(WITH · PLANT PHYSIOLOGY were then transferred to glazed crocks or a large shallow tray containing...

A COMIPARATIVE STUDY OF GRADUAL AND ABRUPT CHANGESIN ROOT TEMPERATURE ON WATER ABSORPTION

R. H. B6HNING AND BHAKDI LuSANANDANNA

(WITH SEVEN FIGURES)

Received December 17, 1951

IntroductionThe effects of low root temperature on various plant processes have long

been known and extensively studied. It is generally recognized that one ofits important effects is the reduction in water absorption of the plant; andthe nearer the root temperature approaches the freezing point of water, thegreater this effect will be manifested. The change in physical state, par-ticularly the increase in viscosity with the decrease in temperature, of bothwater and protoplasm are undoubtedly two basic factors influencing waterabsorption at low root temperatures. The reduction in water absorptionwill affect other plant processes, directly or indirectly, to a greater orsmaller degree depending on the extent of water shortage within the plant,and consequently may result in corresponding injury to the plant. Condi-tions favorable to high transpiration have been reported to bring aboutwilting and injury to the plant from desiccation when its roots are subjectedto low temperature. However, different plant species appear to differ intheir capacity to withstand the effects of low root temperature. KRAMER(7) observed that species which grow in the winter and species native tocooler climates are less affected by low soil temperature than warm-weatherspecies. The statement by KRAMER (7), which will be discussed later, thatplants cooled slowly wilted less severely than plants cooled rapidly, seemsto be the only report in the literature on the comparative effects of rates oflowering root temperature. Under natural conditions, the change in temper-ature varies not only in respect to the level to which the temperature fallsbut also in the rate at which it occurs. Information regarding the effects ofdifferent rates of temperature decrease seems at the present to be limited,and it was the purpose of this investigation to compare the effect on waterabsorption of an abrupt decrease in root temperature with a gradual de-crease in root temperature.

Materials and methodsIn the present investigation three plant species were used, sunflower

(Helianthus annuus L.), tomato (Lycopersicum esculentuin Mill.) varietyRutger, and red kidney bean (Phaseolus vulgaris L.) variety Black Valen-tine. Seeds were allowed to germinate in flats of miioist sand or on clothnetting stretched over the top of a glazed crock of tap water. Seedlings

1 Papers from the Department of Botany and Plant Pathology, The Ohio StateUniversity, Columbus 10, Ohio, no. 545.

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PLANT PHYSIOLOGY

were then transferred to glazed crocks or a large shallow tray containing acomplete culture solution. When they reached the size to be used in the ex-periment, uniform plants were selected and transferred to black paintedone-quart mason jars containing 700 ml. of the complete culture solution,one plant to each jar. At all times the plants were provided with uniformmoderate aeration.

Control of root temperature was made by immersing the jars in whichthe plants were growing in temperature-controlled water baths. The twobaths used in the experiments were rectangular metal tanks, seven feet long,two feet wide, and 10 inches deep, provided with an inlet and an outlet forwater which was maintained at a depth of five inches (the level of the solu-tion in the jars). The bath in which temperature was maintained constantat 250 C had a heating system consisting of a flexible type immersion heateroperated by a bimetal thermoregulator connected to a silent power relay anda cooling system of running water from the water supply line. The otherbath in which temperature was gradually lowered to 50 C had a heatingsystem similar to that of the constant temperature bath and a cooling sys-tem consisting of a thermoregulator-controlled circulating pump whichpumped ice water from a reservoir tank into the bath. By these methods itwas possible to maintain the temperature at any desired level within a rangeof + 0.10 C. Stirrers were kept running all the time during the period ofthe experiment to maintain uniformity of temperature throughout the baths.Temperature gradients in the baths were almost non-existent, the variationin temperature between the two ends of the tanks being no greater than0.50 C. The gradual change in root temperature was made by making asmall change daily in the setting of the thermoregulator in one of the baths.A temperature record was made from the readings of thermometers im-mersed in the water near each end of the tanks.

