Effects of moisture stress during different stages of cotton development were examined in two experiments when temperatures were mild and evapotranspiration less than 6 mm./day. Two varieties of upland cotton, Acala 4-42 and Deltapine Smoothleaf, were affected similarly. Moisture stress at the beginning of flowering reduced growth rate and the numbers of flowers and bolls. During the second half of the flowering period it reduced the percentage of boll retention, boll number, boll weight, seed index, lint index and lint length. Stress during boll development had similar effects and caused earlier maturity. Lint yield was reduced significantly by moisture stress during each of the periods but tensile strength of lint was not affected. Three irrigations, if properly timed so that no appreciable moisture stress occurred, were sufficient for high yields and good quality of lint, and the first could be postponed until after flowering began without any loss in yield.The problem of optimal timing of cotton irrigation has been investigated by several workers. Harris and Hawkins (1942) and Hamilton et al. (1956) found in Arizona that early irrigations, which induced good vegetative growth before fruiting, also increased cotton yields. However, Thornton (1961) found that cotton needed a good supply of moisture during the boll setting period, until about three-fourths of the bolls were mature, whereas moderate drought in the early growing period did little harm. In the experiment reported by Amemiya et al. (1963), all irrigations in the higher yielding treatment were given after the first bloom appeared. Bruce and Shipp (1962) found that the best moisture regime for cotton consisted of a good moisture supply up to three weeks after the commencement of flowering. Experiments in Greece (Christidis and Harrison, 1955) also showed that the three weeks after the beginning of flowering were the most critical period with regard to moisture. Marani and Horwitz (1963) found that the optimum time to apply a single irrigation to cotton was at the beginning of flowering, but the results of Bruce and Romkens (1965) indicate the importance of good moisture conditions during the four weeks after the beginning of flowering.In San Joaquin Valley, California, Stockton et al. (1967) reported that delaying the first irrigation until the beginning of flowering retarded growth and maturity, but increased the yield of cotton, and similar results were reported by Levin and Shmueli (1964) in Israel. Adams et al. (1942) found that an irrigation applied seven weeks after the initiation of flowering was too late to have any beneficial effect on yield. This was confirmed by Bielorai and Shimshi (1963), who found that stopping irrigation 6-7 weeks after first bloom did not affect cotton yield

Two blue-mold resistant cultivars of Nicotiuna tabacum L. were crossed with each other and with the susceptible Israeli local cultivar Mikhal. F1, F, and F3 progenies of these crosses, F1 and Fz of backcrosses and the parental varieties were grown in a series of experiments, in which the seedlings were exposed to heavy natural infection with Peronospora tabacina ADAM. The genetic basis of resistance was found to be identical in the two resistant strains Bel-61-10 and Chemical mutant. A single dominant major gene determined the segregation of resistance versus susceptibility in the crosses of these strains with Mikhal. The level of resistance of resistant segregates was shifted by environment and by quantitatively modifying genes. The index of resistance, which was calculated as a weighted mean of the degree of blue-mold expression of the resistant segregates, differed in the generations of cross progeny. This could be explained by the different expected levels of the modifying genes in these generations. Heritability of the index of resistance was calculated by parent-offspring regression and it was found to be 0.542 in the F3 and 0.227 in F, backcross progenies.

Acid delinting improved and accelerated the germination of cotton seeds at low temperatures in tests conducted both in an incubator (at 12 C and 14 C) and in the field. All the cultivars of Gossypium hirsutum L. and G. barbadense L., tested in these experiments, exhibited this effect, even though seeds of the latter species were covered only by a negligible amount of fuzz. This indicates that acid delinting increases the permeability of the seedcoat.Water was absorbed by the fuzz during the first hours after planting, but later acid‐delinted seeds showed a higher rate of water absorption. The inheritance of the ability of cotton seeds to germinate at low temperatures is probably polygenic and primarily additive, with prominent maternal effects which are associated with characteristics of the seedcoat and the fuzz.

SUMMARYThe effects of moisture stress during various stages of the development of two varieties of upland cotton (Acala 4–42 and Deltapine Smoothleaf) were studied in two irrigation experiments in the Bet-She'an valley, where summer temperatures were high and evapotranspiration in the fully irrigated treatments was up to 8–10 mm./day in July and August. Optimum time for first irrigation in the Bet-She'an valley was two or three weeks before flowering and six or seven irrigations, if timed to prevent moisture stress, were sufficient to give high yields and good lint quality. Lint yield ranged from 270 kg./ha. in non-irrigated to 2000 kg./ha. in the fully irrigated treatments in 1966.