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The effects of three planting patterns (ridge, furrow and flat) and three population densities (31800, 47600 and 71400 plants/ha) on the growth and yield of three sunflower hybrids, Flamme, Upsolveraflor and Isostar, were investigated at the Sultan Qaboos University Experiment Station, 1990-93. Seed and oil yields were higher in the ridge-planted sunflower than those planted in furrow or flat patterns, the order being ridge > flat > furrow. However, the magnitude of yield differences between ridge-planted and flat-planted sunflower was not sufficiently high to compensate for the additional labour and materials required for making ridges. Based on this, planting in the flat was recommended. Although increased population density resulted in taller plants, lodging was generally low and did not constitute a serious problem. Biomass/plant, head weight, receptacle weight, head diameter, number of seeds/head and 1000-seed weight decreased with increase in plant population density. Flamme had its highest seed and oil yield at 71400 plants/ha, while Isostar attained its highest yield at 47600 plants/ha. Isostar outyielded the other two hybrids and appeared to have a potential for commercial production in a desert climate.

The red light (R) to far‐red light (FR) ratio reaching the stem of crop plants is reduced by increasing canopy densities. We sought to investigate whether this light signal that increases resource allocation to the stem affects yield in sunflower (Helianthus annuus L.) plants. The stem of sunflower plants grown outdoors in pots at very low densities was exposed to low R/FR ratios by placing selective plastic filters around this organ. Control plants had a clear filter around the stem and received the R/FR ratio of unfiltered sunlight. In experiments with the open pollinated cultivar Guayacán II, the hybrid ‘Dekalb G100’, and the line MA 566, low R/FR ratios promoted stem extension growth and dry matter accumulation without affecting the ratio between stem dry matter and length. In comparison with the sunlight R/FR ratio controls, low R/FR ratios reduced grain yield and grain number per plant, without affecting grain dry weight. No significant interaction of the R/FR ratio with cultivar or sowing date was observed. The negative correlation between stem growth and yield did not extend to the genetic variability observed for plant stature and grain yield. We propose that the promotion of stem growth by the low R/FR ratios typical of dense stands could reduce the resources available for grain yield in sunflower crops.

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The effect of salinity on biomass production, nodule formation and root acetylene reduction activity (ARA) in alfalfa (Medicago sativa L.) was investigated under field conditions. Seeds of alfalfa cv. Batini inoculated with one of four different Rhizobium strains were planted in washed loamy sandfilled transparent PVC cylinders arranged concentrically in plastic pots embedded in soil with the rims just above the soil surface. Salinization of plants commenced one week after emergence up to 16 weeks. Nitrogen (N)-free nutrient solutions of different concentrations (full strength and 0.05 strength) were also applied during this period. Tap water (0.8 dS nr 1 ) served as control for the nutrient solution treatments as well as the salinity treatments which included 6.4 dS m 1 and 12.2 dSnr 1 prepared with NaCl. The Rhizobium 'Corresponding author. 489 strains were TAL 380, TAL 1372, TAL 1373, and Buraimi. Increased salinity was associated with reduced shoot dry weights, root nodulation and ARA. Of the four Rhizobium strains, the order of efficiency in nodulation was Buraimi > TAL 1372 > TAL 1373 > TAL 380. This order was maintained in biomass the production. The detrimental effects of salinity appeared to be ameliorated by application of nutrient solutions, with the full strength being more effective than the 0.05 strength.