Root Cytokinin Research Articles

A higher plant seven-transmembrane receptor that influences sensitivity to cytokinins

Background: All organisms perceive and respond to a profusion of environmental and endogenous signals that influence growth, development and behaviour. The G-protein signalling pathway is a highly conserved mechanism for transducing extracellular signals, and the superfamily of receptors that have seven transmembrane (7TM) domains is a primary element of this pathway. Evidence that heterotrimeric G proteins are involved in signal transduction in plants is accumulating, prompting speculation that plant 7TM receptors might exist.Results: Using information in the dbEST database of expressed sequence tags, we isolated an Arabidopsis thaliana gene (GCR1) that encodes a protein with seven predicted membrane-spanning domains and other features characteristic of 7TM receptors. The protein shows 18–23% amino-acid identity (46–53% similarity) to, and good colinear alignment with, 7TM receptors from three different families. Its highest sequence identity is with the Dictyostelium cAMP receptors. GCR1 is expressed at very low levels in the roots, stems and leaves of Arabidopsis; it is a single-copy gene which maps close to the restriction fragment length polymorphism marker m291 on chromosome 5. Transgenic Arabidopsis expressing antisense GCR1 under the control of the constitutive cauliflower mosaic virus 35S promoter have reduced sensitivity to cytokinins in roots and shoots, yet respond normally to all other plant hormones. This suggests a functional role for GCR1 in cytokinin signal transduction.Conclusions:GCR1 encodes the first 7TM receptor homologue identified in higher plants and is involved in cytokinin signal transduction. This discovery suggests that 7TM receptors are ancient and predate the divergence of plants and animals.

Fast changes in growth rate and cytokinin content of the shoot following rapid cooling of roots of wheat seedling

Changes in the concentration of cytokinins were studied following root cooling. Simultaneously, the growth rate of the second leaf was monitored with a highly sensitive growth sensor attached to its tip. Cytokinins were separated by thin layer chromatography and immunoassayed using antibodies to zeatin riboside. The extension rate of the second leaf decreased within 15 minutes of cooling the nutrient medium from 24 °C to 4 °C. The concentration of cytokinins in shoots decreased with similar rapidity. In contrast cytokinins in roots increased slightly during the initial period of cooling before declining. The sharp decrease in cytokinin concentrations in shoots 15 minutes after cooling of roots may contribute to the abrupt inhibition of shoot growth.

Conditional transgenic expression of the ipt gene indicates a function for cytokinins in paracrine signaling in whole tobacco plants

This study investigated whether an increased production of the plant hormone cytokinin in roots, the main site of its synthesis and putative signaling organ, can influence developmental events, such as growth of axillary shoot meristems or leaf senescence, in the plant shoot. To this end, transgenic tobacco plants (Nicotiana tabacum L.) were generated that conditionally overproduce cytokinins. These plants harbour the ipt gene under the transcriptional control of a modified 35S promoter that is repressed in plants with high titers of tetracycline repressor protein. De-repression of transcription led to a rapid more than 50-fold increase of hormone concentration. The time course of changes in the steady-state levels of 16 different cytokinin metabolites, as a consequence of IPT enzyme activity, was monitored in different plant tissues. Zeatin riboside was the first and most dramatically increased product; zeatin, dihydrozeatin and glucosides accumulated later. The consequences of enhanced cytokinin synthesis remained mainly restricted to the site of hormone production. For example, de-repression of ipt gene transcription in lateral buds caused the growth of single buds only at the site of tetracycline application. In reciprocal grafts of transgenic plants with wild-type plants, no biological cytokinin effects, i.e. growth of lateral shoot meristems or sequential leaf senescence, were observed in the non-transgenic plant part. Also, the increase in steady-state levels of cytokinins remained restricted mainly to the transgenic part, despite a specific increase of the zeatin riboside concentration in the transpiration stream. These results question the role of cytokinins as a long-range root-to-shoot signal in correlative control of apical dominance and sequential leaf senescence of tobacco, and support the assumption that this hormone is relevant to paracrine signaling.

