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The Role Op Transpiration in the Absorption And THE ROLE OP TRANSPIRATION IN THE ABSORPTION AND TRANSLOCATION OP MINERAL IONS IN PLANTS, AS MEASURED WITH RADIOACTIVE CALCIUM AND PHOSPHORUS DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By RAFIQ AHMAD, B.Sc. (Alig), M.Sc. (Punjab) The Ohio State University 1959 Approved hy Adviser Department of Botany and Plant Pathology ACKNOWLEDGMENT The ■writer wishes to express his gratitude to the faculty and graduate students of the Department of Botany and Plant Patho­ logy, The Ohio State University, for their great help during his stay in the U. S. A. Special thanks are due to Dr. C. A. Swanson for his guidance and encouragement during this investigation and other graduate studies. The writer is also thankful to Dr. R. S. Platt, Jr. for his help in preparing this manuscript and a critical reading of the draft. Thanks are also extended to Dr. C. G. Weis- haupt for critical reading of the manuscript. The writer also wishes to express his appreciation to The University of Karachi, Pakistan, for the grant of study leave, and to the Institute of International Education for the award of a travel grant to pursue these studies. ii TABLE OP CONTENTS Page INTRODUCTION ......................................... 1 LITERATURE R E V I E W ................................... 4 MATERIALS AND M E T H O D S ............................... 8 EXPERIMENTS AND R E S U L T S ............................. 14 Section 1 ....................................... 14 Experiments to investigate whether a low transpiration rate can he growth-limiting Section 2 ....................... .. 2 6 Experiments to determine the rate at which metabolically-absorbed phosphorus^ and c a l c i u m ^ become available for upward trans­ location into shoot Section 3.... ..................... 41 Experiments to determine phosphorus^/ calcium45 ratio in various parts of plants as a function of time DISCUSSION........................................... 48 SUMMARY . ......................................... 59 LITERATURE CIT E D ..................................... 61 AUTOBIOGRAPHY.................. 65 iii LIST OF TABLES Table Page 1 Heights and dry weights of cotton plants sup­ plied with mineral ions during day only or night o n l y ................................ 15 2 Dry weights of bean plants supplied with mineral ions during day only or night only (March 30 harvest l o t ) .......................... 20 3 Dry weights of bean plants supplied with mineral ions during day only or night only (April 8 harvest l o t ) .......................... 21 4 Heights and dry weights of bean plants supplied with mineral ions during day only or night only. 23 32 5 Leaching of P from the roots of cotton plants (experiment 4 A ) ...................... 30 32 6 Leaching of P from the roots of bean plants (experiment 4B)................ 31 45 7 Leaching of calcium ^ from the roots of cotton plants (experiment 4 C ) ...................... 32 45 8 Leaching of calciumfrom the roots of bean plants (experiment 4 C) ...... 33 9 Translocation of metabolically absorbed phos­ p h o r u s ^ from the roots of cotton plant as a function of time (experiment 5 A) ....... 38 10 Translocation of metabolically absorbed cal- cium45 from the roots of cotton plant as a function of time (experiment 5 B ) ........... 38 11 Translocation of metabolically absorbed phos­ p h o r u s ^ from the roots of bean plant as a function of time (experiment 5 C) ....... 39 12 Translocation of metabolically absorbed cal­ cium^ from the roots of bean plant as a function of time (experiment 5 B ) ........... 39 iv LIST OF TABLES (continued) Tahle Page A C "5 0 1 3 Distribution of calcium and phosphorus in various parts of hean plants .............. 43 1 4 Phosphorus^ and calcium^ content in different parts of hean plant expressed as a per cent of the total phosphorus32 ang calcium45 present in each plant at h a r v e s t .......................... 54 1 5 Ratio of phosphorus"^ and calcium^ in different parts of the p l a n t ...................... 58 v LIST OF FIGURES Figure Page 1 Method of support for "bean seedlings growing in a shallow tank of culture s o l u t i o n .......... 10 2 Comparison of growth of cotton plants supplied with mineral ions during the day only or night only . * .................. 1 6 3 Arrangement of hean plants growing in the greenhouse...................................... 19 4 Bean plants showing no difference in heights if supplied with -g-X normal concentration of Hoag- land solution during the day only or night only. 22 5 Comparison of growth of hean plants supplied with mineral ions during the day only or night o n l y ............................................ 24 6 Apparatus designed to leach out non-metaholically absorbed compounds containing phosphorus^ ana c a l c i u m 4 5 from the "outer space" of the roots . 27 32 7 Leaching of phosphorus from excised roots of cotton and hean plants.................. 