Studies on the salt tolerance of vegetable crops in sand culture. IV

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In the present paper, relative salt tolerance and mechanism of salt injury of nineteen species of vegetable crops were discussed with special reference to mineral nutrition, on the basis of the results of the sand culture studies previously reported by the author. The sand cultures were made under glass, using HOAGLAND's solution as the basic solution (control), with NaCl added at various concentrations ranging from 1000 to 16000 ppm. 1. Relative salt tolerance of vegetable crops was evaluated according to the concentration of NaCl in the solution corresponding to a 50 per cent reduction in fresh weight of above ground parts (including fruits for fruit vegetables) or of the edible portion. The results obtained by these two ways did not always coincide with each other. 2. Stimulative effects of 1000 or 2000 ppm treatment on the vegetative growth of such crops, pakchoi, cabbage, radish, chinese cabbage, celery, and tomato might be due to Na in most cases. 3. Percentage of the dry weight of tops, in general, decreased in high salt treatments in fruit vegetables, but did not vary so markedly or slightly increased in high salt treatments in the other vegetable crops, and that of roots of root vegetables tended to increase with increasing salinity. 4. The ability of crops to survive in increasing salinity was almost parallel to the salt tolerance represented by yield decrement. In general, the rate of dead leaves increased markedly as the salt concentration came near to kill plants, and the death of plants generally occurred in consequence of severe dying off from older leaves. 5. Symptoms of salt injury were variable with crop species; symptoms caused by the high osmotic pressure of the nutrient solution, by deficiency of essential elements, or by toxicity of excessive accumulation of Na or Cl occurred singly or accompanying each other, and they affected adversely the quality of certain crops. 6. With increasing salinity, the content of both Na and Cl in leaves increased almost linearly in most crops, but often exponentially in considerably salt tolerant Cruciferous crops. The content of Na in leaves varied widely with species, and tolerant crops tended to accumulate higher concentration of Na in leaves than did sensitive crops, while such relationship could not be found as to Cl in leaves. Direct toxicity of excessive accumulation of Na or Cl in plants could not be considered as the primary cause of salt injury in most crops. Content of Na or Cl in roots was not related to salt tolerance. It was suggested that there might be some important relationships between salt tolerance and the trans port of Na or Cl from roots to tops or Na/Cl ratio in leaves. 7. Content of K, Ca, or Mg in leaves was in antagonistic relation with Na, and as the salinity increased, tended to decrease more markedly in tolerant crops than in sensitive crops in general. Therefore, such interference in absorption of essential cations might not be the primary cause of salt injury in general, but its possibility to accelerate the injury might be of importance occasionally. The degree or the order of the antagonism of each essential cation to Na varied with crop species and with the concentration of NaCl in the solution. 8. Tolerant crops usually showed rather high K content in leaves at control, andshowed significantly higher Na+K content in leaves in the NaCl treat ments than did sensitive crops, while such definite relationships were not found in roots. The K/Na ratio in leaves was not closely related to salt tolerance. The total amount of four cations, Na, K, Ca and Mg, in leaves slighty increased with increasing salinity in most crops, and was not related to salt tolerance. Milliequivalent ratio of Na, K, or Na+K to total cations in leaves was not so closely related to salt tolerance.9.