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whilst it increased them in drought-stressed plants; application of BA had opposite effects.
In these plants pod set positively correlated with photosynthetic rate indicating that the
effect of ABA and BA on pod set may be partially due to their effects on photosynthate
supply (Fig. 14). On the other hand, in the well-watered plants exogenous ABA and BA
affected pod set to an extent greater than that would be expected based on their effects on
photosynthesis alone (Fig. 11A). This implies that direct influences of the two hormones
on the metabolic processes within the pods may exist. The direct effects of the hormones
within the ovary might have been on cell division and on the carbohydrate catalysing
enzyme activity (e.g. acid invertase) as had been reported in maize (Radley, 1979;
Mambelli and Setter, 1998; Setter and Flannigan, 2001; Trouvrie et al., 2003). However, in
soybeans, these possibilities remain open for further investigations.
3.3. Conclusions
Based on the experimental results, it was concluded that:
• Early pod expansion (3-5 DAA) is the most sensitive stage for pod abortion in soybean
exposed to drought stress.
• Sufficient photosynthate supply is vital for setting of pods. Drought-induced reductions
of current and reserve carbohydrate supplies contribute to pod abortion during the
critical, abortion-sensitive phase of pod development.
• Expansion growth of pods is more sensitive to soil water deficits than pod set. Root-
originated xylem sap ABA seemingly controls pod growth, whereas low pod water
potentials disrupt carbohydrate metabolism in the pods and induce pod abortion.
• ABA and BA play important roles in controlling pod set in soybeans. The two
hormones exert their effect on pod set directly via affecting the pod meristem
metabolic processes (e.g. cell division) or indirectly through influencing stomatal
conductance and hence the rate of photosynthate supply.
3.4. Perspectives
Soybeans, as well as other grain legumes, are major source of protein and oil for human
daily diet and livestock feed. Soybean plants produce more flowers than eventual pod set
even under optimal soil moisture conditions. Drought stress during flowering and early pod
development may increase the rate of pod abortion leading to significant yield reduction,
yet the mechanisms of what controls abortion and abscission are still unknown. The results
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