The decrease in volume of culture solution in the containers in which theplants were growing was used as the measure of water absorption. The vol-ume of solution in each jar was measured daily by transferring it to a grad-uated cylinder, and the solution was then filled up to its original volume of700 ml. with distilled water previously cooled to the same temperature. Thetime required for the measurement of water absorption by all the plants ineach experiment was about one and a half hours. Absorption was measuredin the same order throughout each experiment in order that every plantwould have completed its 24-hour absorption period at the time when itsmeasurement was made. Possible sources of error inherent in the methodwere (a) in the loss of water through evaporation as the solution was con-tinuously aerated and (b) in the wetting of the walls of the containers andthe roots when the solution was transferred for measurement. A test con-ducted to determine the magnitude of error from these sources showed thatit was not greater than 2 ml. per day. In order to obtain substantial valuesfor the absolute amount of water absorption and also to minimize the effectof error in the measurement, the plants were used when they were well be-

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BOHNING AND LUSANANDANA: WATER ABSORPTION

yond the seedling stage and had fairly high rates of water absorption. Un-der conditions favorable to rapid transpiration the average amount of waterabsorbed per day per plant of the control sets was more than 100 ml. forbean and 200 ml. for tomato and sunflower.

Prior to the start of each experiment the plants were allowed to remainin the 250 C water bath for a period of not less than two weeks. Gradualand abrupt changes in root temperature were then made in the followingmanner. The temperature of one water bath was lowered 1.5 or 2.00 C eachday until it had been gradually reduced from 25 to 5° C. This rate ofchange in root temperature was considered as the gradual change in thepresent study. Abrupt changes varied in magnitude from 5 to 200 C andwere made by transferring plants from the 250 C bath to a bath cooled tothe desired temperature. In all treatments involving an abrupt change, thechange was made only on the first day, after which the root temperature ofthese plants was gradually lowered until the final temperature of 5° C wasreached. The following temperature treatments were made:

Set Change in root temperatureControl-Constant at 250 C

I-Gradual from 25 to 50 CII-Abrupt from 25 to 20° C, gradual from 20 to 50 CIII-Abrupt from 25 to 150 C, gradual from 15 to 5° CIV-Abrupt from 25 to 100 C, gradual from 10 to 5° CV-Abrupt from 25 to 50 C

Results and discussionIn the present investigation the daily rates of water absorption of the

sets under treatment are expressed as percentages of the rate of the controlset at 25° C for the same 24-hour period. The absorption rate of each settreated was also corrected for its relative rate of absorption to that of thecontrol when both sets were subjected to the same 250 C root temperatureprior to the treatment. This correction factor was used throughout theperiod of the treatment on the assumption that there was no change in thebasic absorptive capacity of the sets except that due to the treatment given.As all plants were exposed to the same conditions of light intensity, ambienttemperature (other than root temperature) and vapor pressure of the at-mosphere, the use of this correction factor will remove the differences dueto the effect of the above factors on transpiration and in turn its effect uponabsorption of water by the plants in the different sets. The method of ex-pressing rate of absorption as stated above was also used by KRAMER (7).

The effect of gradual and abrupt changes in root temperature on waterabsorption of sunflower plants six weeks old is presented graphically in fig-ure 1. During the period of this experiment the weather was mostly clearor occasionally partly cloudy, favorable to a high rate of transpiration. Itwill be noted that the gradual change in root temperature generally resultedin a somewhat higher rate of water absorption than abrupt changes. In all

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PLANT PHYSIOLOGY

cases of an abrupt change in root temperature the absorption rate on thefirst day when the treatment was initiated fell sharply to a level slightlybelow that of the gradual change, and thereafter followed the general trendof absorption of the gradual change. At root temperatures of 80 C and be-low, wilting was observed during the middle of the day from 11:00 A.M. toabout 2:00 P.M. on all plants (root temperature abruptly changed from 25to 150 C and 25 to 100 C respectively). The wilting began on the lowerleaves and progressed to the higher leaves; it was not severe enough tocause drooping of the stems, and lasted for only a brief period of two orthree hours. Plants with root temperature abruptly changed from 25 to 50C wilted severely about one and a half hours after being transferred to 5°

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FIG. 1. Effects of gradual and abrupt changes in root temperature on relative waterabsorption per day of sunflower plants. The measurement of water absorption andchange of root temperature were made at 8:00 A.M. The gradual temperature changefrom 25 to 50 C occurred over a period of 13 days. All sets were kept at 50 C fortwo days.

