A study was conducted on the diurnal changes in hydrocyanic acid, dry matter, and total sugar content in five strains of Dakota Amber selected on the basis of their hydrocyanic acid content. Two strains were tested in 1939 and five strains in 1940. Diurnal variation of hydrocyanic acid in different sorghum strains showed unlike trends during the daytime, although in every case the greatest diurnal drop in hydrocyanic acid content occurred at the close of the day between 6:00 and 8:00 p.m., when photosynthetic activity was low. Similarly, the most rapid diurnal increase in hydrocyanic acid occurred between 8:00 and 10:00 p.m., when reduction in dry matter and total sugars of the leaves had taken place. Hydrocyanic values from sorghum strains sampled at different diurnal periods suggest that unless samples of strains to be compared are taken simultaneously, it may be difficult to get a correct evaluation of the material under study. The diurnal maximum hydrocyanic acid content in the five sorghum strains tested occurred at different quarterly periods. Their diurnal minimums also occurred at different quarterly periods. Similar diurnal hydrocyanic acid trends were found in sorghum strains sampled on a warm, bright, clear day or on a cool cloudy day. Diurnal variations in the trend of hydrocyanic acid content in a sorghum strain sampled at a similar stage of growth in different seasons were similar. The greatest variation for strains between seasons was their mean hydrocyanic acid content. Environment and stage of growth of sorghum plants apparently affected their hydrocyanic content. Dry matter in the leaves of sorghum strains tested began to increase after 4:00 a.m. The diurnal maximum dry matter content occurred between 2:00 and 6:00 p.m. Minimum dry matter content in the sorghum strains occurred about 4:00 a.m. Hydrocyanic acid content in sorghum strains evidently is associated with photosynthesis, being at a maximum during the daytime, shortly after the dry matter in the sorghum leaves was at its maximum. The diurnal variation in the concentration of hydrocyanic acid in sorghum strains is apparently positively correlated with the diurnal variation of total sugar in the leaves. Generally, the hydrocyanic acid change came about later than the change in sugar content. The higher the rate of change in the sugar content, the more rapid was the change in hydrocyanic acid content. Variability in hydrocyanic acid content is also associated with other photosynthetic activities. Diurnal variation of hydrocyanic acid content in sorghum is the function of three variables, as follows: (A) The genetic constitution of strains; strains have been isolated which do have definite hydrocyanic acid levels and diurnal trends. (B) Environmental factors; the growth and development of the sorghum plant influences the hydrocyanic acid content. A higher or lower hydrocyanic acid content level caused by environmental factors did not appear to affect the diurnal hydrocyanic acid trend in a sorghum strain. (C) Photosynthetic activities; assimilation of photosynthetic products causes unlike variations in the diurnal hydrocyanic acid trend within sorghum strains at short intervals of time.

There are sorghum varieties that differ from one another in only one allele that affects maturity. Such pairs of varieties are Texas and Sooner Milos, Hegari and Early Hegari, and Kalo and Early Kalo. These pairs of varieties were crossed to a third variety and the F1 plants were grown and observed for height, duration of growth, tillering, and for yield of grain and stover. All hybrids produced greater yields of grain and stover than the parents. Hybrids that differed in only one allele, in most cases, produced different yields of grain and stover. The alleles that differentiate theMilo, Hegari, and Kalo pairs of varieties differ in combining ability. In one case, reccessive ma4 from Kalo in combination with Kafir produced 37% more grain than dominant Ma4 from Early Kalo. It is apparent from these results that, in sorghum, one allele can have a great influence on the combining ability of a strain.

