A STUDY OF THE ROOT SYSTEM OF BARLEY

Transcription

1 New Phytol. (199) 8, A STUDY OF THE ROOT SYSTEM OF BARLEY. RELATOHPS BETWEEN ROOT DMEO AND NUTRENT UPTAKE BY C. HACKETT* Agricultural Research Council Radiobiological Laboratory, Letcombe Regis, Wantage, Berkshire {Received 14 February 199) SUMMARY Relationships between the number, length, surface area, volume and dry weight of the root members of two varieties of barley {Hordeum vulgare L. vars Maris and ) were examined over the first 4 weeks of growth in water culture. Plants given a complete supply were compared with plants in potassium or phosphorus. Although there were large effects of age, variety and nutrition on all the root dimensions, relationships between the dimensions varied comparatively little. n consequence, effects of age, variety and nutrition on the rate of uptake per unit of root were qualitatively similar whichever dimension ofthe root system was considered. t is shown by reference to descriptions of root systems published elsewhere that constancy in relationships between root dimensions occurs in solid media also. This property of root systems, which appears to result quite straightforwardly from the contrasting morphological characteristics of root axes and laterals, may help to simplify some aspects of the study of absorption by roots. NTRODUCTON The first paper in this series (Hackett, 198) gave some results from an experiment in water culture in which the morphology and absorbing activity of root systems of barley {Hordeum vulgare L.) were examined. Two varieties, Maris and, were grown under three regimes complete, potassium and phosphorus and plants were harvested at i, 2 and 4 weeks from germination. The results presented so far comprise a description of the 4week old root systems in terms of the number, length, surface area, volume and dry weight of their members, each type of root member (i.e. axes, primary laterals and secondary laterals) being dealt with individually. The purpose of the present paper is to describe briefly and in similar terms the younger plants harvested, and then to draw together the data for the various root dimensions of plants of all three ages. The approach used in studying relationships between the root dimensions has been influenced by the need to know much more about the relative importance of alternative root dimensions as measures of the potential absorbing capacity of root systems. Direct investigation of this question is difficult and slow, and it was thought that something useful might be learnt if relationships between the root dimensions themselves could be * Present address: C.S..R.O. Division of Land Research, Canberra, A.C.T. 201, Australia. 102

2 1024 C. HACKETT defined; should the dimensions be fairly constantly related to one another, the study of the overall problem might be simplified. t proved that relatively constant relationships had been maintained between the root dimensions during the experiment, and by comparing rates of uptake per unit of root calculated for each dimension in turn, an attempt has been made to assess the significance of this finding. MATERALS AND METHODS A detailed account of the methods of culture and measurement has already been given (Hackett, 198). n brief, after i week's growth in the dark, plants of the varieties of harley Maris and were grown in the glasshouse under complete supply* and with the supply of potassium and phosphorus reduced to onetwentieth of that in the complete solution. Eight replicate plants were harvested at i, 2 and 4 weeks from the complete treatment and at 4 weeks from the deficiency treatments. At each harvest, 4 replicate root systems were preserved, to be used subsequently for determinations of the number, length, diameter, surface area and volume of the root members. The remaining root systems were dried and weighed. These, together with their shoot systems, were later analysed for potassium and phosphorus byflamephotometry and a molybdenum blue method (King, 195) respectively. The effects of age, variety, and nutrition on the following parameters of the root systems were examined: average length of the members (i.e. total length of root members H total number); average diameter of the members (i.e. the mean of the diameters of each type of root member weighted according to the length of each type present see Hackett, 198); and dry weight per unit volume. Average rates of uptake {R) were calculated by methods analagous to those used in calculating average net assimilation rate from the dry weight and leaf area of plants sampled at intervals. Since the relationship between the plant variables, in this case content (c) and root size {q), could not be determined with confidence, a linear relationship was arbitrarily assumed, and R was calculated from the formula fi,, C2CiXl0ge?2l0g,?i (i) K t t = t is recognized that errors may have been introduced by making this assumption (Radford, 197), but sample calculations using formulae appropriate for nonlinear relations between c and q indicate that any such errors do not materially affect the conclusions which have been drawn from the results. RESULTS Effects of age, nutrition, and variety on relationships between root dimensions The root systems increased greatly in size and complexity during the experimental period (see Tables i and 2 for a detailed description of the plants). At i week, only unbranched seminal axes were present. A week later, these axes bore primary laterals, and a few unbranched nodal axes had. By 4 weeks, there were many nodal axes, several of which were branched; branching in the seminal root system had meanwhile * Major concentrations (meqtiiv./l): Ca++ 1.8, K+ 0.4, Mg++ 0.5, H2PO4O.1, NO~2.i, SO4O.S.

