@2003 Kisman Posted
Science Philosophy PPs702
Term paper
December 2003
Instructors:
Prof. Dr. Ir. Rudy C.
Tarumingkeng (Principal)
Prof. Dr. Ir. Zahrial Coto
EFFECTS OF
DROUGHT STRESS ON GROWTH AND YIELD OF SOYBEAN
By:
Kisman
A361030061
ABSTRACT
Soybean, a major source of
protein and edible vegetable oil, is widely grown in the tropic countries
including
INTRODUCTION
Soybean, Glycine max (L.) Merr.,
the family of Fabaceae, is the major source of the
edible vegetable oils and of high protein feed supplements for livestock of the
world. Soybean is originated from
Soybean seed yield is determined by number of plants per unit area, main-stem nodes per plant, branches with pods per main-stem node, seeds per pod, and seed weight. Large variation in the amount and distribution of soil water often limits the soybean seed yield. On the other words, a soil water deficit due to drought stress during soybean growth will reduce seed yield. According to Bohnert et al. (1995), two major roles of water in plants are as a solvent and transport medium and as an electron donor in the photosynthesis reaction.
Drought stress has become a very interesting topics, since it is the most prevalent stress among environmental stresses. Drought as an environmental stress causes water stress or water deficit. According to Lavisolo and Schubert (1998), drought stress led a reduction of xylem vessel diameter and a modification of conductivity components of transpiration pathway (root, shoot, stomata) that may contribute to the reduction of water flow from root to shoot. In turn, a drought stress affects such physiological processes as translocation at the whole plant level, leaf expansion and gas exchange at the organ level, and photosynthesis at the sub cellular level and at the end, it reduces growth and yield. Figure 1 illustrates how water stress affects physiological and biochemical processes in plants.
Fig.1. Organismal responses to water stress (from Bray, 1993 cited by Bohnert et al.,
1995)
Jones and Jones (1989) defined water stress as the lack of amount of soil water needed for plant growth and development which in certain cells of a plant may affect various metabolic processes. More serious stresses that have the potential for inducing irreversible cell injury in plant are called severe stress. Direct impacts of drought stress to the physiological development of soybean depending on its water use efficiency (WUE) (Earl, 2002). Hoogenboom et al. (1987) noted that water stress occurs when soil water potential (ψ soil) was more negative than -15 kPa. Jones and Jones (1989) reported that the soil water potential differ with each species of plant and its development stages. The degree of water deficit in plants was determined by the harmonic balance between water supply from root zone and water losses, as summarized with a water balance equation ∆W = (P+I) – (O+U+ET), where water supply is precipitation (P) and irrigation (I), and water loss is due to surface runoff (O), deep percolation (U), and evapotranspiration (ET).
In agriculture management involving soybean as a crop, water use efficiency (WUE) is an important physiological characteristic related to the ability of plants to cope with water stress. According to Passioura (1997), grain yield (Y) is function of the amount of water transpired (T), water use efficiency (WUE), and harvest index (HI). Soybean, as a C3 plant, is less efficient in water-use due to high evapotranspiration and low photosynthetic rates.
EFFECT OF DROUGHT STRESS ON VEGETATIVE GROWTH
Soybean root growth and development in relation to shoot growth under field conditions basically consist of four stages (Hoogenboom et al., 1987): (i) rapid root growth beneath plant rows during the vegetative stage, (ii) branching of roots during early reproductive growth, (iii) a decrease of root growth beneath rows and an increase of root growth between rows during pod set, and (iv) cessation of root growth and root loss due to decomposition during physiological maturity.
Soybean root growth shows significant response to drought stress. Under drought stress, soybean root profile is characterized by a low amount of roots in the dry surface layer and a maximum root proliferation in the deeper, wetter soil layer. Under drought conditions, roots grow slowly in the surface layer and more rapidly in the deeper, wetter soil layers. The same phenomenon was found by Garay and Wilhelm (1983) that the effect of water stress on root depends on soil depth. The root growth declined greater in the top soil than that in deeper one, because water uptake per unit root length generally increased with depth in the soil.
