The effect of seven factors, namely genotype, plant maturity, nitrogen fertilizer, phosphorus fertilizer, water stress, light intensity and temperature, on the hydrogen cyanide potential (HCN,) of forage sorghum was studied in three pot experiments.Fivefold differences occurred between genotypes in HCN,, with a breeder's line, X45 106, selected for low HCN, having a maximum of 520 mg HCN kg-' DM (dry matter) compared with 2300 and 2450 mg kg-1 DM for cvs Zulu and Silk respectively. In X45 106, HCN, (mg HCN kg-' DM) declined curvilinearly with age d (days from sowing) (HCNp=8460-320d+ 3.1d2) and linearly in Silk (HCN, = 9020 -1 lOd), but the decline in Zulu was not statistically significant. Nitrogen (equivalent to 200 kg ha-1 of N) increased HCN, (P< 0.001), but more so in full light (100 mg kg-] compared with 1430 mg kg-') thanin 50% shade (190 mgkg-1 compared with 690 mgkg-1). In one experiment, acute water stress appeared to reduce HCN,, but this was confounded with the strong decline due to aging. In another study, acute water stress had no effect on HCN,. Neither the application of superphosphate nor change in light intensity, nor change in temperature had a direct significant effect on HCN, in these studies.Breeding and selection for low HCN, appears a promising approach to ensuring that sorghum plants will provide non-toxic forage from an early stage of growth.
The primary effect of a period of drought on potato plants was to reduce the rate of photosynthesis. Decreases in CO2 uptake could mainly be ascribed to an increase in the residual resistance to CO2 transport. Levels of sugars and of starch-synthesizing enzymes in extracts from tubers of droughted and control plants were similar, and it is likely therefore that the effect of drought on tuber growth is exerted mainly via a reduction in the supply of assimilate.
A field experiment showed the semidwarf cultivar Penjamo 62 to outyield the taller Australian cultivar Gamenya at nitrogen levels up to 896 kg N ha-1. This was attributed primarily to a faster rate of grain growth particularly in the later stages. Lodging resistance of Penjamo 62 accounted for only 16-58 per cent of the differences observed at the lower nitrogen levels. Although these two cultivars showed only small differences in both photosynthesis and in the duration of photosynthetic activity of microplots in the greenhouse, large differences in the efficiency of utilization of post-anthesis photosynthesis for grain production were inferred from growth analysis. Moreover, grain growth of Penjamo 62 was far more sensitive to a post-anthesis drought than that of Gamenya, even though similar levels of water deficit and reductions in photosynthesis were recorded in microplots of both cultivars.
A newly released F1 hybrid wheat (Hybrid Titan) was compared with its parents and three commercial check cultivars in yield trials over 16 site years. Mid-parent heterosis varied considerably (100-127%) over the sites and averaged 111%. However, high-parent and high-check cultivar heterosis levels were much lower (99-119%), most likely a consequence of the low yield of the male parent. A concurrent seeding rate trial showed that lower levels of heterosis resulted if the seeding rate of the hybrid was reduced below that of the parent and check cultivars. Quality tests performed on the harvested grain showed some characters deviating considerably from expected values. The most significant of these were lower milling yield, flour protein content, and water absorption. The T. timopheevi nucleo-cytoplasmic system for producing hybrid wheat was implicated as having likely side effects on yield performance and quality. The study emphasized the need for both parents to be high-yielding and to have good combining ability for both yield and quality.
The productivity of seven multiple rice-cropping systems, utilizing a short-duration cultivar under irrigation, were assessed over 5 years. Systems of one, two and three crops a year were compared. Multiple cropping reduced grain yield, panicles m~2, total dry matter and nitrogen uptake per crop. However, the unfertilized three-crop-per-year system averaged 9/1 t grain, 17-1 t DM and 126 kg N ha"" 1 per annum, without showing a long term decline in productivity. Recovery and utilization of nitrogen fertilizer were generally low compared to other experiments, and were highest for the early dry season cropping period.
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