3 Root dimensions and itptake 1025 progressed to tbe formation of secondary laterals, and tbe total length of root members was more than 100 times that at i week. n view of the final multiformity of the root members, it was surprising that their average length and diameter were rarely affected by age, nutrition, or variety by more than a lactor of 2 (Tables and 4). Constancy of the average length and diameter of the members was particularly noticeable once the seminal axes had branched. Dry weight per unit volume was approximately constant between i and 2 weeks but increased significantly by the fourth week (Table 5). The overall increase between 2 and 4 weeks was approximately 0% which was proportionally greater than the change in the average length and diameter of the members over the same period. Table i. The shoot and root systems of two varieties of barley at i and 2 weeks from germination grown under complete supply {mean values per plant) Shoots No. of leaves on main Dry weight (mg) Roots No. of members Length (cm) Diameter (mm) Surface area (cm^) Volume (cm^) Dry weight (mg) and laterals stem week Maris 25 _ _ 1 os S weeks Maris Proct( Effects of age, nutrition, and variety on the rate of uptake per unit of root The overall contents of the plants are given in Table. Since these varied far more with age and supply than the relationships between the root dimensions (Tables 5), the effects of age and supply on uptake per unit of root were qualitatively similar whichever root dimension was referred to. Taking uptake of potassium by way of example,* this was faster during the period1 2 weeks than 2 4 weeks under complete supply (Table 7), was reduced overall by potassium deficiency (Fig. i), and with the exception of uptake per member in was increased by * Rates of phosphorus uptake may be readily calculated from equation (i) if required since differences in the rates of potassium and phosphorus uptake are directly proportional to the differences in the increments in their content between harvests (Table ) O. 1 SS os 1/)

4 102 C. HACKETT Table 2. The shoot and root systems of two varieties of barley at 4 weeks from germination grown under complete supply and deficiencies of potassium and phosphorus {mean Shoots No. of leaves on main Dry weight (mg) Roots No. of members Length (cm) Diameter (mm) Surface area (cm^) Volume (cm^) Dry weight (mg) and laterals stem values per plant) Potassium Phosphorus Maris Maris Mans and O. and O O O o.i no OO O5 0. and O O and 0.1 * Secondary laterals were present only on the seminal roots of plants grown under complete and phosphorus supply. Table. Average length {cm) of the members of the root systems of two varieties of barley at, 2 and 4 weeks from germination Total root system Maris roots Maris roots Maris week weeks O weeks Potassium Phosphorus L.S.D. between times and supplies (P = OO 0 L.S.D. variety X time and supply

5 Root dimensions and uptake 1027 Table 4. Average diameter (mm) of the members of the root systems two of varieties of barley at 1, 2 and 4 weeks from germination Total root system Maris roots Maris roots Maris week 2 weeks 4 weeks Potassium Phosphorus O59 O54 O.S O O s s O9 0.8 L.S.D. L.S.D. between variety times and x time and supplies supply (P = 0.0s) (P = 0.05) Table 5. Dry weight per unit volume (mg/cm^) of the root systems of two varieties of barley at, 2 and 4 weeks from germination O L.S.D. variety X time and supply (P = 0.05) Total root system Maris O roots Maris 71,. 4,. "5 ^ J roots Maris week 2 weeks weeks Potassium Phosphorus O 102 L.S.D. between times and supplies (P = 0.0s) Table. The potassium and phosphorus contents (mg/plant) of two varieties of barley at 1, 2 and 4 weeks from germination Potassium Maris Phosphorus Maris week.. O.2 O.2 2 weeks.9 55 O weeks Potassium Phosphorus

