r. h. roberts and b. esther · roberts and struckmeyer: top-root ratios 339 lots 19, 20, buckwheat....

THE EFFECT OF TOP ENVIRONMENT AND FLOWERINGUPON TOP-ROOT RATIOS'

R. H. ROBERTS AND B. ESTHER STRUCKMEYER

WITH EIGHT FIGURES

Several papers and many references in other manuscripts have dealtwith the relation of growth of the tops and growth of the roots of plants.These generally present data showing the effect of soil types, moisture ormineral elements upon top-root ratio (2, 4, 10). The idea is quite prevalentin some agricultural circles that the relative amount of roots on a plant isdetermined, if not controlled, by the soil. On the. other hand, there areseveral other factors which can be shown to modify greatly the top-rootratio. -Among these are variety, light intensity (10), photoperiod, partialdefoliation, girdling, fruiting, and growth substances (14). For example,the amount as well as character of the root system of piece-root graftedapple trees varies with the variety of scion used (5, 9). The type oramount of reserves accumulated in perennial plants as the apple has marked.effects upon the subsequent top-root ratio (fig. 1). It has been reportedin the literature that plants grown in short days have relatively few roots(1, 11). This conclusion may be strengthened if the plants are not grownto maturity. Also, having few roots in short photoperiods is an effect whichmay be incidental to the fact that the plants under observation come toflower in short days. Plants which blossom in long photoperiods h-ave beenseen to have fewer roots in long photoperiods (6), indicating a correlatioinbetween flowering and limited root development. This was found to be thecase by WITHROW, who used nitrogen supply as a variable with photoperiod(13). She states: ". . . plants, for the most part, were able to form pro-portionately larger tops as compared to roots under that photoperiod whichbrought about flowering as compared to that photoperiod under which theplants remained vegetative. Nitrogen supply did not usually alter thedirection of this response, although under either photoperiod there wereproportionately larger tops as compared to roots at high nitrogen levels thanwhen the nitrogen supply is limited." Nitrogen is generally credited withincreasing the top-root ratio (2, 4, 10).

The data presented in this paper were collected to measure to whatextent the ratio of top to root is influenced by some factors in the environ-ment of the top; also, to see if the relative amount of root formation is con-sistently related to the function of flowering and fruiting regardless of-whether a plant has a greater ratio of roots in short photoperiods, longphotoperiods, cool temperatures, warm temperatures or other specific cul-tural environment.

Published with the approval of the Director of the Agricultural Experiment Station.

332

Copyright (c) 2020 American Society of Plant Biologists. All rights reserved.

ROBERTS AND STRUCKMEYER: TOP-ROOT RATIOS

Materials and methods

Twenty-nine species or varieties of plants were grown in some or all ofthe following six environments: long and short photoperiods in the green--house, at three minimum (night) temperatures: 550, 650, and 750 F. Plant-

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root development, weight increase, and shoot growth of Wealthy apple trees in pots.1927. Upper row of trees were from a lot shaded the previous year when growing in thenursery; lower row from trees grown in full sunlight.

ings were made in the winter of 1940-41, and many were repeated in thefollowing greenhouse season. The plants used for ratio determinationswere chosen for their previously ascertained responses to photoperiod and

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PLANT PHYSIOLOGY

-temperature (6, 7) to provide a wide range of top and root development aswell as flowering.

Seedling plants were pricked off into flats and later transplanted to potsaccording to usual greenhouse procedure. Prior to potting and transferto the experimental treatments the seedlings or cuttings of varieties whichare responsive to photoperiod were kept in an environment unfavorable toblossoming.

Plants of many of the lots were sampled at intervals to secure data fordifferent stages of development of the plants. As a rule, three plants of alot were taken for a single sample. This number was varied somewhatdepending upon the size or bulk of the specimens. The roots were carefullywashed free of soil and separated from the tops; or, in cases where plantswere started as cuttings, both tops and roots were separated from the origi-nal portion, the latter being discarded.

The samples were brought to dryness at 800 to 850 C. The ratios oftop-to-root were calculated on a dry weight basis.

Results

The influence of temperature, photoperiod, and flowering upon the top-root ratio of most of the varieties of plants studied is shown by table I.

Before summarizing the effects of photoperiod, temperature and fruit-ing upon the top-root ratio, some observations will be made upon the history,special cultural conditions or reactions of some of the lots listed in table I.