34 A5 8 Leaching of calcium from excised roots of cotton and hean plants.......................... 35 9 Translocation of metaholically ahsorhed ions . 40 10 View of hean plants growing in aerated culture solution in controlled environment room .... 42 45 11 Ratio between the activities of calcium and phosphorus^ (Ca/P) in different parts of plants 56 vi INTRODUCTION The precise role of transpiration in the upward transloca­ tion of mineral salts in plants has been the subject of much dis­ cussion. It is known that inorganic ions absorbed by the roots are translocated upward predominantly in the xylem, although the specific channels in the xylem, that is, the cell types through which the bulk of this movement takes place, have never been delineated with certainty. However, it is reasonably probable that most of the ions are carried along by solvent drag forces through the walls and lumina of the tracheids, and, in such species as contain trachea, through these cells as well. It follows from this view that an increase in transpiration rate, to the extent that it accelerates the movement of water up­ ward through these tracheary elements, should similarly increase the rate of transfer of ions from the roots to the shoots. There are, however, many complicating factors which can, and probably often do, obscure this simple relationship. One possible factor is the re-export of ions from the leaves by way of the phloem toward the roots, followed by re-absorption into the xylem. In this case, increased transpiration rates may be ex­ pected to increase the rate of "cycling," but will not result in any appreciable net increase of ions in the shoot. Whether accelerated rates of cycling do result from increased transpiration rates has never been demonstrated experimentally, however. On the other hand, no evidence exists for excluding this possibility. Another possible factor may be the "release rate" from the roots. Transfer of ions from the cortical cells of the roots and other cells external to the xylem into the root and stem xylem may be a rate-limiting step. In this case, accelerating the velocity of upward transport of water in the xylem would merely serve to re­ duce the concentration of the ionic load in the "transpiration stream." The quantity of ions transported to the shoot would be independent, therefore, of the transpiration rate. That this pos­ sible relationship obtains to a degree at least is suggested by the work of Hauber (1953)* It is evident from these considerations that the rate of xylary transport of ions into the shoot as a function of trans­ piration rates involves complex variables, such as the physiological status of the roots; the confused status of the problem in the lit­ erature is therefore understandable. With the hope of clarifying a few of these relationships, a study has been made of the role of transpiration in the upward movement of ions into the shoot. The specific questions to which answers have been sought are as follows; 1. What is the long-term effect of transpiration differ­ entials on the rate of ion accumulation in shoots? Can low rates of transpiration be growth-limiting because of deficient mineral ion supplies reaching the shoot? 2. How rapidly does the fraction of mineral ions absorbed metabolically by the roots (the so-called "inner space" or "apparent non—free space" fraction) become available for upward transport into the shoot? (The assumption is implied in this question that only the ions in the "outer space"'*' or "apparent free-space" of the root cells are carried along by solvent drag forces into the stem, and hence ions absorbed into the "inner space" of the roots are at least temporarily trapped. The question then becomes: How long are the inner space ions trapped?) 3. Is the answer to question 2 significantly different for anions vs. cations? (The negative charge on exchange sites in the "outer space" of the root cells should materially affect the migra­ tion velocity of cations as opposed to anions.) 4. If two isotopes ( P^ and Ca^) are supplied simultane­ ously to the roots, how uniform is the ratio of these isotopes in the different leaves on the plant after a short period of transpi­ ration, and how does this ratio vary with time? It is implicit in this question that if mineral ions (represented here by radioactive phosphate and calcium ions) are transported to the leaves predomi­ nantly in the transpiration stream, the ratio of these ions should be constant for the different leaves unless there are complicating selective exchange reactions in transit or redistribution of the mobile ions from the leaves. ^"Epstein defines "outer space" as "The space to which in­ organic ions have free and reversible access by diffusion." LITERATURE REVIEW This field of study had heen extensively reviewed "by Curtis (1935)» Kramer (l949> 1956), and Epstein (1956).
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