Assessment of transpiration efficiency in peanut (Arachis hypogaea L.) under drought using a lysimetric system

Transpiration efficiency (TE) is an important trait for drought tolerance in peanut ( Arachis hypogaea L.). The variation in TE was assessed gravimetrically using a long time interval in nine peanut genotypes (Chico, ICGS 44, ICGV 00350, ICGV 86015, ICGV 86031, ICGV 91114, JL 24, TAG 24 and TMV 2) grown in lysimeters under well-watered or drought conditions. Transpiration was measured by regularly weighing the lysimeters, in which the soil surface was mulched with a 2-cm layer of polythene beads. TE in the nine genotypes used varied from 1.4 to 2.9 g kg⁻¹ under well-watered and 1.7 to 2.9 g kg⁻¹ under drought conditions, showing consistent variation in TE among genotypes. A higher TE was found in ICGV 86031 in both well-watered and drought conditions and lower TE was found in TAG-24 under both water regimes. Although total water extraction differed little across genotypes, the pattern of water extraction from the soil profile varied among genotypes. High water extraction within 24 days following stress imposition was negatively related to pod yield ( r ²   =   0.36), and negatively related to water extraction during a subsequent period of 32 days ( r ²   =   0.73). By contrast, the latter, i.e. water extraction during a period corresponding to grain filling (24 to 56 days after flowering) was positively related to pod yield ( r ²   =   0.36). TE was positively correlated with pod weight ( r ²   =   0.30) under drought condition. Our data show that under an intermittent drought regime, TE and water extraction from the soil profile during a period corresponding to pod filling were the most important components.     

Kinetics of drought-induced pathogenesis-related proteins and its physiological significance in white clover leaves

To investigate the responses of pathogenesis-related (PR) proteins to the intensity of drought stress and their physiological significance in white clover ( Trifolium repens L.), the change of enzyme activity and its relationship with some physiological parameters were assessed for 28 days under well-watered (control) and water-deficit conditions. Water-deficit treatment gradually decreased leaf water potential (Ψ_w) to −2.33 MPa at day 28. Dry matter significantly decreased from 21 days of water-deficit treatment, while proline and ammonia concentration increased within 7 days. The increase in PR-protein activity was closely related with the decrease in Ψ_w. The β-1,3-glucanase (EC 3.2.1.39) activity in water-deficit leaves rapidly increased for the first 14 days (Ψ_w ≥ −1.67) and then slightly decreased, while the chitinase (EC 3.2.1.14) and cellulase (EC 3.2.1.4) activity continued to increase throughout the experimental period. The enhanced activation of β-1,3-glucanase, chitinase and cellulase for the period of days 0–14 was significantly ( P  ≤ 0.01) related to the increase of proline and ammonia concentrations. The results indicate that the enhanced activity of β-1,3-glucanase, cellulase and chitinase for the early period might be an act of transient tolerance to drought stress, but the activation of these enzymes during terminal stress might be a drought-stress-induced injurious symptom.     

Effects of ultraviolet-B radiation on plants during mild water stress. IV. The insensitivity of soybean internal water relations to ultraviolet-B radiation

The combined effects of ultraviolet-B (UV-B, 280–320 nm) radiation and water stress were investigated on the water relations of greenhouse grown soybean [ Glycine max (L.) Merr. cv. Essex]. On a weighted (Caldwell 1971), total daily dose basis, plants received either 0 or 3 000 effective J m² UV-B_BE supplied by filtered FS-40 sunlamps. The latter dose simulated the solar UV-B radiation anticipated at College Park, Maryland, U.S.A. (39°N latitude) in the event that the global stratospheric ozone column is reduced by 25%. Plants were either well-watered or preconditioned by drought stress cycles. Diurnal measurements of water potential and stomatal conductance were made on the youngest fully expanded leaf. Various internal water relations parameters were determined for detached leaves. Plants were monitored before, during and after water stress. There were no significant differences in leaf water potential or stomatal conductance between treatments before plants were preconditioned to water stress. However, drought stress resulted in significantly lower midday and afternoon leaf water potentials and lower leaf conductances as compared to well-watered plants. UV-B radiation had no additional effect on leaf water potential; however, UV did result in lower leaf conductances in plants preconditioned to water stress. Turgid weight:dry weight ratio, elastic modulus, bound water and relative water content were unaffected by UV-B radiation. Osmotic potentials at full and zero turgor were significantly lower in the drought stressed treatments as compared to well-watered plants.     

