No abstract

Colonization of sorghum by Macrophomina phaseolina in field plots was determined at nitrogen fertilization rates of 0, 56, 112, and 168 kg ha PI in 1988 and 0, 84, 168, and 256 kg ha-' in 1989. Above ground plant tissue and roots were sampled monthly to determine total nitrogen and percent colonization of root segments by natural inoculum. Root infection was not affected by nitrogen treatment, but was affected by growth stage and environment. High root infection occurred before reproductive development (growth stage 3) in 1988 and was associated with hot, dry weather early in the growing season. In 1989, when the weather was cool and wet, root infection began after reproductive development (growth stage 4). The effect of nitrogen treatments on lesion length was determined in sorghum stalks artificially inoculated with M. phaseolina. Lesion lengths were significantly affected by both nitrogen treatments and growth stage. Lesions were significantly longer with all nitrogen treatments at growth stage 9 than with the no-nitrogen treatment, and lesions tended to increase with increased levels of nitrogen fertilization. Significant increases in lesion length occurred between growth stages 5, 7, and 9 in 1988 and between 7 and 9 in 1989. This study demonstrates that nitrogen fertilization affects colonization of sorghum stalks but not root infection by M. phaseolina.

No abstract

Nitrate accumulation in crop plants is intensified by stress from inadequate moisture, but little Information is available about the effect of N fertilization rate on NO3 accumulation in sorghum (Sorghum bicolor (L.) Moench] during a dry period. During the summer drought of 1988, NO3 concentrations in Funks G‐522 DR grain sorghum receiving 0 to 150 lb N/acre as NH4NO3 were measured over five harvests (cutting height 6‐in. above the soil surface), which simulated salvage of a grain crop for animal feed. The objectives of this study were to determine the effects of N fertilization rate on NO3 and dry matter accumulation as well as total N uptake of grain sorghum. Sorghum was seeded 26 April on a Bowie fine sandy loam soil (fine‐loamy, siliceous, thermic Plinthic Paleudults). When sampled on 23 June, NO3 concentration in sorghum tissue was equal to the 2100 ppm NO3‐N concentration that could be lethal to cattle (Bos taurus, Bos indicus) where 50 lb N/acre had been applied, and increased as N fertilizer rate increased up to the 100 lb N/acre rate (2160, 4280, and 5360 ppm NO3‐N for 50, 100, and 150 lb N/acre, respectively, at growth stage 5, or boot stage). When harvested for animal feed on 26 July (growth stage 7, soft dough), the sorghum was safe for consumption by cattle. Percent total N in sorghum tissue increased as N fertilizer rate increased, and dry matter accumulation averaged approximately 7700 lb/acre at growth stage 7. Based on the observations from this study, sorghum producers who consider salvaging a grain crop at the boot growth stage should test the forage for NO3 content if drought conditions exist.

The gene encoding S-adenosylmethionine hydrolase (SAMase) was transferred to tomato (Lycopersicon esculentum, cv. large red cherry) as a means of reducing ethylene biosynthesis in the ripening fruit. S-adenosylmethionine (SAM), the penultimate precursor to ethylene in plants. is converted to methylthioadenosine and homoserine by SAMase thereby reducing the capacity of the transgenic plant to synthesize ethylene. We have used both constitutive and fruit-specific tomato promoters to regulate SAMase gene expression. Whereas the constitutive CaMV 35S:SAMase chimeric gene expressed active SAMase and conferred a 50-60% reduction in ethylene biosynthesis in a leaf disc assay, there was little effect on fruit ethylene synthesis or postharvest ripening physiology. The use of either the tomato E4 or E8 promoters restricted SAMase expression to ripening fruit which caused a substantial (80-90%) reduction in fruit ethylene synthesis and a profound effect on fruit ripening. SAMase expression levels reached 0.1% of total cellular protein as measured on western blots using anti-SAMase monoclonal antibodies. Field trial fruit picked al the mature green stage accumulated less lycopene and were twice as firm as controls over a six week period. Vine-ripened fruit had near-normal levels of lycopene, were firmer at harvest than controls, and did not lose firmness over a two week period. Taste, vitamin content and tomatine content were superior or equivalent to control tomatoes.