The effect of plant water stress on net photosynthesis, leaf growth, yield and yield‐related components were investigated in a single experiment in order to determine in which way water deficits affect sunflower yields. Sunflower plants, grown under controlled temperature regimes, were stressed during budding, anthesis and seed filling by withholding water until the leaf water potential reached ‐1600 and ‐2000 kPa. Leaf area of unstressed plants significantly exceeded that of plants under severe stress during all growth stages investigated. The CO2 uptake rate per unit leaf area as well as the total uptake rate per plant, significantly diminished with stress, while this effect drastically increased during the reproductive phase of the plant. Although this resulted in significantly smaller heads and kernels, it did not affect the number of seeds borne in the inflorescence. Severe stress during anthesis and seed filling resulted in more empty kernels. Moderate and severe stress during budding significantly lowered both grain and oil yields while plants that experienced moderate stress during anthesis and seed filling significantly outyielded those under severe stress.

I. Abstract Little is known about the effect of fertilization on the N uptake of sunflowers. A 42 factorial trial with 0, 60, 120 and 180 kg N ha−1 and 0, 15, 30 and 45 kg P ha−1 was conducted over three years. The N content and concentration of leaves, stems and capitula were determined at three growth stages. High N levels increased the N content and concentration of all plant parts at all growth stages sharply. High P levels increased the N content of all plant components through better growth. P has an inconsistent effect on N concentration but tended to decrease it. After flowering the crop assimilated 20 to 25 % of the total N. This implies that N applied can still be applied and utilized by the crop at a late stage. This should be substantiated by further research.

The relative sensitivity of the cotton plant to water stress throughout the growing season was determined to identify when irrigation will have the greatest beneficial effect. Daily plant water stress for 72 different data sets of water applications was correlated to corresponding yield criteria. The magnitude of significant correlation was interpreted as the degree of sensitivity. Plant water stress during square formation and early flowering resulted in fewer bolls to reach maturity, but this detrimental effect was cancelled by the development of bigger bolls due to greater lint growth. This resulted in better seedcotton and lint yields. Flower and boll senescence which resulted from water stress during flowering peak, however, inhibited seedcotton and lint yields. The most pronounced inhibiting effect stress had on yields, was during boll development well after the end of effective flowering, when it inhibited boll development. Stress during ripening of the bolls increased lint and boll development and consequently enhanced yields. When limited quantities of water is available, preference should be given to irrigation during boll development, then by irrigation when the first flowers appear, followed by irrigation during peak flowering. Water should be withheld from opening of the first bolls.

Standard triaxial compression tests were carried out on an agricultural sandy loam soil to assess the effect of soil dry bulk density and moisture content on soil mechanical properties, namely, cohesion, internal friction angle, Young's modulus of elasticity and Poisson's ratio. Regression analyses were performed to establish mathematical relationships for simulating the variation in these mechanical properties as a function of dry bulk density and moisture content. These relationships were developed for six typical field moisture contents ranging from 3 to 22% d.b., and various dry bulk density levels ranging from 1Á15 to 1Á82 Mg m À3. Results showed that all mechanical properties increased with dry bulk density, except the internal friction angle that was an independent property. Cohesion, Young's modulus of elasticity and Poisson's ratio decreased with moisture content, except that the Poisson's ratio increased after the moisture content exceeded 17% d.b. However, the internal friction angle was also independent of moisture content. The regression equations developed to relate the soil cohesion, Young's modulus of elasticity and Poisson's ratio with moisture content and dry bulk density were quite simple, and had high determination coefficients ranging from 0Á92 to 0Á96. The regression equations indicated that the three mechanical properties varied linearly with moisture content and non-linearly with dry bulk density. Quadratic relationships were detected for the variations in properties studied and dry bulk density. These equations may be used for prediction of the variation in the mechanical properties (except internal friction angle) of sandy loam soils for any combination of moisture content and dry bulk density. Establishing these equations can be considered an advantage from an economical point of view because of avoiding the need of carrying out the costly triaxial compression test. #

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