C, and remained in such condition throughout the day until about 6:00 P.M.when they began to regain turgidity slowly. Portions of some fully devel-oped top leaves were desiccated. However, on the following day at the sameroot temperature, these plants appeared to wilt much less severely than onthe first day, and the degree of wilting was about the same in sets III andIV. Plants in set I (root temperature gradually changed) and set II (roottemperature abruptly changed from 25 to 200 C then gradually changedthereafter) did not show any sign of wilting throughout the range of roottemperature, although their daily rates of absorption did not show veryappreciable deviations from those of the wilting sets.

The results obtained with tomato plants eight weeks old are presentedin figure 2. The weather conditions were favorable for high rates of tran-

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BOHN\'ING AND LUSANANDANA: WATER ABSORPTION

spiration throughout the period of the experiment. The curves indicate anappreciable increase in the rate of absorption in the range of root tempera-ture from 25 to 18° C. It is possible that this range of root temperaturewas the optimum range for water absorption in the particular variety oftomato used. Below 180 C the rate of absorption began to decrease withthe decrease in root temperature. The rate of fall in water absorption wasgreater in tomato than in sunflower. In general the absorption rate of theplants receiving a gradual change in root temperature was greater thanthose of other groups. With the exception of the plants in set II whichshowed an increase in the rate of absorption when the root temperature waschanged from 25 to 200 C, all plants in the other sets showed an abrupt drop

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FIG. 2. Effects of gradual and abrupt changes in root temperature on relative waterabsorption per day of tomato plants. The gradual temperature change from 25 to 5° Coccurred over a period of 13 days. All sets were at 50 C for three days.

in the absorption rate on the first day when their root temperatures wereabruptly changed. The pattern of changes in the absorption rate of thesegroups were similar to those of sunflower plants. Wilting occurred only inthe last set of plants which had the root temperature abruptly changedfrom 25 to 50 C. The wilting began about two hours after the root temper-ature was changed and was severe enough to cause drooping of all leavesand the top portions of the stems. The plants remained in the wilted con-dition throughout the rest of the day and slowly regained turgidity at night.Although the plants wilted very severely no injury from desiccation oc-curred. In the following days the plants in this group again wilted at about11:00 A.M. but regained their turgidity sooner than in the first day (about5:00 P.m.). The relative rate of absorption of the plants in this set showed

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480 PLANT PHYSIOLOGY

a slight increase from the drop of about 70%'o in the first day. When roottemperature was lowered below 13° C guttation could not be observed fromany plant so treated, but it occurred freely from the plants in the controlset. Senescence of lower leaves occurred in all plants with their roots sub-jected to low temperature except those of the last set. The leaves graduallyturned yellow and slowly dried out. All the leaves of the plants in the con-trol set remained in good vegetative condition. About one third of the totalnumber of fully developed leaves of each treated plant were senescent at thetermination of the experiment. The beginning sign of senescence of someof the lower leaves of the last group was apparent from a gradual decreasein intensity of the green color of those leaves. It seemed that these plants

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RO TE14PERATURE, 6ENTIORADEFIG. 3. Effects of gradual and abrupt changes in root temperature on relative water

absorption per day of red kidney bean plants. The gradual temperature change from25 to 5° C occurred over a period of 13 days. All sets were kept at 5° C for three days.

were not kept under the treatment long enough for the change to reach itsmore advanced stage.

