Phytotoxic symptoms

Application of diuron at 3.3kg a.i./ha twice a year resulted in mild foliar symptoms of diuron phytotoxicity on several trees in the third year (Ryan, 1965b). However, when diuron applications were discontinued for 2 years, the phytotoxicity symptoms gradually disappeared. No evidence of simazine phytotoxicity was observed in these experiments. Both diuron and simazine need rainfall or irrigation following application for satisfactory results, but simazine performance appears to be more dependent on moisture. 

The relation between inhibition of the photoinduced responses in isolated chloroplasts and the expression of phytotoxicity remains to be identified positively. Any hypothesis proposed to account for the mode of action must take into consideration that (a) phytotoxic symptoms develop only in the light, and severity is proportional to light intensity (b)... 

Starvation. The early reports that Hill inhibitors limited photosynthesis and that starch disappeared from treated plants, prompted some investigators to refer to these compounds as photosynthesis inhibitors. Photosynthesis is inhibited because ATP and NADFH are not available for carbon dioxide fixation. However, there is little evidence that the plants starve to death. If this were the only process affected, phytotoxic symptoms should resemble those that appear on plants kept in total darkness. Deficiency of photosynthate does limit new growth, but does not account for the morphological alterations that occur within a few hours after treatment. The mechanisms that lead to phytotoxicity appear to be considerably more complex than would result from limiting carbohydrate synthesis by suppression of carbon dioxide fixation (2). ... 

Free Radical Mechanisms. The appearance of phytotoxic symptoms only in the light after treatment of plants with herbicides such as diuron and atrazine prompted some investigators to propose "light-activation1 hypotheses, the formation of toxic substances, or the formation of reactive free radicals. However, except for the strong documentation on the formation of free radicals by bipyridiliums, there is no direct evidence that toxic components are formed from an interaction between a herbicidal Hill inhibitor and light (2). ... 

In plants treated with borates phytotoxic symptoms develop slowly, the main symptoms being withering and scorching. The biochemical mode of action of borates is unknown. According to the investigations of Brebion et al. (1954), sodium tetraborate considerably inhibits chlorophyll synthesis in young wheat plants, indicating one possible cause of its herbicidal activity. 

Phytotoxic symptoms caused by TCA are wilting of the leaves and inhibition of root and stem growth (Mayer, 1957). The foliage temporarily becomes dark green, then chlorosis is manifested. The weeds treated die in 3-4 weeks. Treatment with... 

After treatment characteristic phytotoxic symptoms are developed by plants sensitive to urea herbicides. Acute symptoms are manifested by the withering of leaf tips and leaf edges, chlorosis and water-soaked blotches. Subsequently, chronic symptoms, such as increased chlorosis, wilting, plant-growth reduction, defoliation, and stem collapse are seen. The process ends in the complete decay of the plant (Bucha and Todd, 1951 Minshall, 1954, 1957). 

The recommended rate of application of bentazon is 0.5-3.0 kg/ha, depending on the conditions of application. Its selective application in soybean has been reported by Luib and van de Weerd (1972), in cereals by Behrendt and Sipos (1969) and Menck and Behrendt (1972), and in rice by Mine et al. (1973) and Mine and Matsunaka (1974). Under flooded rice field conditions, bentazon exerts both a foliar and a,root action. The effect of bentazon absorbed through the roots is slowly manifested, while when the preparation acts much more quickly in contact with the leaves. The temperature prevailing at the time of application has an important role in the development of phytotoxic symptoms. In hot weather the effect can be observed in 5-7 days, while in cold weather symptoms may appear only after weeks (Mine and Matsunaka, 1975). 

Amongst many other derivatives (by NH-acylation, sulfonylation, alkylation, arylation) a sulfonylaminocarbonyl-triazolinone with the internal code no. BAY DAM 4493 was synthesized in 1985 (Fig. 2.6.3). It showed not only activity against rice blast Pyricularia oryzae) but also phytotoxic symptoms at application rates of 500 g a.i. ha h... 

GS2 deficient barley mutants isolated under conditions that suppress photorespiration grow without phenotypic aberrations under nonphotorespiratory conditions (2% O2, 0.7% GO2), but mutants with less than 40% of the wild-type GS2 activity show severe phytotoxic symptoms, mainly chlorophyll destruction, when... 

Normal levels of cadmium in plants grown in uncontaminated soil are probably less than 1 ppm in dry matter, but John [309] has quoted a level of 668 ppm cadmium in the dry matter of lettuce leaves from plants grown in cadmium-contaminated soil. This level is certainly exceptionally high, but it is clear that in cadmium, we have an element which is both highly toxic and readily taken up by plants to produce quite high concentrations in the plant without the appearance of phytotoxic symptoms, so that an apparently normal crop may be unsafe for human or animal consumption. It is evident from the results of these experiments that even a small addition of cadmium to uncontaminated soil can produce striking increases in the cadmium contents of a number of widely different kinds of crops. Cadmium is therefore an element which can readily enter mammalian food chains at a dangerous level. 

The results of pot experiments carried out by Crooke suggest that phytotoxic symptoms do not occur in oats unless the nickel content exceeds 100 ppm dry... 

Unfortunately, quite high levels of cadmium (up to 60 ppm dry matter) may be present in lettuce leaves before either visual phytotoxic symptoms or any significant reduction in yield are encountered, so that an apparently normal crop may be unsafe for human or animal consumption. 

This paper [353] also appears to be based on the assumption that as long as phytotoxic symptoms and reduction in yield in crops are avoided, it does not really matter how much metal-contaminated sludge is applied to agricultural soils. On the basis of this view and rather dubious assumptions about the relationship between zinc equivalent and phytotoxic potential, carte blanche has been given in England in recent years for the application of sludges to agricultural land, with little reference to their contents of potentially toxic metals other than zinc, copper and nickel. Since the maximum proposed application is 50 tons dry matter/acre/annum and this would be equivalent to... 

To assess trichothecene effects on intact muskmelon seedlings, six week old seedlings at the 3-4 true leaf stage for each of the five genotypes were carefully removed from the soil medium. The roots were carefully washed in tap water to remove soil particles, then immersed in either solvent or 5y g ml of roridin E for 2 hr only root systems were immersed. After exposure, seedlings were washed and incubated with roots immersed in tap water for 24 hr under continuous light, then examined macroscopically for phytotoxic symptoms. Symptom severity was ranked on a scale of 0 to 5 0 = no visible symptoms and 5 = severe discoloration... 

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