Much evidence indicates that drought stress decreases soybean leaf area. Since growth in leaf area includes both cell division and cell enlargement, on of the subsequences of drought stresses is a reduction in leaf area (De Costa and Shanmugathasan, 2002). According to Sionit and Kramer (1977), water stress decreased total leaf area and leaf weight. Pandy et al. (1984b) also found that increasing drought stress progressively reduced leaf area, leaf area duration (LAD), crop growth rate (CGR), and shoot dry matter.
Another aspect related to leaf area is photosynthesis. Among the processes potentially limiting soybean yield, photosynthesis is one of that has a major impart. Leaf area index (LAI) and activity per unit leaf area are components of field photosynthetic performance (De Costa and Shanmugathasan, 2002). Soybean with large leaf area has a greater potential to contribute more photosynthate to the seeds. Rate of photosynthesis and transpiration declined in soybean as a result of increasing drought stress. Thus, total photosynthesis of water-stressed plants is decreased by reductions in rate of carbon fixation per unit leaf area resulting from premature stomatal closure and non-stomatal inhibition of photosynthetic machinery, and by reductions in photosynthetic surface area caused by decreased leaf enlargement and hastened leaf senescence. Stomatal conductance and photosynthetic rates and transpiration decreased simultaneously in water-stressed soybean plants. As Raper and Kramer (1987) also reported that effect of drought stress on photosynthetic rates of soybean declined rapidly with further reductions in leaf water potential to about -1.8 Mpa, and then continue to decline gradually with decreasing water potential.
Large plants with many nodes on both the main-stem and branches have many potential sites available for pod production. Shoot size at time of flowering i.e., height of the main-stem and length of the branches, number of nodes, and total leaf area have been reported to have a significant effect on soybean yield. Drought stress affected the number of nodes per plant and pods per node, but the response varied among cultivars or with timing of drought stress (Kadhem et al., 1985).
EFFECT OF DROUGHT STRESS ON REPRODUCTIVE GROWTH
Factors regulating seed yield in soybean are not only determined by seed fill period, seed abortion, or seed size, but also determined by flower production (Dybing, 1994). Plants stressed during flower induction and flowering had a short flowering period and produced few flower, as well as the number of pods and seeds because of the abortion of some flower. Failure to set pods may indicate an inherent sensitivity to low water potential (ψw) at critical stages of flower development. Recent evidence indicates that plant water stress have a direct effect on flower water status (ψw, ψs, and ψp) and flower function (Westgate and Peterson, 1993).
Drought stress during early pod formation caused the greatest reduction in number of pods and seeds at harvest (Sionit and Kramer, 1977). Smiciklas et al. (1992) noted that drought stress during flowering and full pod development significantly decreased seed number, and during seed formation reduced seed fill-period. Pods located in branches are more sensitive to drought stress than in top or bottom main-stems. Westgate and Peterson (1993) concluded that drought stress increased the probability of pod abortion and decreased pod set because of inhibiting pod expansion and metabolism. According to Vieira et al. (1992), drought stress during seed development usually interrupts development and shrivels (small, misshapen, undeveloped) seeds. There was a large significant reduction (13-64%) in yield, seed size, and seed number due to drought stress. Kisman and Sudarmawan (2002); Heatherly (1993) reported a similar result that drought stress during reproductive development decreased seed yield, seed number and seed weight. Stress at pod filling shortened the length of maturation period and the pod ripened about one week earlier than those on the other plants. This short period of seed and pod growth along with a reduced leaf area probably reduced accumulation of dry matter in the seeds. The decrease in translocation of dry matter during drought stress to reduced source strength by a reduction of photosynthesis, and reduction in sink strength by inhibiting growth. The rate of dry matter translocation to the seeds may have also slowed resulted in a significant reduction in the total seed weight (Sionit and Kramer, 1977).
