Agriculture Reference
In-Depth Information
wheat has important implications in terms of human nutrition, and should be considered in agro-
nomic and genetic biofortification of wheat with Fe. Various genetic studies revealed the existence
of a close linkage between the remobilization of N and remobilization of Fe from senescing leaf
tissue into grain in durum wheat (Uauy et al., 2006a,b; Distelfeld et al., 2007; Waters et al., 2009).
Kutman et al. (2011) reported that the Fe harvest index (Fe in the grain/Fe in grain plus straw)
strongly depends on N supply in wheat plants.
6.3.5 n
ItroGen
versus
B
oron
Boron deficiency in crop plants has been reported throughout the world (Gupta, 1979; Blevins and
Lukaszewski, 1998). The deficiency of B is generally common for plants grown on light-textured
soils in humid climates where B is readily leached from the soil. Boron in soil solutions usually
occurs as the undissociated boric acid (H
3
BO
3
). Adequate B levels are essential for higher crop
yields and quality. Boron improved the root development in common bean, soybean, and wheat
grown on an Oxisol of central Brazil (Baligar et al., 1998). Boron has been reported to have coun-
teracting toxic effects of Al on the root growth of dicotyledonous plants (Blevins and Lukaszewski,
1998). Boron requirements for reproductive growth are much higher than for vegetative growth of
most crop species (Loomis and Durst, 1992).
Boron has also been reported to be essential for N
2
fixation (Mateo et al., 1986). The application
of B increased the N concentration in chickpea (Yadav and Manchanda, 1979), lentil (Singh and
Singh, 1983), and peanut (Patel and Golakia, 1986), presumably due to the favorable effect of B on
nodulation as nodule counts were found to increase by 37% over no-B control (Patel and Golakia,
1986). Nitrogen in adequate amounts reduces B toxicity in crop plants (Willett et al., 1985).
Warington (1923) is credited with the first definitive proof that B is required by higher plants.
Later, Sommer and Lipman (1926) established B requirements for six nonleguminous dicots and for
one graminaceous plant (barley). However, B requirements are highly variable, and optimum quan-
tities for one plant species could be either toxic or insufficient for other plant species. Based on the B
requirements, plants can be divided into three general groups: (i) graminaceous plants that have the
lowest B demands; (ii) the remaining monocots and most dicots with intermediate B requirements;
and (iii) latex-forming plants with the highest B requirements (Mengel et al., 2001).
Boron deficiency affects the development of meristems or actively growing tissues so that
deficiency symptoms are death of growing points of shoots and roots, failure of flower buds to
develop, and ultimately blackening and death of these tissues. Boron uptake normally decreases
with increasing soil pH (Figure 6.13). In common bean grown on an Oxisol, decreases were 120%
when the soil pH was raised from 4.9 to 7.0. This decrease in B uptake may have been related to
adsorption processes as the soil pH increases. Boron adsorption increased as the pH increased
above 4 and reached a maximum at pH 8-9 before decreasing at higher pH values (Barber, 1995).
Soil texture also affects B adsorption; it is higher in heavy-textured soils compared to light-
textured soils.
Positive relations have been noted between B and N fertilizers for improving crop yields (Hill
and Morrill, 1975; Moraghan and Mascagni, 1991; Fageria et al., 2002). Miley et al. (1969) reported
that B fertilization in adequate amounts enhances utilization of applied N in cotton by increasing
the translocation of N compounds into the boll. Similarly, Smith and Heathcote (1976) found that,
when B deficiency occurred in cotton, the application of 250 kg N ha
−1
depressed yield. However,
this rate of N produced a yield increase when B was applied. Inal and Tarakcioglu (2001) and
Lopez-Lefebre et al. (2002) reported the positive effects of B on uptake and metabolism.
6.3.6 n
ItroGen
versus
m
olYBdenum
Arnon and Stout (1939) established that Mo was an essential nutrient for higher plants using tomato
as a test plant. Molybdenum is indispensable in the nitrogen fixation process to aerobic as well as
