Environmental preconditioning has an important effect on the response of plants to ozone and other oxidants present in photochemical smog (2,4,6,7,9). Ozone injury to plants decreased after a 48-hour prefumigation dark period; no injury occurred when the dark period was extended to 72 hours (4,9). Plants grown at 21.5 Klux were less sensitive to ozone than plants grown at 8.5 Klux. In contrast, plant damage from the photochemically produced phytotoxicant, peroxyacetyl nitrate (PAN), responded quite differently to photoperiod and light intensity (4,9). Temperature affected the response of plants to smog and synthetic oxidants. Plants were less smog-sensitive when grown at cool than at warm temperatures (2, 6, 7).The tobacco leaf spot weather fleck has been shown to be caused by photochemically produced ozone since injury often follows elevated ozone levels, and symptoms of natural and experimental injury are virtually indistinguishable (5,8 Chamber temperature was maintained at 26 to 280 and relative humidity ranged between 80 and 90 %. Ozone was generated electrolytically by passing compressed tank oxygen through dual, corona-type, silent discharge ozonizers.For each fumigation, the various preconditioning treatments were imposed on 4 plants of each variety. The plants were selected for uniform size and vigor from the bulk greenhouse supply. Two methods were used to investigate the preconditioning effects of extended light and extended darkness. Plants were subjected to a combined natural-artificial illumination photoperiod during the 27-hour period before fumigation. Plants exposed to 14-, 42-, and 66-hour dark periods were used to study the effects of extended prefumigation darkness on ozone susceptibility.The lighting cycle for extended light preconditioning was 22 hours of light, 2 hours of dark, and 3 hours of light; for the dark control the cycle was 10 hours of light, 14 hours of dark, and 3 hours of light. All plants were placed in the fumigation chamber for the final hour before fumigation. The 27-hour cycle began at 6 AM and continued under natural illumination until 4 PM. The light-preconditioned plants then were
In comparing the quality of tobacco grown on different plots it is very difficult to keep in mind the percentage of eight commercial grades of 10 17") 30 The grade index of any plot is obtained by multiplying the percentage of each grade by the price in the above schedule, adding the products and dividing by 100.
Numerous analyses of cured tobacco from all tobacco growing sections of the world have been published. The potash percentages from a number of these are assembled in Table 2, which shows that the percentage of potash in the leaf varies widely, from 2.2 per cent to 8.5 per cent of the dry weight of the leaf. Samples from the same locality may differ considerably in this respect.The percentage of potash in the tobacco leaf is not constant, but depends on (1) the amount of potash furnished in the fertilizer or naturally in the soil, (2) the relation of other elements, particularly the bases, and (3) the season. Whether there is a difference in the capacity of different tobacco varieties to absorb potash has not been demonstrated.
Irrigation experiments on Maryland tobacco have been conducted since 1955 on Monmouth fine sandy loam, a typical tobacco soil of Southern Maryland. An overhead sprinkler system was employed. Moisture levels in the soil were studied, using Bouyoucous gypsum blocks and tensiometers. The following conclusions were made from this study:1. In a year of severe drought, 1966, ample irrigation increased yields, unit value and total value. Irrigated yields were 175 per cent of non-irrigated; unit value was 141 per cent, and total value of irrigated tobacco was 247 per cent of non-irrigated.2. Nitrogen fertilizer rates of 100.9, 134.5 and 168.1 kg ha were used. Under irrigation, gains in yield and total value, and some loss in unit value were obtained from the first added increment of fertilizer nitrogen above the base amount. Addition of the second increment produced smaller gains in yield, a greater loss in unit value, and losses in total value.3. In the absence of irrigation, the supply of soil water could limit nitrogen assimilation by the plants. The higher rates of nitrogen fertilization were effective in increasing yield and total value over the lower increments. Even unit value was not adversely affected.4. The Wilson broadleaf variety of Maryland tobacco consistently outyielded the Catterton medium broadleaf variety, but due to lower unit value, probably caused by darker colours, it had a lower total value.5. Plant populations of about 18000 per hectare produced increased yields and gross return over a population of about 12000. Populations of 24000 plants per hectare were excessive in most cases, and produced losses in yield and unit value.6. Comparative response to irrigation depends on severity of drought, but late depletion of soil moisture levels may prevent the full potential development of the crop, and result in lower yields and total value.
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