Yield formation in rice in response to drainage and nitrogen application

Four field experiments were carried out in Tamil Nadu, India, to determine effects of internal drainage (percolation rate) on growth and yield formation in rice, in interaction with nitrogen (N) management. Grain yield response to drainage was positive at all N levels in all experiments. Yields (averaged over N treatments) increased by 14% (Expt. 1), 10% (Expt. 2), 25% (Expt. 3) and 22% (Expt. 4) in response to drainage. Higher yield responses to N application were found under well drained than poorly drained soil conditions. In late-season crops under poor drainage, grain yields decreased in response to high doses of N. The positive yield response to drainage was always associated with increased number of filled grains per panicle (+ 13 to + 17%). increased translocation of stored reserves, increased root biomass at harvest ( + 20 to +36%), increased N concentration in roots and increased root activity (as measured by α-naphthylamine oxidation). Of the yield increment resulting from drainage, 25 to 35% could be accounted for by increased translocation of stem reserves in Expt. 1, 5 to 25% in Expt. 2 and 30 to 40% in Expt. 3. Yield increase in response to drainage was not always associated with more green leaf area, leaf longevity, leaf N content, crop N uptake, nor with a larger dry matter production at given levels of the leaf N pool and global radiation, as quantified with the help of a calibration factor ƒ v.. A literature review is presented leading to two hypotheses to explain the effect of drainage on yield: (1) a possible potassium deficiency is expressed more in reduced (low-redox) soil due to a lowering of K availability per se; or to increased K demand associated with maintaining the root's ‘oxidizing power’; (2) improved root condition resulting from an increase in soil redox potential induces a prolonged synthesis and transport of cytokinins in roots, resulting in an extended photosynthetic activity, enhanced sink strength of grains and more translocation and deposition of carbohydrates in the grains. These hypotheses are not mutually exclusive. They were not explicitly tested in this study.

Root cytokinins, acid phosphatase and nodule activity in drought-stressed mycorrhizal or nitrogen-fixing alfalfa plants

The objective of this research was to study the root and nodule responses to drought of mycorrhizal or nitrogen-fixing alfalfa plants in relation to root cytokinin levels. Double symbiotic plants showed significantly higher root and nodule activities during drought that may be related to smaller decreases in cytokinin content of mycorrhizal plants.

Root Cytokinin Level in Relation to Heat Tolerance of Phaseolus acutifolius and Phaseolus vulgaris

Heat stress influences plant growth and crop production. This research was conducted to determine the importance of root cytokinin level in plant heat tolerance. Tepary bean (Phaseolus acutifolius Gray) and heat tolerant (GNUI 59) and heat sensitive (BBL 47) genotypes of common bean (P. vulgaris L.) were used to determine root and leaf cytokinin levels under 45°C air temperature and either 45 or 25°C root temperature. The concentrations of root zeatin riboside (ZR) were about two‐ to three‐fold higher in tepary bean and GNUI 59 than in BBL 47 under all conditions. Root isopentenyl adenosine ([9R]iP) was much lower than root ZR. High shoot/root temperature (45/45°C) for 5 reduced the root ZR and [gR]iP concentrations in all three genotypes. Heat treatments did not alter leaf ZR and [9R]iP. Pretreatment with exogenous ZR (1 μM) did not show any alleviation from heat injury in terms of photosynthetic activity, even though endogenous ZR levels increased, compared with those without ZR pretreatment. Reciprocal grafting of BBL 47 shoots on tepary roots increased leaf ZR level at high temperature but did not increase photosynthetic activity. Results suggest that root ZR level is not important in determining plant heat tolerance.

The reactivation of tree peony ( Paeonia suffruticosa Andr.) vitroplants by chilling is correlated with modifications of abscisic acid, auxin and cytokinin levels

In vitro rooting of Paeonia suffruticosa was correlated with an arrest of shoot development. A chilling treatment reactivated shoot development of vitroplants, and this was accompanied by an increase of mitotic activity in their apices. Chilling resulted also in the disappearance of abscisic acid from vitroshoots and the accumulation of indole-3-acetic acid in shoots and cytokinins in roots. The winter cold had a similar effect on contents of these three hormones in the mother plant in vivo; however, in contrast to the roots of the mother plant, auxin was never detected in roots of the vitroplants and cytokinin accumulation in chilled vitroshoots was almost 20-fold lower than that in buds in vivo. Despite the reactivation of vitroplants by chilling, shoots did not elongate, developed only one or two leaves and no axillary buds were formed. This limited reactivation was correlated with a rapid decrease of cytokinin and indole-3-acetic acid contents and with a new abscisic acid accumulation in vitroplants. It is considered that, during in vitro culture, explants lost their capacity to regulate hormone metabolism, and this metabolic perturbation was also maintained after transfer ex vitro.

