably encode transporters that participate in internal redistribution. A similar situa-
tion applied with respect to
NH
4
+
absorption (Glass, 2003).
3.5 USE OF CHLOROPHYLL METER
Traditionally, nutrient deficiency/toxicity symptoms, soil testing, plant tissue analysis, and green-
house and field trials have been used to assess N availability for crops (Kitchen and Goulding,
2001; Fageria, 2013, 2014). However, since the early 1990s, handheld chlorophyll meters have
been available to monitor plant N status by measuring the transmittance of radiation through a leaf
in two wavelength bands centered near 60 and 940 nm (Blackmer et al., 1994; Wood et al., 1993;
Souza et al., 2010). Previous research has shown that corn reflectance of green and near-infrared
(NIR) light measured with a radiometer is sensitive to N status (Bausch and Duke, 1996) and can
be used to predict the amount of N fertilizer needed by the crop (Dellinger et al., 2008; Scharf
and Lory, 2009). Walburg et al. (1982) confirmed that corn spectral properties associated with N
deficiency are likely to be apparent by the V12 growth stage, when the crop still has the potential
for large yield responses to added N (Russelle et al., 1983; Scharf et al., 2002). Blackmer et al.
(1996) measured the reflected radiation form from R5 growth-stage corn canopies using reference
areas with nonlimiting N to calculate the relative reflectance. They concluded that the reflected
radiation around 550 and 710 nm provided the best detection of N deficiency in the 400-1000 nm
spectral range.
The use of a chlorophyll meter in topdressing N during a crop growth cycle is an important aid
in correcting N deficiency. After visual deficiency symptoms, it is one of the cheapest techniques
to identify N status in growing crop plants. Instantaneous and nondestructive chlorophyll meter
reading represents an alternative to traditional tissue analysis for diagnosing crop N status, and
this approach has been used in barley (Wienhold and Krupinsky, 1999), corn (Schepers et al.,
1992), rice (Peng et al., 1993), and wheat (Follett et al., 1992; Ziadi et al., 2010).
A chlorophyll meter (Minolta SPAD-soil-plant analysis development) measures the greenness
intensity of plant leaves. The greenness intensity is associated with chlorophyll content, which in
turn is associated with leaf N concentration (Wolfe et al., 1988). In most crop species (rice, wheat,
corn, potato), the relationship between SPAD readings and crop yield was poor at the early crop
development stages, but it improved at later stages because of the greater N deficiency expression as
