Effect of Shading Individual Soybean Reproductive Structures on Their Abscisic Acid Content, Metabolism, and Partitioning

Abstract

Typically, a large proportion of soybean flowers produced, abscise before reaching maturity. Estimates as high as 57 to 82% abscission have been reported for determinate varieties (23, 27), and 32 to 65% for indeterminate types (23).Soybean floral abscission may be either stimulated or reduced by environmental factors. Nutrient deficiency (2), high temperatures, (24), long photoperiods (2, 24) and reduced light intensity (9, 15, 24) stimulate abscission, while increasing the available N (2) and CO2 during flowering (7) (13) in studies of partitioning to rose shoot buds.ABA has been reported to be related to abscission of various plant structures (1). However, recent evidence suggests a role of ABA in the partitioning of photoassimilates to developing fruit and seeds. A positive relationship between exogenously applied ABA and sink activity has been shown in both wheat (6) and barley (22). In peas (3) and soybeans (16), a positive relationship was observed between seed growth rate and ABA concentration. ABA stimulated in vitro sucrose uptake by isolated soybean cotyledons (16) and sugar beet root discs (14). Decreased reproductive abscission in soybean in response to light was shown to be associated with increased partitioning of recently fixed carbon to the reproductive structures (8); in addition, carbon partitioning was greater in flowers destined to set pods than in flowers destined to abscise (4).The association of pod set with carbon partitioning (4, 9), and of carbon partitioning with ABA (3,6,16), suggests that there may be a positive relationship between pod set and ABA concentration of the flower. The objectives of this study were to determine: (a) if shading of individual reproductive structures has an effect on their endogenous ABA levels, (b) if shading of reproductive structures has an effect on ABA catabolism, and (c) if shading has an effect on partitioning of ABA and photoassimilates from leaves to reproductive structures. MATERIALS AND METHODSPlant Material. The semideterminate soybean genotype IX93-100 was grown in four completely randomized blocks in the field at St. Paul, MN in the summer of 1983. This genotype was selected for its relatively long racemes, allowing for easy determination of anthesis date, growth, development, and abscission of individual flowers. Seeds of IX93-100 were donated by C. D. Dybing, USDA-ARS, Brookings, SD.IX93-100 was selected from crosses made by D. E. Green, Iowa State University, of A71-5558-1 and L61-344, a semideter-

Intra-Raceme Variation in Pod-Set Probability Is Associated with Cytokinin Content in Soybeans

“…Thus the results on the effect of IAA on abscission of soybean flowers are inconsistent. Yarrow et al (1988) suggested that ABA might reduce abscission of soybean flowers by favoring reproductive structures with allocation of photosynthate. Investigations on the interactive effects of these hormones, as well as the effects of environmental factors, are needed to clarify the detailed mechanisms controlling reproductive abortion of soybeans.…”

“…Possible physiological factors affecting abortion include quantity of certain plant hormones (Huff and Dybing, 1980 ;Beckmann, 1981 ;Heindl et al, 1982 ;Spollen et al, 1986b ;Yarrow et al, 1988), deficiency in or competition for carbohydrates and nutrients (Brevedan et al, 1978;Brun and Betts, 1984 ;Antos and Wiebold, 1984 ;Heitholt et al, 1986aHeitholt et al, , 1986b) and the quality and quantity oflight in the plant canopy (Heindl and Brun, 1983;Brun et al, 1985;Myers et al, 1987).…”

Effects of Source/Sink Ratio and Cytokinin Application on Pod Set in Soybean

“…By reducing this abortion, we might be able to increase pod and seed numbers, thereby increasing the yield (Kokubun, 2004). Many previous studies have focused on two possible factors that might be involved in regulating the abortion; availability of photoassimilate or nutrients (Brevedan et al, 1978;Antos and Wiebold, 1984;Brun and Betts, 1984;Heitholt, 1986aHeitholt, , 1986b and quantity of certain hormones (Huff and Dybing, 1980;Heindl et al, 1982;Spollen et al, 1986aSpollen et al, , 1986bCarlson et al, 1987;Yarrow et al, 1988;Kokubun and Honda, 2000).…”

Roles of Auxin and Cytokinin in Soybean Pod Setting

“…By reducing this abortion, it might be possible to increase the pod number, thereby increasing the yield (Kokubun, 2004). Numerous precedent studies showed two putative factors involved in controlling the abortion: availability of photosynthate or nutrients for pod development (Brevedan et al, 1978;Antos and Wiebold, 1984;Brun and Betts, 1984;Heitholt et al, 1986a, b) and availability of certain hormones (Huff and Dybing, 1980;Heindl et al, 1982;Spollen et al, 1986a, b;Carlson et al, 1987;Yarrow et al, 1988;Kokubun and Honda, 2000;Yashima et al, 2005). Within individual racemes, the pod set percentage of basal flowers is considerably higher than that of distal ones (Wiebold et al, 1981;Spollen et al, 1986a;Saitoh et al, 1999): this phenomenon appeared to be associated with the endogenous cytokinin content of flowers; the basal flowers contained a higher percentage of cytokinin than the distal flowers (Kokubun and Honda, 2000).…”