Lot 1, Sunflower. During the latter part of the growing period theminimum night temperature of 550 F. in the cool hours could not alwaysbe maintained. The effect of the treatment during the early phases ofgrowth did, however, produce plants very much unlike those in the warmerhouses (fig. 2).

Lot 2, Alfalfa. The plants made responses typical of alfalfa; that is,plants placed in a warm location remain weakly vegetative after early lim-ited flowering; the medium temperature, long-day plants blossom, but thesegenerally abort (4) ; and the cool, long photoperiod plants fruit well althoughslowly. The cool, short-day plants grow slowly and remain strongly vege-tative (fig. 3). An interesting feature of this graph is the change in top-root ratio as the flowering state is passed by the warm -plants and as thecool, long photoperiod plants come to flower.

Lots 3, 4, Spinach. Spinach plants "go to seed" very quickly in warmand medium temperatures, especially in long but to some degree also in shortphotoperiods. Plants remain in a rosette stage of growth for months whengrown in cool, short photoperiods; the cool, long-photoperiod plants produceseed stalks at a young age, and these often grow to several feet in height.The top-root ratios of the cool temperature plants seeded on November 29are shown by. figure 4. Again, the seasonal change in top-root ratio as flow-ering occurs can be noted and also the extremely high top-root ratio of thefruiting long photoperiod plants.

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Lot 12, Cineraria. The high top-root ratio of this short-day plant, asit came to flower (fig. 5), is in contrast to the ratio situation in long-dayplants; compare with figures 3 and 4.

Lot 13, Millet. The appearance of the plants at sampling time is shown

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FIG. 2. Helianthus cucumerifolius, var. Orion. A, cool, short photoperiod; B, cool,long photoperiod; C, medium, short photoperiod; D, medium, long photoperiod; E, warm,short photoperiod; F, warm, long photoperiod, plants with poor flower development.

by figure 6. The high top-root ratio in short photoperiods is typical ofshort-day plants.

Lot 16. Salvia. Old plants were heavily top and root pruned January 8.The ratio of naew growth of top and root-s on March 18 was 2.4 for vegetativeplants in long, photoperiods anid 4.0 for flowering plants in short photo-periods at a warm temperature.

337


PLANT PHYSIOLOGY

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FIG. 3. Top-root ratio of alfalfa, strain H-822-1. Warm temperature plantsblossom early and become vegetative later. Cool, long-day plants come to flower afteran early vegetative period.

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ROBERTS AND STRUCKMEYER: TOP-ROOT RATIOS 339

Lots 19, 20, Buckwheat. This is the first indeterminate plant reportedon. It is also a plant which forms blossoms when very young, and in largenumbers. As would be expected if flowering plants have a limited rootdevelopment, the top-root ratio is very high.

Lot 21, "Apple of Peru." This indeterminate plant might be expectedto be valuable in determining the influence of temperature upon top-rootratio as it blossoms in both long and short photoperiods and in each tempera-ture being used. Its value for this purpose is questionable until more isknown of the effect of slower blossoming in the cool temperature upon top-root ratio.

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FIG. 5. Top-root ratio of Cineraria cruenta. The blossoming plants in short photo-periods had uniformly fewer roots in relation to the top than the non-blossoming plantsin long days at each temperature.

Lot 25, Corn. This plant is sometimes considered as being a short-davspecies. The Mexican varieties are short-day types, but those grown innorthern latitudes blossom earlier in short photoperiods only at cool tem-peratures for the species, as 60 65° F. In a warm environment of 750 F.varieties such as Wisocnsin Golden Glow show little response to photoperiodso this plant should be classed as an indeterminate type. On March 14 themedium temperature plants in long and short photoperiods were both intassel and had top-root ratios of 8.6 and 9.5, respectively.

Lots 26-30, Peas. Alaska peas nieed to be planted early for successfulcropping. Later maturing sorts as Prince of Wales can be planted laterwith equal success. Early peas nieed cooler weather conditions. Possiblv


PLANT PHYSIOLOGY

this varietal difference may be due to the rooting habits. The early flower-ing sort, Alaska, had a much larger top-root ratio especially in the warmertemperature where fruits were produced early.

