Abscisic Acid and Its Relationship to Seed Filling in Soybeans

Abstract

ABSTRACrThe effect of exogenous abscisic acid (ABA) on the rate of sucrose uptake by soybean (Glycine max L. Men.) embryos was evaluated in an in vitro system. In addition, the concentrations of endogenous ABA in seeds of three soybean Plant Introduction (PI) lines, differing in seed size, were commpared to their seed growth rates. ABA (10-molar) stimulated in vitro sucrose uptake in soybean (cv 'Cly') embryos removed from plants grown in a controled environment chamber, but not in embryos removed from field-grown plants of the Sink strength describes the capacity ofan organ to accumulate photoassimilates, and is the product of sink size and sink activity (25). Plant hormones are probably involved in determining both sink size and sink intensity. While cytokinins, gibberellins, and auxins generally promote growth processes, ABA has most often been considered an inhibitory substance (24). However, promotive effects of ABA in sink tissues have now been reported. ABA enhanced sugar content in roots of intact bean plants, possibly by stimulating sugar transport from the shoot to the root (11). In some cases, applications of ABA to ears of wheat (7) and barley (22) have stimulated 14C assimilate transfer from the flag leaf to the ear. However, King and Patrick (12) saw no stimulation of 14C transport to wheat ears after injection of ABA into the grains. Correlations between rate of growth and ABA accumulation in major sinks have been reported for wheat (13,14), grapes (5), pea (3), bean (10), and soybeans (4, 15). The relatively high levels (up to 12.2 #g g-' fresh weight) (15) and timing of ABA accumulation in soybean seeds (1,4,15) suggest a role in seed growth above simply preventing precocious germination. Browning (3) found that ABA accumulated more rapidly in a lare-seeded pea line and suggested a role for ABA in controlling cotyledon enlargement during linear seed fill. It has also been suggested that ABA may stimulate phloem unloading (8, 18) by decreasing the proton motive force across the sieve tube plasmalemma. The embryo-seed coat interface of the soybean seed (21) In addition, the endogenous ABA concentrations of seed coats, cotyledons, and embryonic axes of three field-grown soybean PI lines differing in seed size were compared to their seed growth rates during filling. MATERIALS AND METHODSGrowth Chamber Study. Plant Culture. Variety 'Clay' soybeans (Group 00 maturity) were grown in a controlled environment chamber in 1-L plastic pots in a 1.5:1.5:1 (v/v/v) mixture of vermiculite, peat, and sand. Pots were watered twice daily with automatic drip irrigation and given 250 ml of Peters 20-20-20 (200 ppm) twice weekly. A 14-h photoperiod with a 25/19C day/night temperature regime was provided. The photosynthetic photon flux density at the top of the canopy was 800 jE m-2 s-1.In Vitro Sucrose Uptake. During mid pod fill (seed dry weight 40% of final seed dry weight) pods were removed. Seed coats were surgically removed from a number of seeds and groups of three embryos were placed in 15 ml of sterile 10 ...

Growth Regulators Have Rapid Effects on Photosynthate Unloading from Seed Coats of Phaseolus vulgaris L.

“…Some control of sink unloading residing in the source itself would provide the means for source leaves to influence sink unloading according to total photosynthate supply. Circumstantial evidence that such a mechanism operates in the whole plant for ABA is provided by a report that soybean varieties with larger seed size contain higher ABA levels in seed coats (17). Interestingly, preliminary results of our own examining the effects of BAP and ABA injection into developing Phaseolus seeds in situ indicate that higher seed weights are produced with both growth regulators only when plants are severely defoliated in order to create a limited photosynthate supply for seed growth.…”

Tansley Review No. 27 The control of carbon partitioning in plants

“…Egli et al (1987) have demonstrated experimentally that physical constraints can limit the duration of seed development in soybean, but it is not certain that this is a factor under normal growth conditions. The level of abscisic acid (ABA) has been shown to reach a peak in the seed of soybean at the time when the growth of the seed is most rapid and large seeded genotypes have 50 % more ABA than small seeded genotypes (Schussler, Brenner & Brun, 1984). Also exogenously applied ABA (1 X 10"' M) was shown to stimulate the uptake of sucrose by embryos; whether rapid growth was regulated solely by ABA still needs to be resolved.…”

Distribution of Abscisic Acid in Maize Kernel during Grain Filling

“…The level of ABA in seeds has been found to increase during development usually coincidently with accumulation ofdry mass and reaches a maximum prior to physiological maturity (8,11,13,16,17). Typically, the ABA concentration in cereals is generally lower than found in legumes (8, 11, 13 16, 17).…”

“…the sink strength of kernels), which may be under hormonal control. ABA has been implicated in the regulation of sink strength in small grains (2,11,13,16,22), soybeans (17), and some horticultural crops (3,8,14,25,26) through mediation of phloem unloading. Similar information has not been reported for maize.…”