Mineral Concentrations and Cytokinin Activity in the Xylem Exudate of Grafted Watermelons as Affected by Rootstocks and Crop Load.

Mineral concentrations and cytokinin activity in the xylem exudate of watermelon grafted to squash (Cucurbita maxima × C. moschata 'Shintosa') were compared with those of watermelon grafted to bottle gourd (Lagenaria siceraria Standl. 'Sakigake') and non-grafted watermelon in relation to fruit quality. In addition, the effect of fruit number on mineral concentrations and cytokinin activity in the xylem exudate was also investigated.1. Watermelon plants grafted to squash (W/squash) showed more vigorous growth than watermelon plants grafted to bottle gourd (W/bottle gourd) and non-grafted watermelon. The node order of first pistillate flower was lowered by grafting to bottle gourd. The flesh of the mature fruit was firmer and the textural quality was lower in W/squash than they were in W/bottle gourd and non-grafted watermelon plants. Hence, grafting to squash plants lowered melon quality in comparison to melons from the W/bottle gourd graft combination or to those from the non-grafted watermelon plants.2. Concentrations of NO3-N, P, K, Ca and Mg in the xylem exudate collected at 3 different growth stages: at anthesis, 14 days after pollination and fruit mature, were determined. The mineral absorption rate of W/squash was faster throughout the growth period in comparison with the rate of W/bottle gourd; whereas, the mineral absorption rate of W/bottle gourd was slower as the fruit matured.3. The effect of crop load on mineral concentrations in exudates of both W/bottle gourd and W/squash at 14 days after pollination was undetectable. But as the fruit matured, the mineral concentrations in the exudate of W/bottle gourd seemed to decrease more so in the 2-fruited treatment than those in the 1-fruited one; there was no significant difference in mineral contents between the 1- and 2-fruited treatments of W/squash.4. The cytokinin activity, mainly ribosylzeatin, in xylem exudate collected at 14 days after pollination was significantly higher in W/squash than in W/bottle gourd scion/stock combination and in the non-grafted watermelon. Two-fruited treatment decreased the cytokinin activity in both rootstocks. These results suggest that the root cytokinin is rapidly affected by the assimilate supply translocated from the shoot.5. It was concluded that the functions of W/squash roots, for example, mineral uptake and cytokinin synthesis, were considered more active and tolerable to heavier crop load than those of W/bottle gourd and non-grafted watermelon roots.

The effects of endogenous cytokinin content on benzyladenineenhanced nitrate reductase induction

Foliar application of benzyladenine (BA) has been shown to enhance nitrate‐dependent induction of nitrate reductase (NR; EC 1.6.6.1) in etiolated wheat (Triticum aestivum L.) seedlings. Whether similar enhancement occurs in light‐grown plants, or whether endogenous cytokinin content affects this enhancement is unknown. Since the cytokinin content of etiolated plants probably differs from that of light‐grown seedlings, the NR response of each to exogenous root‐ or shoot‐applied BA in wheat (cv. Red Bob) was examined. Endogenous cytokinins present in untreated control tissues prior to BA application and changes that occurred after a 22 h (12 h dark followed by 10 h of light) period were determined using a combined HPLC‐immunoassay method. Shoot application of BA enhanced the induction of NR in etiolated seedlings in a concentration‐dependent manner but failed to enhance NR induction in light‐grown plants. Root‐applied BA enhanced NR induction in both etiolated and light‐grown seedlings. Endogenous root cytokinin levels were similar in both etiolated and light‐grown plants. In contrast, shoots of 6 day‐old light‐grown seedlings contained at least 20 times the amount of total cytokinins measured in shoots from etiolated plants of the same age. Total cytokinin content of the light‐grown plants diminished after the 22‐h period while that measured in etiolated seedlings increased. The responsiveness of seedlings to BA was correlated with endogenous cytokinin levels in that enhancement of NR induction by exogenous BA was low in tissues which contained high concentrations of cytokinin at the time of BA application. These results may prove useful in interpretation of gene responses to exogenous plant growth regulators.

