Agriculture Reference
In-Depth Information
Logically, much of the research has concentrated on root temperature as it is directed at the site of
nodule development, but shoot temperatures are also important with respect to assimilating flow to
and from the nodules (Gibson and Jordan, 1983). Studies with soybean (Schweitzer and Harper, 1980)
and Trifolium subterraneum (Eckart and Ragues, 1980) concluded that shoot temperature is a major
factor influencing nitrogenase activity. For example, soybean held at 27°C in the dark maintained the
same level of nitrogenase activity as those kept under light for 72 h, but cooling the shoots to 18°C
led to a marked drop in activity. The effect was attributed to a greater mobilization of carbohydrate.
Optimum temperatures for nodulation in dry bean, obtained from isolated root and whole plant
studies, vary from 25°C to 30°C (Barrios et al., 1963; Small et al., 1968), and nodulation is mark-
edly reduced at 12°C or 33°C (Graham, 1981). Pankhurst and Sprent (1976) reported the highest
nitrogenase activity in isolated nodules at 20°C. Graham and Rosas (1979) reported that peak rates
of N 2 (C 2 H 2 ) fixation increases from 33.8 µmol C 2 H 2 produced per plant per hour at 35/25°C day/
night temperature regime to 73.08 µmol C 2 H 2 produced per plant per hour at 25/15°C day/night
temperature. Liu et al. (2013) reported that N 2 fixation was very sensitive to low temperature and
photosynthetic rates. Potentially, cultivar breeding aiming at cold resistance and a higher photosyn-
thetic rate could increase N 2 fixation in practice (Liu et al., 2013).
7.6.2 s oIl m oIsture
Moisture availability is one of the most important factors determining crop production. Seasonal
water supply affects growth potential and N 2 fixation (Giller and Wilson, 1991; Peoples et al., 1992).
Crop yield can be reduced both at very low and very high levels of moisture. Many studies have
shown that biological N fixation can be adversely affected by both waterlogging and soil dehy-
dration at critical times during the development and growth of legumes (Sall and Sinclair, 1991;
Pena-Cabriales and Castellanos, 1993). Excess moisture reduces soil aeration and, thus, the supply
of O 2 available to roots. With poor aeration, the activities of beneficial microorganisms and water
and nutrient uptake by plants can be seriously inhibited, though aquatic plants and rice are adapted
to and function well even when soils are saturated. Soil moisture deficits can cause the stomata in
the leaf to close, reducing transpiration and helping maintain hydration of protoplasms, but also
reducing photosynthesis. Moisture stress also causes reductions in both cell division and cell elon-
gation and, hence, in growth (Fageria et al., 2011; Fageria, 2013).
The storage terms relate only to the portion of soil moisture available to the plant. Field capacity
and a wilting coefficient or a permanent wilting point are the practical upper and lower limits of
water availability for crops. The upper limit of soil water availability to plants is often considered
the water contents after the saturated soil has freely drained for 2-3 days or the wetted soils have
been subjected to pressures in the range from 5 to 30 kPa (kilopascals) or 0.05 to 0.3 bars (Unger
et al., 1981). The lower value is generally applicable to light-textured soils and the higher value to
heavy-textured soils. Root growth is better when the water content in the soil is around field capac-
ity. Hence, dinitrogen fixation will also be maximum when soil moisture is at field capacity.
Worrall and Roughley (1976) reported that nodulation is affected by a reduction in root hair infec-
tion and suppression of nodule development at low soil water potential, that is −3.6 × 10 5 Pa (Pascal).
Rewatering stressed plants to a rapid recommencement of root hair growth, followed by infection and
nodulation initiation. In soils with low water potential, there is poor movement of rhizobia (Hamdi,
1971), and nodule development may be restricted to sites close to the crown. Gallacher and Sprent
(1978) reported that the development of nodules initiated prior to the imposition of water stress is
retarded by low water potential, although development is renewed when the stress is alleviated.
7.6.3 s oIl a CIdItY
Soil acidity is a serious problem worldwide for agricultural production. According to Edwards et al.
(1991), acid soils constrain agricultural production in more than 1.5 Gha worldwide, with the scope
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