As the fifth report on studies of nitrogen nutrition for muskmelon, this report deals with the results of experiment to clarify the influence of increased magnesium concentration in nutrient solution upon the growth of melon and the absorption and assimilation of two forms of nitrogen, NH4-N and NO3-N, in the tissue of it. The results can be summarized as follows. 1. Magnesium deficiency appeared in the melons of three plots, NMg0, AMg0 and AMg5, and the growth of such as stem length and fresh matter weight of melon of those plots was remarkably lower than that of other plots. Furthermore, magnesium deficiency of melon tended to appear more easily on AMg-series than on NMg-series. 2. From the analytical results of mineral nutrients, the critical concentration of Magnesium deficiency of melon is supposed to be 0.25%. On AMg-series in comparison to NMg-series, the absorption of magnesium was not so much even when the magnesium concentration invreased excessively, for there is an antagonism between NH4+ and Mg2+. 3. From the formal fractionation of magnesium, the content and distribution ratio of both fractions of FNaCl and Face, of AMg-series were comparatively lower than those of NMg-series, and especially, the Face, pectic magnesium was remarkably lowest. These facts indicate that hte content of Face in AMg-series does not increase, so far as the supplying form of nitrogen is not changed. 4. Magnesium deficiency delayed the reduction of NO3-N, the assimilation of NH4-N into amino acids, and the protein synthesis of amino acids in melon. However, glutamine content was still high even when the magnesium concentration of AMg-series increased to 25 ppm or 100 ppm. This fact indicates that melons absorbed NH4+ and accumulated glutamine in order to counteract and store it. 5. In AMg-series, reduced sugar content was high even when magnesium concentration increased. It is Upposed that sugar did no t act smoothly as the energy source as well as the source of carbon skeleton. This was caused by the use of NH4-N as a sole nitrogen source, so that the retardation of melon by NH4+ could not be removed through increasing magnesium concentration.
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