Response of Trifolium repens to UV-B radiation: morphological links to plant productivity and water availability

This study used nine populations of Trifolium repens L. (white clover) to investigate possible relationships between plant morphological attributes and responses to ultraviolet-B (UV-B) radiation. Plants were exposed to 0 or 13.3 kJ·m(-2) ·day(-1) UV-B for 12 weeks. Drought was applied in parallel to these treatments during the last 4 weeks of the experiment to test whether limited moisture availability would alter morphological UV-B responses. UV-B affected plant morphology under well-watered conditions, reducing leaf size by 15%, leaf number by 5% and stolon elongation by 19%. The number of leaf primordia in the apical bud was decreased by 4% under UV-B, and by 12% under drought. In drought-exposed plants, leaf size was reduced by 50%, leaf number by 30% and stolon elongation by 60%. In addition, drought reduced specific leaf area (SLA) by 33% and increased leaf percentage dry mass (PDM) by 40%. UV-B-induced reduction in plant biomass in the T. repens populations was associated with higher plant productivity and this was further linked to larger leaf size as well as to lower PDM. In conclusion, the findings suggest that morphological attributes conferring fast potential growth under productive conditions carry a cost in the form of lower biomass accumulation under UV-B.     

Effects of UV-B radiation on growth and motility of four phytoplankton species

A depletion of the stratospheric ozone layer would result in an increased UV-B radiation, which could have harmful effects on marine organisms. The aim of this study was to determine the effects of an enhanced UV-B radiation (280–320 nin) on the motility and growth in four Swedish phytoplanklon species. The different plankton species were exposed to different doses of UV-B radiation during growth. The growth of the motile dinoflagellates, Gyronidium aureolum Hulburt (Ba 6), and Prorocentrum minimum (Pav.) P. Schiller (Ba 12), was more sensitive to UV-B radiation than the non-motile diatoms Dityhim brightwellii (P. West) Grun (Ba 15) and Phaeodactylum tricornutum Bohlin (Ba 16). One week of UV-B radiation 2 h daily (159 J m⁻² day⁻¹), had a dramatic effect on the growth of the dinoflagellates, while the diatoms were nearly unaffected. On the other hand, when given higher intensity of UV-B radiation (312, 468 and 624 J m⁻² day⁻¹) during the initial phase of growth, also the growth of the diatom, D. brightwellii, was inhibited. Not only the growth but also the swimming speed of the dinoflagellates C. aureolum and P. minimum were affected by UV-B radiation. The speed decreased rapidly after 1–2 h of UV-B radiation (312 J m⁻² day⁻¹), and after longer irradiation times the dinoflagellates lost their motility. G. aureolum exposed to UV-B radiation, regained normal speed after two weeks of visible light.     

Growth and physiological responses of canola (Brassica napus) to three components of global climate change: temperature, carbon dioxide and drought

Elevated CO₂ appears to be a significant factor in global warming, which will likely lead to drought conditions in many areas. Few studies have considered the interactive effects of higher CO₂, temperature and drought on plant growth and physiology. We grew canola ( Brassica napus cv. 45H72) plants under lower (22/18°C) and higher (28/24°C) temperature regimes in controlled-environment chambers at ambient (370 μmol mol⁻¹) and elevated (740 μmol mol⁻¹) CO₂ levels. One half of the plants were watered to field capacity and the other half at wilting point. In three separate experiments, we determined growth, various physiological parameters and content of abscisic acid (ABA), indole-3-acetic acid and ethylene. Drought-stressed plants grown under higher temperature at ambient CO₂ had decreased stem height and diameter, leaf number and area, dry matter, leaf area ratio, shoot/root weight ratio, net CO₂ assimilation and chlorophyll fluorescence. However, these plants had increased specific leaf weight, leaf weight ratio and chlorophyll concentration. Elevated CO₂ generally had the opposite effect, and partially reversed the inhibitory effects of higher temperature and drought on leaf dry weight accumulation. This study showed that higher temperature and drought inhibit many processes but elevated CO₂ partially mitigate some adverse effects. As expected, drought stress increased ABA but higher temperature inhibited the ability of plants to produce ABA in response to drought.     