Figure 3 shows the results of the experiment with red kidney plants sixweeks old. The plants were exposed to the same atmospheric conditions asthe tomato plants. Variation in root temperature within the range of 25 to200 C caused very little change in the rate of absorption. When the roottemperature decreased below 200 C, the rate of absorption began to declinemore markedly. The rate of absorption of the plants receiving a gradualchange in root temperature was generally higher than those receiving anabrupt change in root temperature. At 100 C the decrease in the absorp-tion rate was approximately 50 to 60%, which was much greater than in




sunflower and tomato. Although the decrease in the absorption rate at roottemperatures below 100 C was less conspicuous than above 10° C, the de-crease progressed steadily even while the root temperature was maintainedat 50 C. Wilting did not occur in the plants in sets I, II, and III throughoutthe entire course of the experiinent. Plants in set IV wilted severely abouttwo hours after the root teilmperature was changed abruptly from 25 to 10°C, and the wilting continued through the rest of the day until early at nightwhen the plants gradually recovered. On the following day the injury dueto desiccation of several whole leaflets or portions of some leaflets could beobserved in all plants. The extent of the injury from desiccation was ap-proximately 20% of the total foliage of each plant. The uninjured portionof the foliage showed slight wilting again on the following two days andremained turgid thereafter despite being kept at lower temperature; how-ever, the rate of absorption continued to decline. Plants in set V whichwere transferred abruptly from 25 to 5° C were affected in the same wayby the sudden drop of root temperature as those in set IV, but the wiltingand injury from desiccation were much more severe than in the former set.Desiccation occurred in most of the fully developed leaves as well as insome of the young leaves and appeared to continue into the second day, andthe injury damaged approximately 90% of the total foliage of the plants.The rate of absorption in this set continued to fall at a fast rate to 6.5% atthe end of the third day when the root temperature was maintained at 50 C.

Several possible causes of reduction in water absorption at low root tem-perature have been given by KRAMER (8), and the more important ones areas follows: 1. Retardation of root elongation. Continual root growth isnecessary to bring the absorbing regions of roots into contact with soil wa-ter in the undepleted portion of the soil. 2. Decrease in rate of movementof water from soil to roots. 3. Decreased permeability of cells. 4. In-creased viscosity of protoplasm and of the colloidal gels in the cell walls.5. Increased viscosity of water. 6. Decreased metabolic activity of the liv-ing tissues of the roots, which probably affects the permeability of proto-plasmic membranes and the active-absorption mechanism.

The first two causes would not limit absorption in the present work inas-much as the plants grew in solution, and thus the absorbing surfaces of theroots were constantly in contact with water. This point seems to be borneout by the work of ARNDT (1) who found that under conditions favorable torapid transpiration, cotton plants wilted when the soil teinperature was notlower than 17 to 20° C; but when growing in solution culture, wilting oc-curred at lower temperatures (10 to 17° C).

Two physical changes directly resulting from the effect of low tempera-ture would be the increase in viscosity of water and of protoplasm. Thespecific viscosity of water is 0.4987 at 25° C, 0.5608 at 20° C, 0.6363 at 150C, 0.7297 at 100 C, 0.8475 at 50 C, and 1.0000 at 00 C. It will be seen thatthe viscosity of water increases at an increasing rate as the temperatureapproaches its freezing point, and the viscosity increases at an increasing

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482 PLANT PHYSIOLOGY

rate from 10 to 00 C. The increase in viscosity results in a decrease in thediffusion pressure of water, and undoubtedly retards the rate of diffusion ofwater into root cells and through the cells of various tissues of the roots.The viscosity of protoplasm is also known to be greatly increased at lowtemperatures. KRAMER (6) cited the work of Weber and Hohenegger indi-cating that the viscosity of protoplasm in the root cells of Phaseolus seed-lings was about four times as great at 2.00 and three times as great at 5.00C as at 19 to 220 C. HEILBRUNN (4) showed that the protoplasmic viscos-ity of Amoeba increased greatly in the temperature range of 12 to 150 Cand continued to increase but at a slower rate in the range of temperaturebelow 120 C; the viscosity between 5 and 100 C changed only slightly and

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FIG. 4. A comparison of the effects of gradual and abrupt changes in root tempera-ture on water absorption of sunflower plants. On the curve showing the effect of anabrupt change each point represents the rate of absorption of a different set of fiveplants in which the roots were abruptly cooled from 250 C to the specified temperature.

