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Royal Veterinary and Agricultural University Copenhagen (2004)

Physiological regulation of pod set in soybean (Glycine max L. Merr.) during drought at early reproductive stages

LIU FULAI

Titre : Physiological regulation of pod set in soybean (Glycine max L. Merr.) during drought at early reproductive stages

Auteur : FULAI LIU

Université de soutenance : Royal Veterinary and Agricultural University Copenhagen

Grade : Doctor of Philosophy (PhD) 2004

Résumé partiel
Water scarcity is a major factor limiting crop productivity worldwide. Yield in soybean (Glycine max L. Merr.), a staple oilseed crop, is highly affected by drought stress, particularly when the stress occurring during flowering and early pod expansion. The yield loss is due mainly to an increased rate of pod abortion resulting in a less number of seeds per unit area. However, until now the underlying mechanisms causing pod abortion are still poorly understood. The overall objective of this study was to investigate the physiological basis for pod abortion of soybean exposed to drought stress during early reproductive development. Early pod development of soybean is characterised by active cell division in the young ovules and is marked by rapid pod expansion ; both processes are very sensitive to drought stress. Drought-induced carbohydrate deprivation and change in the concentration of endogenous abscisic acid (ABA) of the plants could have significant effects on pod growth and development, and may thus be involved in inducing pod abortion. To test these hypotheses, four pot experiments with soybean (cv. Holladay) were conducted in an environmentally-controlled glasshouse during 2002 and 2003. The fraction of transpirable soil water (FTSW) was used as a measure of the soil water status in the pots. The first two experiments were designed to investigate the critical stage for pod abortion and the associated changes of biophysical and biochemical factors, viz. plant water relation characteristics, photosynthesis, endogenous ABA and carbohydrate concentrations under drought stress. The third experiment was set up to investigate the effect of drought stress on pod ABA concentration and to further study the relationships of pod set to the aforementioned factors at the critical, abortion-sensitive stage. Finally, in the last experiment, manipulation studies were carried out in order to verify the postulations formulated during the former experiments. The results showed that severe drought stress significantly decreased pod set up to 40% and the critical stage for pod abortion was 3-5 days after anthesis (DAA) when cell division was active in the ovaries. Drought at later stages when pod filling had begun reduced seed size but had no significant effect on pod set. Pod water potential decreased by drought, however pod turgor was maintained at similar level to the well-watered controls. ABA concentration increased significantly in the xylem sap, leaves, and pods of drought stressed plants. Xylem-borne ABA and leaf ABA were seemingly the source of ABA accumulated in the drought-stressed pods. Carbohydrate metabolism was disrupted by drought stress in both leaves and floral organs. In leaves, drought stress decreased photosynthetic rate, starch and sucrose concentrations but increased hexoses (glucose + fructose) concentrations indicating a source limitation. In flowers and pods, drought stress increased sucrose and hexoses concentrations but decreased starch concentration, soluble invertase activity, and hexoses to sucrose ratio indicating that the capacity of the pods to utilise the incoming sucrose was impaired by drought stress. As a consequence of both source and sink restrictions, non-structural carbohydrate (sucrose + hexoses + starch) accumulated in the pods was significantly reduced under drought stress.

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