Download PhD Thesis - Fiva

determining the yield potential of a crop (Egli and Bruening, 2001). Modification of source
activity of soybean during flowering and early pod expansion usually results in a
corresponding change in pod number and this has been well described under optimal soil
moisture conditions (Egli and Yu, 1991; Board and Tan, 1995; Andrade and Ferreiro,
1996; Board and Harville, 1998). In the present study, the results showed for the first time
that drought stress disrupted carbohydrate metabolism in both leaves and floral organs
resulting in both source and sink restrictions (Paper II). These could be clearly seen from
the changes of the carbohydrates compositions and soluble invertase activity in the two
organs (Fig. 10) (Fig. 4; Paper II). In source leaves, it was observed that drought stress
decreased photosynthesis, starch and sucrose concentrations but increased hexoses
(glucose + fructose) concentrations, in line with earlier findings of Huber et al. (1984)
(Fig. 4; Paper II). The change of sugar composition in soybean leaves under drought
stress might have significant impact on the whole plant physiology. For instance,
accumulation of hexoses will favour leaf survival by an effect of osmotic adjustment and
restrict assimilate export, which however together with the decreased sucrose and starch
concentrations in the leaves indicative of a strong source limitation, whereby the export
rate of sucrose to the developing sinks (pods) is reduced. A low rate of current
photosynthate supply at the critical stage of pod development could be lethal for setting of
pods. This was obvious the case in the present study where we found pod set to be linearly
correlated with leaf photosynthetic rate when the latter reached below a threshold value
(Fig. 11A). This result confirms earlier findings in well-watered soybeans grown under
conditions of source limitations (Egli and Yu, 1991; Board and Tan, 1995; Andrade and
Ferreiro, 1996; Board and Harville, 1998).
Drought stress also disturbed carbohydrate metabolism in the flowers and pods. In these
organs, drought stress increased sucrose and hexoses concentrations but decreased starch
concentration (Fig, 4; Paper II). Drought stress also decreased soluble invertase activity
and hexoses to sucrose ratio in the pods (Figs, 5 and 6; Paper II). These findings indicate
that the capacity of the developing pods to utilise the incoming sucrose was impaired by
drought stress (Paper II). Similar results have also been reported for drought-stressed
maize ovaries (Andersen et al., 2002) and anthers of drought-stressed rice (Sheoran and
Saini, 1996), implying that a common effect of drought stress on carbohydrate metabolism
within crop reproductive structures may exist.
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