was about three times as great as at 250 C. The marked increase in viscos-ity of protoplasm at low temperature will certainly create some increase inresistance to the movement of water through the cells. KRAMER (5) foundthat the rate of absorption of sunflower plants in potometers at 6° C roottemperature was increased two and one half times when the roots were ex-cised, and the plants regained their turgidity very rapidly. He concludedthat the root pressure mechanism was not the essential part in absorptionand that the high resistance to water movement across the tissues into thexylem of the root was effective in slowing down the entrance of water. Thesame investigator (6) showed that the rate of water movement through deadsunflower roots attached to a vacuum pump also decreased with decreasing



BOH NINIG AND LUSAN-AN'DANA: WATER ABSORPTION48

temperature, but not to the same extent as in living roots, and stated thatkilling of the cells resulted in collapse of the protoplasts and disorganizationof the strands of cytoplasm passing through the cell walls, thus lesseningthe resistance to water movement.

Although it is certain that the decrease in root temperature will affectthe metabolic activity of the living tissues of the roots, which may in turnaffect the rate of absorption, it is unlikely that the decreased metabolic ac-tivity will significantly influence the rate of absorption in rapidly transpir-ing plants. In the present investigation the plants with their root tempera-ture gradually lowered were exposed to low temperature much longer thanthe plants with their root temperature abruptly changed, and yet the rates

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FIG. 5. A comparison of the effects of gradual and abrupt changes in root tempera-ture on water absorption of tomato plants. On the curve showing the effect of anabrupt change each point represents the rate of absorption of a different set of fourplants in which the roots were abruptly eooled from 250 C to the speeified temperature.

Of absorption fell to practically the sarne level. In general the results of thepresent investigation show that there was greater reduction in the rate ofabsorption at temperatures below 10° C in sunflower (fig. 4) and tomato(fig. 5) than at higher temperatures. This finding seems to agree with thechanges in viscosity of water as discussed previously and shown in figure 7.In bean,7 however, the mlOSt rapid decrease in absorption was in the ternpera-ture range of 10 to 17° C corresponding closely to changes in viscosity ofprotoplasm as discussed above and diagramlmed in figure 7. The curve repre-senting absorption of water by bean plants seems to have the same generalshape as would a curve drawn from figure 7 interrelating the effect of theviscosity of both water and protoplasm. It thus appears that the increase

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PLANT PHYSIOLOGY

in the viscosity of water and of the protoplasm of the root cells are twomajor factors causing the decrease in the rate of water absorption at lowroot temperature.

KRAMER (7) in an extensive study on the rate of absorption of water byvarious species of plants in cold soil found that serious reduction in waterabsorption was not limited to temperatures near freezing, but might occur insome species at temperatures between 10 to 200 C. He observed that ab-sorption was reduced more in species which normally grow in warm soil,such as cotton and watermelon, than in species such as collards and northernpines which normally grow in cooler climates. He also stated that speciesdifferences with respect to the effect of low root temperature are probably

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ture on water absorption of red kidney bean plants. On the curve showing the effect ofan abrupt change each point represents the rate of absorption of a different set of fourplants in which the roots were abruptly cooled from 250 C to the specified temperature.

due to fundamental differences between species in the reaction of proto-plasm to low temperature, possibly differences in the degree of change inviscosity and permeability of protoplasm. In this investigation the resultsalso tend to show the presence of species differences in protoplasmic reac-tion to low temperature. It will be seen from figure 6 that the change inrate of water absorption by bean plants in relation to the change in roottemperature seems to follow more closely the change in viscosity of proto-plasm than the change in viscosity of water (fig. 7), indicating that in thisspecies perhaps a more pronounced change in viscosity of protoplasm ac-companied cooling. However, in sunflower (fig. 4) and tomato (fig. 5) thechange in rate of water absorption appears to follow the change in viscos-

484




ity of water (fig. 7), indicating the probability that only a slight change inviscosity of protoplasm occurred in these species at low temperature andthat certain protoplasmic changes might have occurred which may have re-duced the viscosity changes in protoplasm as a limiting factor in water ab-sorption. The change in rate of water absorption of bean appears to followthe general trend of changes in rate of transpiration of watermelon and cot-ton as reported by KRAMER (7), whereas that of sunflower and tomato tendsto resemble the curve of the rate of transpiration of collards which canthrive in cold soil (7).

