Soluble ADPglucose-az-glucan 4-a-glucosvltransferase (starch synthetase), ADPglucose pyrophosphorylase, UDPglucose pyrophosphorylase and phosphorylase were assayed in extracts from developing kernels of maize (Zea mays). Normal, waxy and amylose-extender maize at stages of development ranging from 8 days to 28 days after pollination were studied. Shrunken-4 maize at the 22-day stage was also studied. There is adequate activity of both ADPglucose pyrophosphorylase and starch synthetase at all stages of development to account for the synthesis of starch. Thus all starch could be synthesized via the ADPglucose pathway. High levels of UDPglucose pyrophosphorylase and of phosphorylase activities were also found at all stages of development. The possible role of phosphorylase in starch synthesis could not be discounted. The levels of phosphorylase, ADPglucose pyrophosphorylase, starch svnthetase, and UDPglucose pyrophosphorylase activities in shrunken-4 kernels were about 20 to 40% of that found in normal maize kernels. It appears that the mutation in shrunken-4 affects the activities of more than one enzyme. The defective starch svnthesis seen in this mutant could be due to the low activities of ADPglucose pyrophosphorylase and starch synthetase rather than the low activity of phosphorylase.Biosynthesis of a-1 ,4-glucosidic linkages of starch in higher plants is generally considered to be catalyzed by ADPglucosea-,4-glucan 4-a-glucosyltransferase (starch synthetase) (13). It has recently been shown that some forms of this enzyme extracted from spinach, maize, and potato can synthesize a-1, 4-glucosidic linkages in the absence of added primer (7,11,12 Carbohydrate Determinations. To 0.5 g of frozen kernels were added 5 ml of 75% (v/v) ethanol. The kernels were thawed and ground in the ethanol and then heated for 20 min in a boiling H20 bath. After cooling, the suspension was centrifuged at 10,000g for 10 min. The supernatant fluid was decanted, and the starch precipitate was extracted a second time as above. The supernatant fluids were combined, evaporated to dryness, and dissolved in 1 ml of H20. This solution was used for analyses of reducing sugars (10), total soluble sugars (6), and sucrose (6).
In two cultivars of bean (Phaseolus vulgaris L., Redkloud and Redkote) the older fruits growing at the base of racemes aborted less frequently than the younger ones above them. When older fruits at the base of racemes were removed, the abortion rate of the younger ones was reduced and their abscisic acid (ABA) concentration was lowered. Thirteen days after fruit removal, 36 to 45% of the younger fruits remained viable on treated plants while less than 12% of the younger fruits were viable on control plants. On these intact controls the ABA concentration of young fruits was at least twice that of defruited plants. A similar difference was found when the ABA content was expressed on a per fruit basis, suggesting a direct regulatory influence of older fruits over the ABA content of younger fruits.Premature fruit abscission, often of a large percentage of fruits on a plant, is a common phenomenon that has been reported for a number of plant species including apples (9), beans (15), and cotton (4,8). In beans, abscission appears to be the last step in the process of fruit abortion, which is characterized by cessation of seed development, flattening of pod walls, and loss of green color (15). Events leading to fruit abortion may include a decrease in the concentration of auxins (9) and an increase in the concentration of ethylene and ABA (4,8). However, neither the regulation of these hormonal changes within a fruit nor the coordination of fruit abortion with over-all plant development is well understood.In the companion article, fruits were shown to inhibit the growth of axillary buds of bean plants (15). The present report describes the competitive inhibition of young fruits by adjacent older fruits of bean plants. MATERIALS AND METHODSBean plants (Phaseolus vulgaris L. cv. Redkloud and cv. Redkote) were grown in a controlled environment growth chamber. The conditions of plant growth and methods of ABA analysis were described in the companion paper (15). The effect of older fruits on the abortion rate and ABA content of the younger ones was tested as follows. Fruits within each raceme were classified into categories of "older" (those at the base of the raceme), "younger" (those in the upper part of the raceme), and "aborting" RESULTS Effect of Older Fruits on the Growth and Abortion of YoungerFruits. The relative age of developing bean fruits within a raceme affected their rate of abortion: older fruits at the base of the raceme aborted less frequently than the younger ones above them. When the older fruits were removed from each raceme, the abortion rate of the younger fruits greatly decreased in both cultivars (Table I). On treated plants (older fruits removed), 36 to 45% of the younger fruits remained viable, while on controls the number was below 12%. Only 18 to 33% of the older fruits aborted on control plants. The effect of older fruit removal was highly significant, as shown by analysis of variance, on the number of viable younger fruits (F = 45.5, P = 0.001). Over 82% of the total variation in young...
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