The subsequences of drought stress during seed growth differ depending on when the stress occurs. If drought stress occurs at or soon after pollination, embryo abortion often occurs reducing the total number of seeds (Westgate and Peterson, 1993). The effect was primarily due to carbohydrate limitation from decreased photosynthesis. Embryo abortion can also be induced by reducing light intensity at pollination and supplementary sucrose solution. If drought stress occurs later during the dry weight accumulation phase of development, abortion generally does not occur, although seed size may be reduced due to a shortened seed filling period. Sionit and Kramer (1977) reported that drought stress during the pod filling stage produced the smallest seeds but did not reduce the number of pods or total number of seeds below the controls.
According to Desclaux and Roumet (1996), drought stresses may also modify the phenology of plants, and thus affect the yield components. A signal by which an early switch of plant development from vegetative to reproductive stage is triggered by drought stress. Appearance of stem-nodes initiated during stresses was delayed, resulting in a small number of nudes produced, whereas flower and pod appearance were hastened. Each reproductive stage was shorter under stress, mainly due to the appearance of new organs that prevented the emergence of organs belonging to the earlier ontogenetic stages. The seed filling stage and the final stage in seed abortion began earlier in stressed plants and the duration of the maturation period was significantly reduced by stress during seed filling, leading to accelerated senescence.
EFFECT OF DROUGHT STRESS ON SEED YIELD
Soybean is the most
sensitive to drought stress during pod filling. Yield loss is due primarily to
a decrease in pod number per plant. The overall decrease of pod number was due
to an increase in flower and pod abortion. Drought stress often occurs in
soybean production areas in
Figure 2. A flow diagram showing processes of the effect of drought stress on seed yield `(modified from Fukai and Cooper, 1995).
High seed yield is basically contributed by three reproductive mechanisms; development of flowers and pods, number of seeds per pod, and seed filling. Drought stress may adversely affect all three, but the magnitude of effects will vary with the timing of the stress. Among the yield components, the numbers of pods per plant were the most severely affected by drought stress, the number of seeds per pods per pod. In addition, because drought stress affected seed formation more than total dry matter yield, the harvest index declined as the drought stress increased (Pandy et al., 1984a).
The amount of water uptake into the plants determines the leaf water potential. Pandy et al. (1984a) concluded that increased drought stress followed by decreased cumulative leaf water potential, increased cumulative canopy and air temperature difference, and decreased seed yield and total shoot dry matter. According to Lavisolo and Schubert (1998) explained that drought stress modified water flow rate due to a reduction of xylem vessel diameter that may contribute to reduction of assimilate and further promoted a reduction of seed yield. Pandy et al. (1984b) noted that as drought stress increased: leaf area, leaf area duration, crop growth rate, and shoot dry matter progressively reduced. In turn, the adverse effects of drought stress on these growth attributes led to low seed yield. The flow chart showing the process of the effect of drought stress on seed yield is shown in Figure 2.
PROSPECT OF RESEARCH FOR DROUGHT TOLERANT SOYBEAN
Research in varietal improvement to obtain drought tolerant varieties is important to improve soybean production. One opportunity likely to be more effective is by focusing on the physiological-genetic along with physio-morphological traits determining the growth and yield under drought. Morphological traits known to have an effect on drought tolerance are leaf morphology (stomatal and cuticular resistance) and root penetration, while physiological factors are osmotic adjustment and related traits. Molecular works have been successful in identifying the physio-morphological traits controling plant responses to drought stress (Ribaut et al., 2001).
CONCLUSIONS
Based on the discussion above, it can be concluded that,
- The effect of drought stress on soybean growth and yield depend both on the degree of stress and on the stage of growth at which stress occurs.
- Drought stress during vegetative growth reduces root growth and root density. In turn this limits the water uptake, influencing metabolic relations and then reducing the organ growth in plants such as leaf growth, total leaf area, leaf area index, and net assimilation rate.
- Drought stress during reproductive growth reduces flowering period and number of flowers, as well as number of seeds and pods due to the flowers abortion.
- Drought stress decreases soybean yield.
- Improving drought tolerance in soybean can be done by applying understanding drought physio-morphological related traits with breeding and molecular works.
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