Hemp. The data on top-root ratio for Cannabis sativa var. Femarring-ton was not included in table I because of the lack of columns for male andfemale plants. This is a short-day plant at the three temperatures used,although some blossoming occurs on the older plants in long days at hightemperatures. The particular item of interest in its response is the mark-

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FIG. 6. German millet. A, cool, short photoperiod; B, cool, long photoperiod; C,medium, short photoperiod; D, medium, long-photoperiod; E, warm, short photoperiod;F, warm, long photoperiod.

edly limited rooting of the male plants (:fig. 7, D). (This may be a leafrelationship as the female plants have persisting green leaves and a greaterleaf area.) The top-root ratios March 27 are shown with figure 7 for plantsfrom seed planted November 28 and given environmental treatmenits begin-ning January 24.

EFFECT OF PHOTOPERIOD UPON TOP-ROOT RATIO

An examination of table I shows that the longr-day plants (lots 1-11)have a consistently high-er top-root ratio in long photoperiods, that is, at a

given temperature. The short-day species (lots 12-18) have just as con-

340



sistently a higher top-root ratio in short photoperiods at the same tempera-ture. In the case of indeterminate lots (lots 19-36) there is no pattern ofphotoperiod effect upon top-root ratio.

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FIG. 7. Top-root ratios of Hemp, var. Femarrington. Numbers are top-root ratios.A, cool, short photoperiod (budding), 6.4; B, cool, long photoperiod (vegetative) 4.9;C, medium, short photoperiod (female blossoms) 7.4; D, medium, short photoperiod (maleblossoms) 14.5; E, medium, long photoperiod (vegetative) 8.3; F, warm, short photo-period (female) 9.5; G, warm, long photoperiod (budding) 9.2. Not pictured, warm,short photoperiod (male) 20.2.

EFFECT OF TEMPERATURE UPON TOP-ROOT RATIO

Again from table I, it is seen that many of the plants had greater top-root ratios in a warm temperature. Among these are spinach, stock, blue

341


PLANT PHYSIOLOGY

grass, cabbage, Cineraria, German millet, Jap buckwheat, Nicandra andPrince of Wales peas. On the other hand some plants as geranium and thephlox, Miss Lingard, have a larger top-root ratio in a cool temperature. Theother species under study showed no consistent response to influence of tem-perature.

EFFECT OF FLOWERING UPON TOP-ROOT RATIO

Since neither photoperiod nor temperature were consistently related totop-root ratio throughout the list of plants being grown, the question re-mained as to whether the formation of blossoms and fruits was associatedwith a greater top-root ratio at a given temperature, in the case of thosekinds which show a consistent temperature, influence. The data of table I,as well as the data for hemp, show a consistently greater amount of top inrelation to the root in the case of flowering plants. From figures 3-5 aswell as successive samplings of several other lots as the season progressedit is clear that the high top-root ratio was not the result of the photoperiodand temperature combination in which the plants were growing but was acondition which arose as thg plants developed flowers and fruits. It is thusclear that coming to flower reduces root extension in relation to the weightof top produced.

DiscussionThe data presented consistently show an effect of the environment sur-

rounding the top upon the top-root ratio. This is also consistent with the

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FIG. 8. Cross-sections of roots of tobacco var. Maryland Mammoth. A, nonfloweringplant; B, flowering plant. Changes in structure accompanying flower production arecomparable to those found in the stem.

well-known influence of shade, partial defoliation, and girdling (of some spe-cies) in reducing root extension. The characteristic differences in amountof root formation and top-root ratio of different varieties are marked. Alsothe influence of blossom induction and fruiting in checking root growth pre-

342



sents a problem when considering the role and efficiency of root systems.The question which is particularly raised is-at what stage of growth shouldsamples be taken.

The effect of flowering on the top-root ratio may well be related to thereduced cambial activity and limited development of phloem tissue whichaccompanies blossoming (8). Partially completed studies of the relationof flowering to root anatomy have shown that the structure of the roots ofannuals is similarly modified by the reproductive state of the top (fig. 8).The question arises naturally as to the possibility of altered transport fol-lowing a reduction in phloem development being involved in reduced rootgrowth which follows from shading, partial defoliation or girdling.

The fact that an environmental factor as length of photoperiod or tem-perature does not have a consistent effect upon top-root ratio points to thepossibility that the internal conditions of the plant as elaborated or storagematerial, rather than purely external factors would be the real determinantsof root production. This could even be the case, as CURTIS (3) has sug-gested, where soil conditions are credited with influence upon the roots.The composition and make-up of the top as an influence upon roots is alsoindicated by the seasonal changes in top-root ratio of plants under uniformsoil, temperature and light conditions.