Nitrate-Regulated Growth and Cytokinin Responses in Seminal Roots of Barley

The influence of nitrate availability on growth of seminal roots, and root cytokinin levels, was studied in barley (Hordeum vulgare L. cv Golf). Nitrate was continuously supplied to initially N-starved seedlings at relative addition rates (RA) of 0.03 to 0.21 per day (standard cultures) or at RA 0.09 per day in split root cultures with the nitrate additions distributed in ratios of 100:0 or 80:20 to the two subroots. Data were collected both during a phase of acclimation (first 10 days of N additions) and in the acclimated stage (>10 days after onset of N additions). Limitation of whole-plant growth was observed at RA <0.15 per day. The lateral root frequency increased with RA in plants of equal chronological age. However, the lateral root frequency was related to root size rather than to RA; roots of uneven age but having comparable total root lengths also had comparable lateral root frequencies. Growth of individual subroots in split root systems during acclimation was proportional to the fraction of the total N addition that was fed to the root. All subroots had comparable relative growth rates in acclimated plants, and their lateral root frequency correlated with total root length in the same manner as in standard cultures. Onset of N additions in a 80:20 split root culture resulted in doublings of zeatin riboside (ZR) levels in shoots and in the "80" root, whereas the response of the "20" root was small. No effect of perturbed nitrate availability on xylem translocation of ZR was observed. The ZR levels remained higher in the "80" root during acclimation but returned to the level of the "20" root after acclimation. Root cytokinin levels and xylem translocation in acclimated standard cultures were unaffected by RA in the lower range but increased at high RA. Arguments for involvement of cytokinins in the nitrate-regulated growth response are discussed.

Early responses of drought-resistant and -susceptible tomato plants subjected to water stress

Three-week-old seedlings of one drought-susceptible tomato cultivar (Lycopersicon esculentum cv. “New Yorker”) and two drought-resistant species of tomato (Solanum pennellii andLycopersicon chilense) were subjected to various degrees of PEG 8000-induced water stress from −0.017 to −1.0 MPa for a duration of 24 h so that their early responses to water stress could be compared. Such a comparison would determine if there was a relationship to root cytokinin levels following sudden induction of water stress in the drought-resistant species. Transpiration rates of leaves were monitored throughout the 24-h period, shoots were evaluated for leaf water potential (LWP), and roots were extracted for levels oft-zeatin riboside (t-ZR) and dihydrozeatin riboside (DHZR) using a monoclonal antibody enzyme immunoassay. Transpiration rates were evaluated gravimetrically by difference every 6 h up to 24 h. Transpiration rate decreased with increasing PEG levels and passage of time in all three species, measured at 6 and 12 h, logarithmically in the case of the twoLycopersicon species and linearly in the case ofSolanum. From 12–18 h (while plants were in darkness), transpiration rate was a function of the level of PEG only and not time in all three species. When light resumed from 18–24 h, only 5.pennellii showed no further decrease in transpiration rate over time with increasing PEG. Drought-susceptibleL. esculentum had a stronger linear decrease in LWP with increasing PEG 8000 concentration than the other two species.L. esculentum also had a higher initial transpiration rate than did either of the drought-resistant species. The two drought-resistant species showed less change in LWP with 5.pennellii having a small decrease andL. chilense having little change. OnlyS. pennellii exhibited a decrease in roott-ZR levels, which may imply a role for root cytokinin within the first 24-h exposure to water stress in this species.L. esculentum exhibited no change in roott-ZR. The levels oft-ZR inL. chilense were less than that ofL. esculentum but showed only a slight decrease with increasing PEG.S. pennellii andL. chilense, although both drought-resistant tomato species, showed different patterns of response with respect to pattern of decline in transpiration rate, LWP, and roott-ZR levels.

Acidification stress of root environment as related to endogenous cytokinins and gibberellins in oak seedlings

Activity of endogenous cytokinins and gibberellins was determined in the shoots and roots of oak seedlings following the submersion of roots in water acidified with sulphuric add to pH 3.0, 2.5, 2.0, 1.7 and 5.6 (control) for 6 and 72 h. After 6 h the cytokinins increased in the shoots and in roots at pH 2.5 and 2.0, respectively. None of the tested pH values decreased the cytokinin activity. The acidification of solutions exhibited no remarkable influence on gibberellin activity. Only slight stimulation of the gibberellin activity was exhibited at pH 2.0. After 72 h a slight decrease in the activity of cytokinins was observed in the shoots at pH 2.5, remarkable decrease was revealed at pH 2.0 and 1.7. Nevertheless, in the roots stimulation of cytokinin activity appeared up to pH 2.0 (the highest at pH 2.5), and significant decrease was shown at pH 1.7. The activity of gibberellins in shoots showed similar dependence of changes as the activity of cytokinins in roots. Acidification stress exerted no influence on the gibberellin. activity in roots.

