Quantitative investigation on nitrogen metabolism in mulberry leaves
1990
Yamashita, T. (National Inst. of Sericultural and Entomological Science, Tsukuba, Ibaraki (Japan)) | Ohsawa, R.
Rubisco, the enzyme responsible for photosynthetic CO2 fixation, is the most abundant nitrogenous compound in the mulberry leaves and is the dominant nutrient for the rearing of silkworms. Hence, Rubisco in mulberry leaves has to be kept at a level sufficient not only to sustain tree growth but also for satisfying the nutritional requirement of silkworms. The formation and loss of Rubisco were monitored during two distinct phases of mulberry growth, i.e. from flushing to the full development of leaves in spring and from the onset of the autumnal senescence to the falling of the leaves. The level of Rubisco protein increased along with leaf development in spring, and the increase was more rapid in the leaves which opened later. In the leaves that opened in early June, the level of Rubisco protein was maximum within one month after the leaves opened. At that time, the ratio of Rubisco-N to total-N in these leaves reached a value of 0.43. In the leaves which had completed their development, the gradual increase in the levels of Rubisco protein lasted for a while, and then the rubisco protein was mobilized during leaf senescence as a result of the sequential emergence and development of new leaves. After termination of the new growth in early October, the Rubisco protein content remained steady for a certain period of time. Then, in early November, it began to decrease, but its decline was so gradual even for the older leaves in lower shoot positions that the fallen leaves still retained more than half of the rubisco protein level observed at the onset of autumnal leaf senescence. Thus, the characteristic redistribution of Rubisco nitrogen within a plant was observed during two distinct phases in the life cycle of mulberry leaves
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