All of these observations combine to raise a question as to the essentialvalue of studies of the top-root ratio when attempting to determine the roleand function of roots in plant growth and development. It appears thatthe "efficiency" of a root system must be measured by other techniques thanrecording top-root ratio; possibly by such studies as those by WENT (12).He has concluded "that less than ten per cent. of the root system . . . is re-sponsible for more than 50 per cent. of their (tomato) growth rate."

SummaryTwenty-nine species or varieties of plants were grown in long and short

photoperiods at minimum (night) temperature of 550, 650, and 750 F. ina uniform soil.

Different species have characteristic top-root relations.The top-root ratio was found to vary with changes in the physiological

conditions associated with the seasonal development of the plant.A given external environment such as length of photoperiod or tempera-

ture was found not to have a similar effect upon the top-root ratio of differ-ent species. Many but not all of the species grown have more roots in rela-tion to the top in cooler temperatures than in warm. Some have fewer rootsin relation to top when the temperature is cool. Likewise, some kinds havesmaller top-root ratios in short photoperiods and others in long photoperiods.

The top-root ratio was uniformly larger when the plants came to flower;that is, blossoming plants consistently have fewer roots in relation to theamount of top at a given temperature than non-flowering plants of a species.

It appears that the composition and reserve conditions within the top isobviously a large, if not the controlling, factor in the production of rootsand so, the top-root ratio.

343


PLANT PHYSIOLOGY

A measurement of the top-root ratio would seem to offer less than issometimes expected towards a solution of the problem of the role or efficiencyof roots in plant growth.

DEPARTMENT OF HORTICULTUREUNIVERSITY 01 WISCONSIN

MADISON, WISCONSIN

LITERATURE CITED

1. BONNER, JAMES. Experiments on photoperiod in relation to the vege-tative growth of plants. Plant Physiol. 15: 391-325. 1940.

2. CRIST, JOHN W., and STOUT, G. J. Relation between top and root sizein herbaceous plants. Plant Physiol. 4: 63-85. 1929.

3. CURTIS, 0. F. Stimulation of root growth in cuttings by treatmentwith chemical compounds. Cornell Univ. Agr. Expt. Sta. Mem.14. 1918.

4. HARRIS, FRANK S. The effect of soil moisture, plant food, and age onthe ratio of tops to roots in plants. Jour. Amer. Soc. Agron. 6:65-75. 1914.

5. ROBERTS, R. H. Some stock and scion observations on apple trees.Wisconsin Agr. Expt. Sta. Res. Bull. 94. 1929.

6. , and STRUCKMEYER, B. ESTHER. The effects of tempera-ture and other environmental factors upon the photoperiodic re-sponses of some of the higher plants. Jour. Agr. Res. 56: 633-677. 1938.

7. and . Further studies of the effects oftemperature and other environmental factors upon the photo-periodic responses of plants. Jour. Agr. Res. 59: 699-709. 1931.

8. STRUCKMEYER, B. ESTHER. Structure of stems in relation to differen-tiation and abortion of blossom buds. Bot. Gaz. 103: 182-191.1941.

9. SWARBRICK, T., and ROBERTS, R. H. The relation of scion variety tocharacter of root growth in apple trees. Wisconsin Agr. Expt.Sta. Res. Bull. 78. 1927.

10. TURNER, THOMAS WYATT. Studies of the mechanism of the physiologi-cal effects of certain mineral salts in altering the ratio of top growthto root growth in seed plants. Amer. Jour. Bot. 9: 415-445. 1922.

11. WEAvER, J. E. and HIMmE, W. J. Relation between the developmentof root systems and shoot under long- and short-day illumination.Plant Physiol. 4: 435457. 1929.

12. WENT, F. W. Effect of the root system on tomato stem growth. PlantPhysiol. 18: 51-65. 1943.

13. WITHROW, ALICE P. The interrelationship of nitrogen supply and pho-toperiod on the flowering, growth and stem anatomy of certain longand short day plants. Butler Univ. Bot. Studies VII: 1-25. 1945.

14. ZIMmERMAN, P. W., and WLCOXON, F. Several chemical growth sub-stances which cause initiation of roots and other responses in plants.Contrib. Boyce Thompson Inst. 7: 209-229. 1935.

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r. h. roberts and b. esther · roberts and struckmeyer: top-root ratios 339 lots 19, 20, buckwheat....

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