On the biogenesis of cytokinins in roots of Phaseolus vulgaris

Roots of intact bean plants were supplied with [(14)C]adenine by pulse-chase experiments. The rate of incorporation of radioactivity into tRNA and oligonucleotides of roots as well as the content of radioactive labeled cytokinin nucleotides in these RNA fractions were determined. On the average, 1/70 of the radioactivity incorporated into tRNA was localized in N(6)(Δ(2)isopentenyl)adenosine. The half life of tRNA was estimated to be 65-70 h. Shortly after the pulse period, oligonucleotides contained zeatin riboside at a ratio of 1:800, on the basis of radioactivity. The half life of these oligonucleotides was determined to be about 8 h. The main free radioactive cytokinin of roots and leaves was zeatin. Comparing the rate of degradation of (14)C-labeled tRNA and the oligonucleotides of roots and the rate of appearance of radioactive cytokinins in roots and leaves, we found strong indications for their dependency. The results contradict the hypothesis of de novo synthesis of cytokinins in roots of intact bean plants.

Effect of Disbudding on Root Cytokinin Export and Leaf Senescence in Tomato and Tobacco

Bud removal and decapitation (disbudding) of plants of tomato (Lycopersicon esculentum Miller) and tobacco (Nicotiana tabacum L.) resulted in an increase in both the concentration and the total flux of cytokinin in bleeding xylem sap. There was a retardation in the rate of chlorophyll loss from leaves of disbudded plants of both species. Ultrastructural examination of the chloroplasts of disbudded tomato plants revealed a maintenance of chloroplast integrity, compared with chloroplasts from control plants. It is suggested that the delaying of leaf senescence observed in disbudded plants is due to an increased availability of cytokinin to these leaves.

The Influence of the Root System on the Metabolism of 2,3[3H]-Gibberellin A20 by Shoots of Phaseolus coccineus L. var. Prizewinner

Summary Evidence was sought to help substantiate the hypothesis of a root-shoot interlinked gibberellin biosynthetic pathway in light-grown seedlings of Phaseolus coccineus . Application of [ 3 H]-gibberellin A 20 to apical buds of seedlings of P. coccineus resulted in negligible transport of this compound into roots. A possible role for root-produced cytokinins as factors regulating gibberellin metabolism in apical buds of P. coccineus was investigated by examining the effect of root excision and cytokinin application on [ 3 H]-gibberellin A 20 metabolism in shoot apical buds of this plant. The results obtained provided little support for roots as a major site of metabolism of shoot-produced gibberellin A 20 or for an involvement of root-synthesized cytokinins as regulators of gibberellin A 20 metabolism in apical buds.

The Role of Cytokinins in Tuber Development in Dahlia variabilis

Summary Fibrous root cytokinin activIty in Dahlia variabilis was examined in relation to tuber development both under long-days and short-days. Root cytokinin activity increased during tuber initiation but decreased thereafter. The initial high cytokinin levels may benefit early events of tuberization, namely cell division and differentiation, of tuberforming roots which were placed immediately above roots at the cotyledonary node. Subsequent events of tuber growth, accumulation of dry matter, may have benefited by low root cytokinin levels which influence the shoot hormonal composition leading to preferential translocation of dry matter into tubers.

The Importance of Roots in Regulating the Senescence of Soybean Primary Leaves

AbstractThe role of roots in regulating primary leaf senescence of 14‐day‐old soybean seedlings was investigated. Compared with intact seedlings, the senescence of primary leaves is accelerated by removal of the root system but delayed if apical bud and the first trifoliate leaf are removed. No difference in senescence was found between intact seedlings and seedlings without roots, apical bud, and first trifoliate leaf. Lateral roots seem to play a predominant role in regulating primary leaf senescence. However, neither root nodules nor primary root play any function in senescence.Results indicate that benzyladenine (BA) at optimal concentration (2 mg/1) completely replaces the roots to prevent the senescence of primary leaves, whereas gibberellic acid (GA) and abscisic acid (ABA) accelerate. The effect of indole‐3‐acetic acid (IAA) to replace roots in preventing senescence depends on the season the young seedlings are grown. Additional, though indirect, information of acropetal transport of ABA is provided.In conclusion, it seems that cytokinins in lateral roots play a predominant role in leaf senescence and the normal supply of root cytokinins is important in leaf metabolism.

