Effects of drought on nodulation, nitrogen fixation, growth and yield
Drought may affect various physiological and morphological aspects of the soybean plant, which in turn affect nodulation, nitrogen fixation, growth and yield. The effect of drought stress on a plant may depend on various factors including the stage, severity and duration of stress. Although significant levels of osmotic adjustment have been reported in soybean (Likoswe and Lawn, 2008), osmotic adjustment is of little benefit for leaf survival.
Water stress may have varied effects on the plant, ranging from visually unnoticeable effects to wilting and death. Export of photoassimilates
Table 9.1. Effect of water stress (applied at the beginning of growth stage R6) and the timing of stress relief on yield and yield components of soybean grown in soil-filled pots (adapted from Brevedan and Egli, 2003).
Pods Seeds Seed size Seed yield per plant per plant (mg per seed) (g per plant)
Non-stressed (control) Early stress relief (returned to non-stress watering level after 3 days of stress) Late stress relief (returned to non-stress watering level after 6 days of stress) Continuous stress (pots received 40% of the water needed to bring non-stressed control to pot capacity) until maturity
from the leaves is affected by water stress (Ohashi et al., 2000). With a soil moisture deficit, the chlorophyll content is lowered (Velu, 1999) and various physiological parameters such as photosynthetic rate, net carbon assimilation efficiency, total area of stomatal apertures per unit leaf area, transpiration rate and WUE are decreased considerably (Ghosh et al., 2006b). Water stress even for short periods (3-13 days) during the seed-filling stage (R6) rapidly reduces the carbon exchange rate (Brevedan and Egli, 2003), thereby resulting in earlier maturity, smaller seed size and lower seed yields (Table 9.1). Pod abortion occurs in drought-stressed soybean (Liu et al., 2004), which may be due to a low availability of photosynthate in leaves and an impaired ability of pods to utilize sucrose.
Nodulation is adversely affected by drought or low moisture stress. Drought affects the number, size and weight of nodules. Furthermore, the pattern of nodule formation on roots (i.e. whether on tap or lateral roots) is also influenced by drought. Nitrogen fixation, in terms of percentage nitrogen fixation and the amount of nitrogen fixed, is influenced greatly in soybean by moisture availability and is decreased with water stress (Mohamed, 1995; Ohashi et al., 2000; Ray et al., 2006). Genotypes of soybean do vary in their ability to tolerate drought in terms of nitrogen fixation (Serraj et al., 1997; Sinclair et al., 2007). Furthermore, drought tolerance in some genotypes (e.g. Jackson) may be due to a large nodule size (King and Purcell, 2001), which helps with better photosynthate and water allocation, relative water content and water supply for ureide export.
In soybean, water stress decreases biomass production (Ohashi et al., 2000; Hajare et al., 2001), seed yield (Purcell et al., 2004), root surface area (Benjamin and Nielsen, 2006), root length, plant height, leaf area, dry weight of all plant organs, seed yield and the number of branches, flowers, pods and seeds (Ghosh et al., 2000a; Noureldin et al., 2002b), with the effects increasing with rises in the intensity and duration of the water deficit. The reduction in biomass production due to moisture stress is the greatest at the pod formation and seed-filling stages (Hajare et al., 2001). When drought stress occurs between the initiation of flowering and seed fill, the total seed yield is decreased. This is mainly because of reductions in branch vegetative growth and consequent reductions in seed number and branch seed yield, rather than due to any effect on main-stem seed yield (Frederick et al., 2001). Drought stress during the reproductive phase decreases pod set, which may be due to decreases in water potential and increases in the abscisic acid content of flowers and pods 3-5 days after anthesis (Liu et al., 2003). The above-ground biomass and seed yield of the soybean have been found to be reduced by 16% and 4%, respectively with deficit irrigation at the R2 stage and by 6% and 28%, respectively, with deficit irrigation at the R5 stage (Karam et al., 2005). Dogan et al. (2007b) reported that water stress at the R3, R5 or R6 results in substantial yield reductions compared with full irrigation, with the greatest reduction with water stress at the R6 stage.
Effects of waterlogging on nodulation, nitrogen fixation, growth and yield
Like drought, excessive soil moisture or waterlogging also has adverse effects on soybean plants (Thomas and Sodek, 2005). Waterlogging may be caused by heavy rainfall or over-irrigation and the problem is more common in fine-textured soils. Furthermore, excessive soil moisture is also experienced in paddy fields due to the formation of a hard pan owing to puddling operations. This excessive soil moisture induces growth losses in the succeeding soybean crop, particularly during vegetative growth. The problem is more severe in crops sown on a flat bed. Crops sown on raised beds or ridges do not generally experience the adverse effects of excessive soil moisture (van Cooten and Borrell, 1999; Seong et al., 2000a). However, the height of the raised bed or ridge also matters. In a rice field in Korea, the total dry matter accumulation was found to be severely decreased until the growth stage of R5 when soybean was sown at a 10-cm ridge height as compared to at ridge heights of 30 and 50 cm (Seong et al., 2000a).
In the case of flooding soon after sowing, a rapid in-rush of water into soybean seeds results in physical disruption of seeds, consequently reducing seedling emergence as well as seedling growth (Nakayama et al., 2005). Due to excessive soil moisture under poor drainage, crop growth and seed development in soybean are adversely affected (Rao et al., 1999; Seong et al., 1999) and soybean cultivars do differ in response to excessive soil moisture (Seong et al., 2000b). Furthermore, decreased contents of nitrogen, phosphorus, potassium, calcium, magnesium and copper in soybean leaves have also been reported due to excessive soil water stress (Seong et al., 1999), which may be due to decreased plant growth.
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