No abstract

No abstract

SUMMARYOne pot and five field experiments were made to study different aspects of the competition between R. exaltata and maize.The growth of young maize plants was not inhibited by being grown together in pots with young R. exaltata plants. In the field the soil tended to be somewhat wetter when the two species were grown together than when maize was grown alone, and was wettest with R. exaltata grown alone. Maize grain and total yield decreased and shoot yield of R. exaltata increased with R. exaltata plant density on both irrigated and unirrigated blocks of land, but yields were not much affected on either block by increase in plant density of maize or in nitrogen supply; maize yield was increased by irrigation but that of R. exaltata was not. Maize plant arrangement did not greatly affect maize grain and total yield or R. exaltata shoot yield, nor did arrangement of R. exaltata plants have much influence on their depression of maize yield, but R. exaltata caused a greater decrease in the grain yield of a short than of a tall maize cultivar.R. exaltata plants germinating at the same time as the crop plants did not have much effect on maize grain yield if they were removed by 8 weeks after the seedlings emerged, but decreased it considerably if allowed to remain for 12 weeks or more; weeds sown 2 or more weeks after the maize emerged hardly grew and had little effect on maize yield. When maize and R. exaltata were grown together leaf area of the maize was little affected up to the time of flowering, but was decreased after flowering, while leaf area of the weed was greatly depressed. Up to 7–8 weeks after seedling emergence more of the ground area was covered by foliage when maize was grown with R. exaltata than when it was grown alone, but later the ground was completely covered by foliage in both cases. Dry weight of grain and shoot of maize increased and that of shoot of R. exaltata decreased when the weed plants were shortened with growth regulators.

In maize (Zea mays L.) grown under normal conditions in Rhodesia, prevention of pollination or removal of the ears after flowering caused premature senescence of the leaves above the ear, preceded by the appearance of a purplish red color. In plants from which the ears had been removed the concentration of sugars and starch increased markedly in both upper and lower leaves, the increase being greater in the upper leaves.The removal of flowers or of developing fruits has been observed to delay senescence in a number of annual plants (5). In contrast, we have noticed in Rhodesia that the removal of developing ears causes leaves of maize plants to wither prematurely. Two experiments were conducted to investigate this problem. METHODSIn both experiments the locally bred single-cross hybrid 59H17, known commercially as SR52, was grown under normal conditions on fertile soil at a density of 31,000 plants per hectare (12, 500 plants per acre).In the first experiment, we investigated the effect of ear removal or prevention of pollination on leaf senescence. There were four treatments, viz., a control, ear removal 13 or 27 days after silk emergence, and covering the silks during the period of pollen shedding. Plots consisted of 10 plants in four replicates.At the time of flowering the area of every leaf was measured on 20 plants. Subsequently, the green, i.e., unwithered, leaf areas were estimated at weekly intervals until 8 weeks after flowering, and the green leaf area above and below the ear was calculated (1).In the second experiment we studied the effect of ear removal on carbohydrate accumulation. This experiment consisted of two treatments in three replicates, namely, a control and ear removal 19 days after flowering.Samples of leaf laminae were harvested from both above and below the ear on three occasions, namely, 7, 17, and 28 days after ear removal. Each sample consisted of 10 laminae, obtained by collecting either the middle two of the laminae above the ear, or the middle two of the laminae below the ear from each of five plants. Parts of some laminae had withered by the time of the third harvest, and these parts were discarded.The samples were always collected shortly before noon and within half an hour were placed in a forced draught oven at a temperature of 110 to 120 C for 20 min and then dried for about 6 hr at 60 to 70 C.The dried samples were finely ground and analyzed for total nonstructural carbohydrates with Clarase 300 (9). Since fructosans do not occur in maize (2), the results represent total sugars plus starch, expressed as glucose. In addition, reducing and nonreducing sugars were determined after extraction with alcohol (8); the replicate samples for the third harvest were analyzed separately, but for the first two harvests composite samples from the three replicates were used for the sugar determinations. Starch was calculated by difference: (total nonstructural carbohydrates minus total sugars) x 0.9. All analytical results are expressed as percentages of the dry matter. RESULTSExper...

1998). Comparison of conventional and organic apple production systems during three years of conversion to organic management in coastal California.Abstract. Conventional and organic semidwarf Granny Smith apple production systems were compared during three years of conversion to certified organic management. Because of differences in fruit load with hand thinning compared with chemical thinning, apple tonnage was higher in the organic production system (OPS) in 1989 and 1991. The organic system was higher than the conventional system in number and weight of fruit per tree, but smaller in average fruit size. Using grower-receivedfarmgate premiums of 38% (1990) and 33% (1991) for unsorted, certified organic apples, comparative cost accounting showed greater net return per hectare for the OPS. The OPS required higher material and labor inputs in all years.Greater terminal growth in the conventional production system (CPS) in 1991 was the only significant difference in growth indicators between systems. N was generally higher in leaf and new wood bark tissues in the CPS. P was generally higher in the leaf and new wood bark tissues in the OPS. No decline in yield was associated with increased weed biomass in the OPS. There was no difference in fruit damage caused by codling moth between production system treatments (codling moth granulosis virus and pheromone-based mating disruption vs. synthetic insecticide). In 1991, secondary lepidopterous pests (apple leafroller and orange tortrix) caused greater fruit scarring in the CPS. In all years, tentiform leafminers caused greater leaf damage in the CPS. Apple leafhopper density and leaf damage were greater in the OPS in 1990 and Soil nutrient levels showed few significant changes during conversion to organic management. Soil bulk density and water holding capacity were useful indicators of changes in soil physical characteristics. Potentially mineralizable nitrogen andmicrobial biomass-C were more sensitive indicators of system change than total N or organic C. Two soil biological ratios, the respiratory ratio and biomass-C/total organic-C, were similar in the two production systems. Earthworm biomass and abundance increased in the OPS in the third year. The introduction of Lumbricus terrestris into the OPS greatly increased litter incorporation rates.