An abrupt decrease in root temperature generally resulted in an abruptdecrease in water absorption. Although the rate of absorption under the

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FiG. 7. Change in viscosity of water and protoplasm in relation to change in tem-perature. The curve for viscosity of protoplasm is based on the works of HEILBRUNN(4) and Weber and Hohenegger as cited by KRAMER (6).

gradual change of root temperature was generally higher than the rates un-der abrupt changes, the difference was not very great except when suchabrupt change brought about some permanent injury to the plants as in thecase of bean plants. KRAMER (7) observed that elm, privet and sunflowerplants cooled to one or two degrees above freezing in four or five hours oreven over night wilted severely, while plants cooled over a period of four orfive days wilted only slightly, and the rapidly cooled plants also transpiredless than plants which were cooled slowly to the same temperature. In thepresent investigation, none of the plants showed any sign of wilting whenthe roots were gradually cooled from 25 to 5° C, or when abruptly changedfrom 25 to 20° C and from 25 to 150 C then gradually cooled to 5° C. Bean

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PLANT PHYSIOLOGY

plants wilted severely when the temperature was changed abruptly from 25to 100 C and from 25 to 50 C, while sunflower and tomato plants wiltedseverely when the temperature change was from 25 to 5° C; however, in thefollowing days all the plants gradually recovered from the effect of suchabrupt temperature changes, with the exception of the bean plants in set V(abrupt temperature change from 25 to 50 C) which were very severely in-jured by desiccation and did not appear to recover during the period of theexperiment. BARNEY (2) also reported that the transpiration rate of lob-lolly pine seedlings was generally reduced when the soil temperature wassuddenly decreased, and then the transpiration rate gradually increased to-ward a stable rate at that temperature. In the present work, it will benoted that the rate of absorption of the wilted plants was not much lowerthan that of the turgid plants, and it seems unlikely that the difference inwater absorption at the same temperature between the two groups of plantswas the cause of such greatly contrasting effects of the gradual and abruptchanges in root temperature. It is possible that transpiration rate mayhave had considerable influence in bringing about the contrasting resultsbetween the two groups of plants. The plants subjected to gradual changein root temperature showed a gradual decrease in rate of absorption. Thedecreased rate of water absorption would cause an increasing water deficitand a decrease in vapor pressure of water within the plants, which conse-quently would result in a decrease in the transpiration rate. The plantscould remain at a low root temperature without apparent sign of wiltingprobably because the transpiration rate was continually decreasing as theroot temperature was gradually lowered, and it did not therefore exceed theabsorption rate enough to cause wilting. An abrupt decrease in root tem-perature might have resulted in an unbalanced condition between the rateof absorption and the rate of transpiration. The vapor pressure of waterin the leaves, however, might not have changed as abruptly; and the plantswould continue to transpire near the usual rate while water absorption wasgreatly retarded, thus wilting would have ensued. It might be expectedthat the high transpiration rate would not continue long, and would begreatly reduced as wilting advanced. The wilting of the plants was lesssevere on the following days, and this might be due to the gradual shiftingof the transpiration rate toward an equilibrium with the reduced rate ofwater absorption. However, in the case of bean plants the loss of waterfrom wilted leaves seemed to continue at such an appreciable rate that con-siderable damage from desiccation occurred to the foliage. TAGAWA (11)found that the stomates in bean plants opened more widely when the roottemperature was abruptly lowered from 20 to 100 C, 50 C, and 00 C andremained widely opened while the leaves wilted; no injury from desiccationwas reported probably because the experiment was of short duration. Con-tinued opening of the stomates in the wilting leaves could cause furtherdiminution in water content of leaf cells to the extent that extensive injuryfrom desiccation occurred to the leaves.