Crops vary widely in their ability to compete with bindweed. This is indicated by the survival of bindweed over the periods of controlled cropping reported in this study (Table 1). Under certain conditions, a crop and bindweed may grow together more or less normally without any apparent competition. During the seasons of ample moisture on fertile soil, spring-sown oats and bindweed develop normally as companion plants with neither apparently reducing the available supply of essential growth elements to the detriment of the other. The same conditions often exist where corn is growing on bindweed-infested land. When the essential growth factors, such as soil moisture or soil nutrients, are not readily available in quantities sufficient for the development of both the crop and weed, competition develops and competitive forces are reflected in the reactions of the plants. This competition ultimately determines the dominance of the associated species or individuals. Field observations indicate clearly that where competition develops for soil moisture, bindweed competes successfully with practically all crop plants. This study indicates that in southwestern Minnesota, competition for available soil moisture is not normally the prime competitive factor in controlling field bindweed growth with crop plants. The soil moisture data obtained in 1939 and other seasons indicate that bindweed, growing normally and undisturbed, reduces soil moisture almost as rapidly as where a crop of rye is growing in competition with it. Therefore the growth of bindweed would not be expected to be greatly impaired if the rye was competing for this factor alone. When soil moisture was maintained at a high level by a preceding period of intensive cultivation, bindweed growing in competition with sorghum or soybeans produced abnormal growth and was forced into dormancy, indicating quite clearly that soil moisture was not the critical factor. Further, when the rye, soybeans, or sorghum was harvested, bindweed resumed active growth without additional soil moisture. This study indicates that, where soil moisture can be conserved or is ample and where essential soil nutrients are plentiful, light is the prime factor around which competitive forces develop. Furthermore, selected crops used in this study can be manipulated by cultural practices so that competition for light becomes a limiting factor in bindweed development and an important practical control measure. The choice of crops and the cultural practices used in their production are most important factors in predetermining the outcome of this controlled competition. Only when the bindweed is shaded sufficiently by the canopy of the crop are these forces operative. Fall-sown rye or wheat, when preceded by a period of intensive cultivation, intercepts such a high percentage of available sunlight that the subsequent growth and development of the bindweed are hindered. Alfalfa has the inherent ability to compete more successfully with bindweed for soil moisture and soil nutrients than any other crop included in this study. It is also an excellent competitor for sunlight. Maintenance of stands is perhaps the limiting factor in the use of alfalfa in bindweed control as the balance of competitive forces eventually eliminates weaker members of the crop population as well as the weed. The use of summer-planted competitive crops allows the farmer to take advantage of a time when bindweed can be cultivated most effectively i.e., during the period of greatest growth vigor. Millet, sorghum soybeans, and Sudan grass demonstrated their ability to germinate and grow rapidly when planted in midsummer and to maintain a deep, dense shade canopy above their attenuated bindweed companions. Hemp and sunflowers lack the uniformity and rapidity of growth to meet these requirements as is shown by the extent of bindweed survival. Soybeans and alfalfa show a lower daily fluctuation in shade value than do any of the crops of the grass family that were studied. Plants whose leaves roll or fold in response to transpiration deficits show greater daily variations in shade value than do those lacking this response phenomenon.

The effects of various placements on the relative response of crops to the phosphate in nonammoniated and ammoniated superphosphate were studied in greenhouse pot cultures. In experiments at five locations, the state experiment stations of Alabama, Arkansas, Mississippi, and South Carolina, and the Plant Industry Station at Beltsville, Md., the effects of placement on response of millet, Sudan grass, and sorghum to nonammoniated superphosphate and superphosphate ammoniated to 2, 3, 4, or 5% under various conditions, were studied on six acid soils and one alkaline soil. The placements were (1) complete mixed fertilizer containing phosphate, nitrogen, and potash, mixed with all the soil, (2) the fertilizer mixed with 5% of the soil in a layer halfway down the pot; and (3) the fertilizer applied in a 3-inch circular layer 1 1/2 inches below the surface without mixing with the soil. In another experiment at Beltsville, millet was grown on four of the same soils used in the other experiments and on another acid soil. All the soils were fertilized with nonammoniated superphosphate or with ammoniated superphosphate (4%) prepared under conditions least conducive to phosphate reversion. Placements were the same as before except that the localized placement was a continuous band 0.5 inch wide 2 inches below the seed. Nitrogen and potash were applied separately in solution form. On the acid soils mixing with all the soil tended to be the least effective of the three methods of placement for both types of phosphate. The localized placements (unmixed layer and band) gave results with the nonammoniated superphosphate about equal to the method of mixing with 5% of the soil (mixed layer), with Sudan grass on Newtonia and Grady soils, and with millet on the Atwood soil. Results in these placements with the nonammoniated phosphate were slightly to markedly superior to the mixed layer placement with Sudan grass on Cecil soil and with millet on Newtonia, Grady, Cecil, and Sassafras soils, especially at the lower rates of application. The localized placements were inferior to the mixed layer placement on the Hartsells and Sunnyside soils with millet. The ammoniated superphosphates tended to give better results on the acid soils in the mixed layer placement than in the localized placements. The rate of application modified this effect somewhat in the millet tests on the Newtonia, Grady, Cecil, and Sassafras soils. The effect was more pronounced at the higher rates of ammoniation. It persisted even when the ammoniation conditions were least conducive to phosphate reversion. Observed placement effects arose mainly from the position of the phosphate since results obtained when the potash and nitrogen were applied separately were similar in trend to those obtained when all three nutrients were applied together. Results on the single alkaline soil tested were similar but less definite. The results reported in this paper point to the need for consideration of the placement factor in field comparisons of nonammoniated and ammoniated superphosphates, especially when more than 3% of ammonia have been added to the latter. A placement optimum for the nonammoniated superphosphate, or fertilizer containing it, may not be the best placement on a particular soil-crop combination for the ammoniated material.