Cytokinin and inhibitor activity in roots and stems of sugar maple seedlings through the dormant season

Cytokinin and inhibitor activity were followed in the roots and stems of 2-year-old maple seedlings exposed to normal overwintering conditions from early November through April. The production of new roots was also monitored, and all measurements were repeated with dormant control seedlings placed in a greenhouse from the beginning of the experiment and provided with warm temperatures and extended photoperiods. The level of cytokinin-like substances increased in the roots and stems of the outside group in March after a large surge in root activity that began in late February and continued through April. Cytokinins peaked just before budbreak, with the highest levels detected in the lateral roots. Three bands of activity were present on the chromatograms at Rfs 0.1, 0.6, and 0.9. Large cytokinin peaks were also observed in the non-chilled seedlings and were associated with two periods of active root growth and abnormal bud flush in the fall and spring. Abscisic-acid-like activity was detected in the roots and stems of both greenhouse and exposed seedlings, but it remained essentially constant in the stems. Changes that occurred in the roots were not associated with emergence of the buds from the dormant condition.

Redistribution of Endogenous Cytokinins in Geotropically Stimulated Ribes nigrum Roots and Shoots

Summary The occurence of cytokinins in extracts of both roots and shoots of Ribes nigrum was demonstrated by soybean callus bioassay and gas-liquid chromatography. Quantitative estimation of cytokinin activity in extracts from upper and lower halves of both roots and shoots which have been kept horizontal, and showed positive and negative curvatures after 4 h respectively, indicated a redistribution of endogenous cytokinins during geotropic stimulation. The ratio of the total cytokinins in roots in upper and lower halves was 2.9 : 1, whereas in shoots it was found to be 1 : 3.5, in favour of upper and lower halves respectively. In wheat roots, IAA was inhibitory to root-elongation within the first one and a half hours, whereas kinetin had a stimulatory effect during the same time. ABA was inhibitory at all times although the lowest concentration of 0.001 mg/l showed a non-significant stimulatory effect. GA 3 stimulated root-elongation of wheat seedlings within 2 hours. In wheat coleoptiles, IAA was inhibitory within the first hour, whereas GA 3 showed a stimulatory effect during the first one and a half hours. Kinetin had an inhibitory effect in the first hour, followed by a stimulatory effect after 2 hours. 100 mg/l had an inhibitory effect at all times. ABA inhibited the coleoptile at higher concentrations, whereas the lowest concentrations of 0.001 and 0.01 mg/l gave rise of a slight increase in coleoptile growth. This growth was less than that of the controls. 60 and 50% of 10 mg/1 kinetin treated roots and shoots of intact Vicia faba seedlings developed positive and negative geotropic curvatures respectively, whereas 50 and 100 mg/l kinetin prevented the process in both organs. These results suggest the possibility of a participation of endogenous cytokinins in addition to auxin in the development of both positive and negative geotropic curvatures.

Physiological Responses of Apple Trees to Supraoptimal Root Temperature

AbstractUngrafted apple rootstocks were grown in sand cultures at constant root temperatures between 20°C to 40°C. Temperatures of 30°C and above reduced root and shoot growth. Serious damage to the leaves occurred at 35°C and above.The O2 consumption, CO2 evolution and respiratory quotient (RQ) of the roots showed maximum values at 35°C.Different rootstock cultivars varied greatly in their susceptibility to damage by supraoptimal root temperatures apparently due to anaerobic respiration. The more susceptible ones differed from resistant types in the larger amount of ethanol they accumulated in their roots at supraoptimal root temperature, and the more severe reduction in the malic acid content of the roots at such temperature. Acetaldehyde was also found in roots and leaves at supraoptimal root temperatures, whereas the organic acid content of the leaves tended to decrease.Supraoptimal root temperature also caused a reduction of cytokinins in both roots and leaves accompanied by a reduction in the leaf chlorophyll content. This could be prevented by the application of kinetin or benzyladenine to the leaves.In a short experiment a rise in root temperature up to 40°C caused an increase in transpiration and a decrease in the resistance of the leaves to the passage of water vapor, whereas in prolonged experiments transpiration reached a maximum and leaf resistance a minimum at 30°C. The leaf water potential increased also with increasing root temperature. Leaf temperature increased with increasing root temperature, irrespective of increasing or decreasing transpiration rates.