486




Although lowering of root temperature will generally result in a decreasein the rate of water absorption, the effect seems to vary with the plant spe-cies. Some plant species appear to be able to withstand the effect of lowroot temperature better than others. CLEMENTS and AMARTIN (3) foundthat the transpiration rate of sunflower plants varied very little with soiltemperatures between 13 and 370 C, but fell rapidly as the temperature wasdecreased below 13° C. SCHROEDER (10) observed that cucumber plantsmight be severely injured by wilting when the root temperature dropped be-low 160 C. RALEIGH (9) found that muskmelon plants growing in solutionculture wilted when the root temperature was lower than 180 C. The re-sults of the present work also indicate that there are differences among theplant species used in the experiments, with respect to ability to withstandthe effect of low root temperature. Bean plants appeared to be more sus-ceptible to injury caused by abrupt decrease in root temperature than theother two species, and sunflower seemed to be less affected than tomato orbean. The rate of absorption of bean plants fell rather rapidly even atrelatively high temperatures, while the absorption rate of sunflower plantsbegan to drop rapidly only at relatively low temperatures.

SummaryThe change in rate of water absorption under gradual and abrupt

changes in root temperature within the range of 25 to 50 C was studied bymeasuring daily the decrease in volume of the culture solution in which sun-flower, tomato and red kidney bean plants were growing. In general, thegradual decrease in temperature resulted in a gradual reduction in the ab-sorption rate, and the abrupt changes in temperature resulted in a corre-spondingly abrupt decrease in the absorption rate to a value slightly belowthe absorption rate of the plants which had been gradually cooled to thatsame temperature.

The absorption rate of bean plants showed greater reduction in the tem-perature range of 10 to 17° C than at lower temperatures and appeared tofollow the trend of reported changes in viscosity of protoplasm, indicatingthe probability that in this species the change in viscosity of the protoplasmis an important factor in controlling water absorption. The absorption rateof sunflower and tomato plants, however, showed greater reduction at tem-peratures below 10° C than at higher temperatures and tended to conformmainly with the reported changes in viscosity of water, indicating that cer-tain protoplasmic changes may have occurred, which might have reducedthe viscosity changes in protoplasm as a limiting factor in water absorption.

All plants with root temperature gradually decreased from 25 to 50 Cshowed no sign of wilting. Tomato and sunflower plants wilted severely onthe day when the temperature was dropped abruptly from 25 to 50 C andgradually recovered on subsequent days with little or no permanent injury;but bean plants wilted severely and were greatly damaged from desiccationof the foliage when the temperature was abruptly dropped from 25 to 100 C

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and from 25 to 5° C, and showed very slow recovery of the uninjured por-tion of the foliage.

DEPARTMENT OF BOTANYTHE OHIO STATE UNIVERSITY

COLUMBUS, OHIO

LITERATURE CITED

1. ARNDT, C. H. Water absorption in the cotton plant as affected by soiland water temperatures. Plant Physiol. 12: 703-720. 1937.

2. BARNEY, C. W. Effects of soil temperature and light intensity on rootgrowth of loblolly pine seedlings. Plant Physiol. 26: 146-163.1951.

3. CLEMENTS, F. E. and MARTIN, E. V. Effect of soil temperature ontranspiration in Helianthus annuus. Plant Physiol. 9: 619-630.1934.

4. HEILBRUNN, L. V. Protoplasmic viscosity of Amoeba at different tem-peratures. Protoplasma 8: 58-64. 1930.

5. KRAMER, P. J. The intake of water through dead root systems and itsrelation to the problem of absorption by transpiring plants. Amer.Jour. Bot. 20: 481-492. 1933.

6. KRAMER, P. J. Root resistance as a cause of decreased water absorp-tion by plants at low temperatures. Plant Physiol. 15: 63-79.1940.

7. KRAMER, P. J. Species differences with respect to water absorption atlow soil temperature. Amer. Jour. Bot. 29: 828-832. 1942.

8. KRAMER, P. J. Plant and Soil Water Relationships. McGraw-HillBook Co., New York. 1949.

9. RALEIGH, G. J. The effect of culture solution temperature on water in-take and wilting of the muskmelon. Proc. Amer. Soc. Hort. Sci.39: 487-488. 1941.

10. SCHROEDER, R. A. The effect of root temperature upon the absorptionof water by the cucumber. Missouri Agr. Exp. Sta. Res. Bull. 309.1939.

11. TAGAWA, T. The influence of the temperature of the culture water onthe water absorption by the root and on the stomatal aperture.Hokkaido Imp. Univ. Jour. Faculty Agr. 39: 271-296. 1937.

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