ASSIMILATION OF AMMONIUM AND NITRATE NITROGEN FROM SOLUTION CULTURES BY ROOTS OF PANDANUS VEITCHI HORT., AND DISTRIBUTION OF THE VARIOUS NITROGEN FRACTIONS AND SUGARS IN THE STELE AND CORTEX

Abstract

Analysis of the exudates and tissues of the roots has furnished the following results: Ammonium is assimilated instantaneously and rapidly as it enters the root, where it is converted either into amide or amino N or more gradually into protein. Nitrate is not assimilated as rapidly as ammonium and the amts. of the various fractions of soluble organic N are not as high, particularly in the terminal root tissues, as those found usually in roots grown in ammonium cultures. The amts. of protein or insoluble N are greater in the roots of the plants grown in nitrate than in those receiving ammonium nutrition. The authors'' tentative explanation of this condition is that roots grown in ammonium cultures, through the rapid formation of amino acids utilize and exhaust very rapidly the supply of carbo-hydrates and possibly of other substances essential for protein synthesis. With roots of plants grown in nitrate cultures, nitrate assimilation and synthesis of amino acids being slow, carbohydrates and other substances essential for protein synthesis are not depleted but are always present in sufficient amounts to promote protein synthesis, the ultimate stage of N metabolism. Organic N, newly synthesized in the roots, either from ammonium or nitrate, is transported through the tissues of the stele to the proximal regions of the root, following the path of water and mineral salt conduction in the roots. The channels which the hydrolytic products of stored N follow are the same, if the authors'' interpretation of the results obtained in minus-N nutrient culture is correct. Certain of these studies made on root exudates indicate that, except for a great difference in the amounts of soluble organic N between plus-N and minus-N cultures, no other lines of demarcation can be drawn with the present analytical technique between newly synthesized and stored fractions of soluble organic N. No appreciable quantity of proteins was found in the exudate of roots; assimilated N is apparently translocated as soluble organic and not as insoluble organic or protein N. The distr. of reducing sugars and sucrose in the tissues of the stele and cortex shows that sucrose is present in great amts. in the stele but is lacking almost completely in the cortex. Reducing sugars, possibly, enter the tissues of the stele through the terminal tissues of the cortex of the main root. The amts. of red. sugars in the stele are greatest at the point of entrance or terminal region, but decrease in the upper regions of the stele. At different levels along the root there is in the stele an indirect relationship between the amts. of red. sugars and sucrose. In the terminal portion of the stele, which may serve as paths of entrance, red. sugars are higher than sucrose, but in the proximal region of the main root, sucrose increases rapidly in the stele, whereas red. sugars decrease in about the same ratio, apparently indicating that there is a gradual, rapid conversion of red. sugars to sucrose. Red. sugars are presumably the sugars of downward transport, sucrose being stored in the stele. If sucrose ever enters the transport stream in the roots of Pandanus veitchii it may follow a course similar to that in the sugar beet, i.e., an upward direction. The tissues of the stele of the roots of P. v., on account of their ability to synthesize and store sucrose, are comparable to the root tissues of the sugar beet, discussed by Colin.