6 1028 C. HACKETT phosphorus deficiency (Fig. i). Effects of nutrition were quantitatively alike with respect to root surface area, volume and dry weight. Varietal effects on content were smaller than those of age and nutrition, and rates of uptake were therefore more dependent on the root dimension considered. Even so there was a consistent indication in Table 7 that had absorbed faster than Maris between i and 2 weeks under complete supply and slower between 2 and 4 weeks. DSCUSSON t has long been apparent that the fresh weight, and hence the volume, of young cereal root members is closely related to their dry weight, but relationships between these and other root dimensions have not been extensively investigated. Results in the present paper show that not only dry weight per unit volume of root but also the average length and diameter of the members change comparatively little with time during the early stages of growth; hence, approximately constant relationships are maintained between the total number, length, surface area, volume and dry weight of the members. Similar relationships between root dimensions are evident from data published elsewhere, in these instances for plants grown in solid media. Thus, Eig. i of May, Chapman, and Aspinall (195) shows an approximately constant relationship between the number and length of barley root members grown for 5 weeks in perlite, and Table of Weaver, Kramer and Reed (1924) shows a constant relationship between the length and surface area of wheat roots grown for 10 weeks in the field. The underlying mechanism by which these relationships are maintained is unknown, but some explanation can be offered in morphological terms. Constancy of the average diameter of the root members grown in the present experiment appeared to be due to a combination of the following characteristics of the root systems: the preponderance in length of primary laterals at 2 and 4 weeks (Tables i and 2), their similarity in diameter on seminal and nodal roots (Table 2 and Part, Table ), and their uniformity in diam Table 7. rate of potassium uptake per unit of root by two varieties of barley grown under complete supply Maris //g/day/member /(g/day/mm length //g/day/mm^ surface area ^g/day/mm^ volume figldayjmg dry weight /^g/day/member //g/day/mm length //g/day/mm^ surface area //g/day/mm^ volume /ig/day/mg dry weight 1 2 weeks 2 4 weeks 1 4 weeks eter along their length. The constancy in average length of the members was not on the other hand so readily explicable from the data available, but a rough numerical description of the growth and branching of a single root indicated that it was dependent on the

7 Root dimensions and uptake 1029 following: the timing of branching, the negative correlation between orders of branching and their rate of extension, and the positive correlation between orders of branching and the rate of increase in number of their members. The characteristic expression of these qualities in the barley root system could be seen to lead straightforwardly to a balance between the number and length of members. Owing to the existence of close relationships between the dimensions of the root systems studied, alternative measures of their size were roughly equivalent as bases for expressing the absorbing activity of the root systems (Table 7 and Fig. i). Since it is easier to determine some root dimensions than others, this finding appeared to have useful implications. However, the effects of variables on the rate of uptake per unit of root 200r 100 No. of mem Root length Root surfac Root volume >. D "0 Maris rftm K P m K P Fig.. The effect of complete supply (C), potassium deficiency ( K), and phosphorus deficiency ( P) on the average rate of potassium uptake per unit number, length, surface area, volume and dry weight of the root memhers of two varieties of barley over the period 1 4 weeks from germination, relative to Maris under complete supply, units for which are given in Table 7. depended to some extent on the root dimension considered, and it was therefore important to know whether the dimensions simplest to determine are those most directly related to the process of absorption by roots. The experiment provided no evidence on this question, but it was shown recently (Russell and Sanderson, 197) that the uptake of phosphorus and strontium by intact segments of diflferent types of barley root member is closely related to the volume of the segments. The relationship with surface area, though not widely different from that with volume, is less close. While these findings cannot be applied with confidence to whole root systems, the difference in uptake on the two bases was in reasonable accord with that seen in the present paper. The results from both investigations when taken together suggest that both volume and dry weight are measures of young barley root systems which refiect their capacity to absorb s from wellstirred or fiowing solutions. Since coordinated development of root systems occurs in soil as well as water culture.

8 100 C. HACKETT it is likely that volume and dry weight reflect the absorbing capacity of young root systems in this medium also when transport of s to the roots is not limiting uptake. However, transport of s to plant roots is often restricted in soil, due for example to localized distribution in the profile, as after fertilizer placement, or to adsorption on soil particles. n such circumstances, the practical significance of the constancy in relationships between root dimensions could be much diminished. Root distribution and root hair development might then be characteristics of major importance. Thus, whilst the coordination of root growth discussed in this paper may simplify some aspects of the study of the morphology and performance of root systems, it is unlikely to lead to the identification of a single and universal measure of their absorbing capacity. ACKNOWLEDGMENTS Again should like to thank Dr R. Scott Russell for his encouragement in this work and, together with Dr M. G. T. Shone and Dr D. T. Clarkson, for commenting on drafts of this paper. Mr B. O. Bartlett and Mr K. B. Gunn provided statistical and chemical assistance, which is very gratefully acknowledged. REFERENCES HACKETT, C. (198). A study of the root system of barley.. Effects of nutrition on two varieties. New Phytol., 7, 287. KNG, E. J. (195). Micro Analysis in Medical Biochemistry, rd edn. London. MAY, L. H., CHAPMAN, F. H. & ASPNALL, D. (195). Quantitative studies of root development.. The influence of concentration. Aust. J. biot. Sci., 18, 25. RADFORD, P. J. (197). Growth analysis formulae their use and abuse. Crop Sci., 7, 171. RUSSELL, R. S. & SANDERSON, J. (197). Nutrient uptake by diflerent parts of the intact roots of plants. J. exp. Bot., 18, 491. WEAVER, J. E., KRAMER, J. & REED, M. (1924). Development of root and shoot of winter wheat under field environment. Ecology, 5, 2.

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