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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...

MATERIALS AND METHODSTwo adenosine diphosphoglucose: a-, 4-glucan a-4-glucosyltransferases were extracted from kernels of waxy maize harvested 22 days after pollination and separated by gradient elution from a diethylaminoethyl-cellulose column. Both fractions could utilize amylopectin, amylose, glycogen, maltotriose and maltose as primers. The rate of glucose transfer from adenosine diphosphoglucose to rabbit liver glycogen of fraction II was 78% of the rate of glucose transfer to amylopectin, but with fraction I the rate of transfer of glucose to rabbit liver glycogen was 380% of that observed to amylopectin. Glucan synthesis in the absence of added primer was found in fraction I in the presence of 0.5 M sodium citrate and bovine serum albumin. The unprimed product was a methanol-precipitable glucan with principally a-1,4 linkages and some a-1,6 linkages, and its iodine spectrum was similar to that of amylopectin.Either sugar nucleotide transferases or phosphorylase could be involved in the initiation and synthesis of starch in higher plants (3,4,10,(13)(14)(15). Multiple forms of ADP-glucose:a-1,4-glucan a-4-glucosyltransferase have been separated from spinach leaves, one of which in the absence of added primer catalyzes the synthesis of a glucan with principally a-i ,4 linkages and some a-1 , 6 linkages (12). The unprimed activity was stimulated over 1000-fold by BSA3 and high concentrations of some salts (12). In maize kernels, Tsai and Nelson (15) have found evidence for the presence of multiple forms of phosphorylase, two forms of which are capable of synthesizing a polyglucan in the absence of added a-1 ,4 glucan primer.The present communication reports the existence of two forms of ADP-glucose:a-1 ,4-glucan a-4-glucosyltransferase (ADP-glucose-starch glucosyltransferase) in waxy maize, one of which can catalyze the formation of a polyglucose with properties similar to amylopectin in the absence of added primer. Rates of the unprimed reaction were up to eight times faster than the primed reaction at physiological concentrations of ADP-glucose.

A comparison was made of the germination, growth, and leaf CO2‐exchange rates of redroot pigweed (Amranthus retroflexus L.) and common lambsquarters (Chenopodium album L.) under varied temperature regimes. It was found that redroot pigweed had greater rates of germination, growth, and CO2‐exchange rates at high temperatures, whereas common lambsquarters performed better at low temperatures. The implications of such characteristics for competition between these species of weeds and their effect on crops are discussed.

Two dry bean (Phaseolus vulgaris L.) cultivars were grown in controlled‐environment chambers under standard light [390 μ Einsteins m‐2 sec‐1 (400 to 700 nm) = approx. 22,000 lux] and shaded light [55 μ Einsteins m‐2 sec‐1 (400 to 700 nm) = approx. 3,200 lux] intensities. Experimental observations included measurement of gas exchange and photosynthetic and respiratory enzyme activity along with examination of plant morphology, leaf anatomy, and chloroplast ultrastructure.Shading reduced leaf number, area, and thickness. Photosynthesis/unit area of shaded leaves was decreased by an average of 38%. Transpiration was not significantly affected. Increased intracellular resistance of the shaded leaves was more important in reducing CO2 uptake than was the increase in stomatal resistance. The increase in mesophyll resistance was reflected both in the biochemical reactions of the leaf (activity of all measured enzymes being reduced by 70%) and in the ultrastructure of the leaf chloroplasts (reduced quantities of starch in spite of extensive grana).

Phaseolus vulgaris L. plants exposed to 5 C for a single night exhibited severe reductions in photosynthesis the following day. However, the response was manifest only if the roots as well as the shoots were cooled. Photosynthetic reductions were accompanied by a parallel drop in transpiration, a rise in stomatal and mesophyll resistance to CO2 uptake, and a decrease in leaf water potential. Activities of ribulose diphosphate carboxylase and malate dehydrogenase were unaffected. Starch retention within chloroplasts of cold‐treated leaves accompanied a drop in photosynthesis, but the presence of starch did not reduce photosynthesis. The data suggest that rather than feedback inhibition, the photosynthetic reduction of potted bean plants following an exposure to cold is due to stomatal resistance brought on by temporary water stress.