Agriculture Reference
In-Depth Information
is composed of climatic and soil factors, which exert a great influence on plant growth and, conse-
quently, yield. Climatic factors such as temperature and moisture supply play an important role in
crop production. Similarly, soil physical, chemical, and biological properties are directly related to
crop productivity (Fageria, 2013). Dinitrogen fixation is significantly affected by environmental fac-
tors. The most important environmental factors that affect dinitrogen fixation are soil water content
(Serraj et al., 1999; Pimratch et al., 2008), soil mineral N content (Jensen, 1987; Voisin et al., 2002),
and soil temperature (Halliday, 1975; Lie, 1971; Lira et  al., 2003; Liu et  al., 2013). When these
conditions are constant, however, the response of N 2 fixation can vary among species and cultivars
(Liu et  al., 2013). Aside from cropping history, environmental factors such as soil moisture, pH,
and nutrient content can influence the persistence of rhizobia in the soil (Keyser and Munns, 1979).
People and Herridge (1990) reported that a large number of legumes can be produced in
N-impoverished soils without the addition of fertilizer N, and in many soils without depleting soil
N reserves. These authors further reported that these desirable characteristics of legumes can be
realized only when large amounts of atmospheric N 2 are fixed. The successful formation of a func-
tional symbiosis is dependent on many physical, environmental, nutritional, and biological factors
and cannot be assumed to occur as a matter of course. If these factors are in the favorable range for
legume growth, dinitrogen fixation will also be at an optimum level. Some of these factors can be
manipulated in favor of N 2 fixation. These factors are discussed in the following sections.
7.6.1 t emperature
Temperature is one of the most important climatic factors affecting the growth and development
of plants. It affects various growth and metabolic processes in plants. In general, the rates in plant
processes are restricted when temperatures are too low to reach their maximum at somewhat higher
temperatures and decrease again when temperatures are too high. Crop species react differently to
temperature throughout their life cycles. Each species has a defined range of maximum and mini-
mum temperatures within which growth occurs and an optimum temperature at which plant growth
progresses at its fastest rate (Hatefield et  al., 2011). Vegetative development usually has a higher
optimum temperature than reproductive development. The progression of a crop through phono-
logical phases is accelerated by increasing temperatures up to the species-dependent optimum tem-
perature. Exposure to higher temperatures causes faster development in food crops, which does not
translate into an optimum for maximum production because the shorter life cycle means a shorter
reproductive period and a shorter radiation interception period (Hatefield et al., 2011).
As with most plant tissues, the nodule metabolism increases with temperature, resulting in Q 10
values of approximately 2.0 (Layzell et  al., 1983). Frame and Newbould (1986) reported that for
white clover ( Trifolium repens L.) the temperature requirement to grow and fix N ranged from 9°C
to 27°C with an optimum of 26°C. Gibson and Jordan (1983) reported that legumes from temperate
regions nodulate rapidly at 28-30°C root temperature, but the optimum temperature for N 2 fixation
is 20-24°C and tends to decline as the plant ages. These authors also reported that legumes of a
tropical or subtropical origin exhibit a temperature optimum for nodulation and N fixation in the
range of 25-30°C. The maximum temperature for nodulation is about 36°C and the minimum is
about 15°C (Lindermann and Ham, 1979). However, Mengel et al. (2001) reported that maximum
N-fixing rates have been obtained at a high soil temperature (33°C). These authors further reported
that the potential N 2 -ixing capacity of free-living bacteria is thus highest in subtropical and tropical
regions.
The minimum or maximum temperatures that can support N 2 -ixing nodules vary greatly with
the legume species, and even within a single species. Variation has been reported in the tolerance
of symbioses for N 2 fixation at extremes in temperature (Thomas and Sprent, 1984). Consequently,
it seems that genetic potential may exist for realizing significant increases in N 2 fixation (Layzell
and Moloney, 1994). Weisz and Sinclair (1988) reported that the changes in nodule activity with
temperature are inversely correlated with changes in the nodule's resistance to O 2 diffusion.
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