A fundamental problem in experimental work with living tissues is the selection of samples in such a way that differences in composition which result from the treatment to which the material is exposed shall represent the effects of the treatment alone. There should be no opportunity for other processes to intervene and obscure the results. Complete isolation of the system under study can be effected, in the case of leaf tissue, only by excision from the plant. This has the disadvantage that the leaves are then merely "surviving organs" and soon begin to undergo changes that are ultimately lethal. Nevertheless, for a reasonable length of time, such leaves can be assumed to behave in a manner that reflects the normal course of metabolic events.The technique of excised leaf culture as applied in this laboratory (4, 5) consists in the selection at random of a group of approximately equal samples from a quantity of leaves picked from the plants at the same time, and therefore, presumably, in the same biological and chemical condition. One or more samples are at once prepared for analysis, in order to provide a point of departure for the series, and the others are subjected to treatment. Samples are subsequently withdrawn from time to time for analysis, the data obtained being computed in terms of equal quantities of fresh tissue as weighed at the start of the experiment. Biological variation is minimized by the use of moderately large numbers of leaves per sample. Although the results are expressed in concentration units, they refer to originally equal and similar lots of tissue. Accordingly, the differences from sample to sample furnish, within the limitations of biological and statistical variation, a measure of the actual magnitudes of the changes that have occurred. Errors arising from translocation are eliminated and, presumably, the only phenomena that affect the data are those associated with photosynthesis, respiration, absorption of culture solution, transpiration, and chemical transformations occurring within the cells.The diurnal variation of acidity characteristic of crassulacean plants is a phenomenon admirably suited for study by this general method. The entire cycle of changes is normally completed within 24 hours, a period sufficiently short to justify the assumption that irreversible catabolic changes which may be initiated are of minor significance in comparison with the 477 www.plantphysiol.org on August 5, 2020 -Published by Downloaded from
In general, the experimental procedure used to determiine the availability of nitrate or ammonium ions to plants has involved culture solutions that provide but one of the two ions. In experiments of this sort in recent years, care has been taken to maintain the pH of the solution within a range found appropriate for absorption by plants. The work in this field by Dr. J. W. SHIVE and his associates at the New Jersey Agricultural Experiment Station has been outstanding; TIEDJENS and ROBBINS (11), for example, have shown that tomato and soybean plants grown at pH 7.9 in ammonium salt culture solution, or at pH 4.0 in nitrate solution, were almost equally well developed.The effect upon the detailed chemical composition of tomato plants grown under the two conditions has been studied in this laboratory by CLARK (2), who observed that the organic acid content of leaves and stalks was greatly depressed in plants grown in ammonium salt solution as compared with those grown in nitrate solution. Protein and several soluble nitrogenlous components were, on the other hand, increased in the plants grown on amiimonium salts, the production of glutamine in particular being enormously stimulated. Detailed comparison of the effects of the two different nutrient conditions was made difficult, however, by the fact that the plants grown with ammonium salts were much smaller than those grown with nitrate, although both were setting fruit at the time of harvest.In order to follow more closely the effect upon tissue composition of nitrate versus ammonium salt nutrition, it is desirable to grow the plants in a series of culture solutions of similar over-all composition, but with the relative proportion of nitrate to ammonium ions varied in regular steps from all-nitrate nitrogen to all-ammonium nitrogen. Such an experiment 205www.plantphysiol.org on May 7, 2018 -Published by Downloaded from
When Bryophyllum calycimum leaves are detached from the plant and cultured in water in darkness at ordinary room temperature, the course of the changes in composition with respect to the organic acids, the protein and the starch depends upon the time of day at which the samples were collected (5). If leaves are taken in the afternoon at a time of low acidity and low protein content, there is a prompt increase both in acids and in protein and a corresponding decrease in the high starch content. Alternatively, if the leaves are collected in the morning at a time of high acidity and protein content, both acids and protein fall and the starch increases. These changes take place within the first 24 hours and appear to be expressions of the normal metabolism of this species. After 24 hours, certain other changes begin to make their appearance; in particular, for leaves collected in the afternoon, the acids and the protein, after reaching maxima, begin to fall and the starch may increase during the second day of the culture period. It seems possible that this reversal in the direction of the chemical changes represents the initiation of the catabolic changes that inevitably follow excision of a leaf. Accordingly, in order to study this possibility further, an experiment was carried out in which the culture period at 200 C. was prolonged to 6 days and, in addition, parallel cultures were carried out at 90 C. and at 10 C. in the hope that, at the lower temperatures, the reactions might proceed more slowly and certain of the details might be revealed more clearly. The present paper gives a report of this experiment.
it cannot be claimed, however, that a great deal of light has been shed upon the chemical mechanisms involved, despite the manifest and widely recognized importance of the problem. Nevertheless, the development in recent years of reasonably satisfactory and accurate methods for the analysis of plant tissues and, more especially, the ildentification of MAYMiK's alleged isomer of matic acid (the so-called "crassulacean malic acid") as isocitric acid (5, 10, 11) have led to the hope that renewed study might reveal helpful information. In this series of papers, accordingly, the results of analytical investigations, chiefly of the leaves and stems of Bryophyllum calycinum, www.plantphysiol.org on May 11, 2018 -Published by Downloaded from
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.