The parasitic weed striga had significantly greater effects on sorghum yield components than non-parasitic weeds. Grain reduction caused by striga amounted to 65% compared with 32% reduction caused by non-parasitic weeds. Comparable reductions were also manifest in straw yield, grains/head and plant height of sorghum. Early removal of striga (a week after its emergence), and thereafter at regular weekly intervals, may help to improve crop yields.In Sudan, Sorghum bicolor cultivars commonly grow in an agricultural background of mixed non-parasitic weeds. However, the incidence of Striga hermonthica Benth., a parasitic weed of sorghum, has recently become of frequent occurrence under arable conditions. Striga seeds normally germinate when stimulated by root exudates from sorghum or from other (non-host) plants (Andrews, 1947). As a result of a striga attack, yields of grain sorghum can be reduced to 30% or even to zero, especially during seasons of low rainfall (Andrews, 1945;Basinki, 1955;Last 1960a;Kambal, 1979). Thus it is of considerable importance to know the time at which parasitic weeds begin to affect the growth and yields of sorghum. An experiment was therefore conducted to find out, first, how late in the growth of a sorghum/striga mixture the control of striga could be delayed before the parasitic weed produces an irreversible depression of host yield? Second, what degree of crop recovery is possible if the striga population is removed after it has started to reduce the growth of sorghum? And, third, how sorghum yield reduction, caused by non-parasitic weeds, relates to that caused by the parasitic weed striga? MATERIALS AND METHODSSeeds of two sorghum cultivars, Feterita and Safra, chosen because of their susceptibility to striga attack and collected from Gedaref area, were sown in a split-plot with completely randomized block design. The cultivar treatments were allocated at random to the main plots and the weed treatments to subplots, whose dimensions were 4.8 x 5 m. Six sub-plot treatments, replicated four times, included (A) weedy check -affected by both parasitic and nonparasitic weeds, (B) cultivated check -free of both parasitic and non-parasitic weeds, (C) non-parasitic weeds only, (D) striga only, (E) striga removed 4 weeks

S U M M A R Y Nitrogen-atrazine combinations were significantly more effective in stimulating higher yield components of sorghum and depressing yield components of striga than sole treatments of either nitrogen or atrazine. Over all treatments nitrophoska at 129 kg N/ha in combination with atrazine at 1.75 kg/ha gave 224, 180, 155 and 151% increases in yield components over the control for head, straw, 1000 grain weights and plant height of sorghum, and a decrease in 83, 88 and 90% of striga density at 8, 12, and 16 weeks from sowing the sorghum. The synergistic effect of nitrogen-atrazine is demonstrated.

SUMMARYChanging the geometry of planting under irrigated conditions from the traditional pattern of regular rows (with plants equidistant from each other) into double rows (with plants arranged in row pairs) promoted plant competition. This resulted in a significant reduction in overall yield components of sorghum but a marked increase in shoot yields of striga. A better yield of sorghum was obtained in ridged plots than in flat plots, associated with less striga infestation and a slightly higher soil moisture content under the ridged plots.

Microscopic examinations and chemical studies were performed to study the response of two legume crops, hyacinth bean (Lablab purpureus (L.) Sweet and kidney bean (phaseolus vulgaris L.) to the filed dodder (FD) (Cuscuta campestris Yuncker) parasitism. Hyacinth bean, showing no effective resistance mechanisms, was found to be a highly susceptible host to FD. However , kidney bean, displaying resistant reactions towards the parasitism of FD, was found to be an incompatible host. The possible reasons for the resistance of kidney bean to FD were anatomical (hypersensitivity) and chemical (high contents of phenolic acids and lignin) stimulated defence mechanisms , which developed during the actual intrusion of FD haustorial cells inside its tissues.

Boric, fumaric, gibberellic (GA), indoleacetic (IAA), and succinic acids at concentrations of 0.05, 0.5, 5, 50, or 100 ppm in a basic sucrose and agar medium stimulated pollen germination and tube growth of date palm (Phoenix dactylifera L.). Both germination and tube growth increased with increasing concentration of boric acid, GA, and IAA without injurious effects. Maximum pollen germination was observed at 0.05 ppm succinic acid and 0.5 fumaric acid.