Use of white clover as an alternative to nitrogen fertiliser for dairy pastures in nitrate vulnerable zones in the UK: productivity, environmental impact and economic considerations

Perennial ryegrass and perennial ryegrass/white clover permanent dairy pastures are compared with respect to productivity, environmental impact and financial costs in nitrate vulnerable zones (NVZ) in the UK. With appropriate management, and utilisation of recommended perennial ryegrass and white clover cultivars, white clover is likely to stabilise at around 20% of total dry matter production in a mixed pasture. Plant dry matter production and milk production from a perennial ryegrass/white clover pasture are likely to be similar to that from a perennial ryegrass pasture receiving 200 kg N ha⁻¹ annum⁻¹ and around 70% of that obtained with perennial ryegrass supplied with 350–400 kg N ha⁻¹ annum⁻¹. Nitrate, phosphorus and methane losses from the system and decreases in biodiversity relative to a grazed indigenous sward are likely to be similar for a perennial ryegrass/white clover pasture and a perennial ryegrass pasture receiving 200 kg N ha⁻¹ annum⁻¹: nitrate leachate from both systems is likely to comply with European legislation. Greenhouse gas emissions resulting from nitrogen (N) fertiliser production would be avoided with the perennial ryegrass/white clover pasture. Within NVZ stocking rate restrictions, white clover can provide the N required by a pasture at a lower financial cost than that incurred by the application of N fertiliser.     

Keeping a positive carbon balance under adverse conditions: responses of photosynthesis and respiration to water stress

Drought and salinity (i.e. soil water stress) are the main environmental factors limiting photosynthesis and respiration and, consequently, plant growth. This review summarizes the current status of knowledge on photosynthesis and respiration under water stress. It is shown that diffusion limitations to photosynthesis under most water stress conditions are predominant, involving decreased mesophyll conductance to CO₂, an important but often neglected process. A general failure of photochemistry and biochemistry, by contrast, can occur only when daily maximum stomatal conductance ( g _s) drops below 0.05–0.10 mol H₂O m⁻² s⁻¹. Because these changes are preceded by increased leaf antioxidant activities ( g _s below 0.15–0.20 mol H₂O m⁻² s⁻¹), it is suggested that metabolic responses to severe drought occur indirectly as a consequence of oxidative stress, rather than as a direct response to water shortage. As for respiration, it is remarkable that the electron partitioning towards the alternative respiration pathway sharply increases at the same g _s threshold, although total respiration rates are less affected. Despite the considerable improvement in the understanding of plant responses to drought, several gaps of knowledge are highlighted which should become research priorities for the near future. These include how respiration and photosynthesis interact at severe stress, what are the boundaries and mechanisms of photosynthetic acclimation to water stress and what are the factors leading to different rates of recovery after a stress period.     

Interaction between sublethal pollution by sulphur dioxide and drought stress. The effect on photosynthetic capacity

Five-year-old Picea abies L. plants were grown in growth cabinets in the presence (3.1 μmol m⁻³) or absence of SO₂. After 5 weeks, the photosynthetic capacity of mature needles produced in the year was the same in both conditions. Trees were then submitted in situ to drought stress by withholding water. The decline of leaf photosynthetic capacity was greatest in the presence of SO₂. Chlorophyll decreased only when trees were submitted to dehydration in the presence of SO₂; however, this al-one could not account for the large decline in photosynthetic capacity observed under that condition. Needle water content was the lowest during dehydration in the presence of SO₂. It is concluded that the critical factor in the interaction between pollution by SO₂ and drought stress is the greater dehydration of the tissue found in stressed plants grown in the presence of SO₂. The large decline in photosynthetic capacity under such conditions might be due to this greater dehydration.     

Growth enhancement and antitranspirant activity following seed treatment with a derivative of 5-hydroxybenzimidazole (Ambiol) in four drought-stressed agricultural species

The biostimulating action of seed treatment with the synthetic antioxidant, Ambiol (2-methyl-4-Edimethylaminomethyl-5-hydroxybenzimidazole dihydrochloride) on subsequent growth and transpiration of seedlings was studied. To study growth and transpiration responses, seeds of four agricultural species, soybean ( Glycine max L.), rapeseed ( Brassica napus L.), winter wheat ( Trilicum aestivum L.) and corn ( Zea mays L.), were soaked in Ambiol for 24 h, using the following concentrations: 0, 0.01, 0.1, 1, 10 and 100 mg 1¹. The subsequent seedlings were subject to simulated soil drought, using computer-automated root misting chambers. The influence of Ambiot on transpiration rate under simulated air drought was studied by growing plants under low humidity in a controlled humidity chamber. Response to Ambiol varied, depending on its concentration, the species used and the environment. Compared to untreated plants, 10 mg 1-⁻¹ Ambiol reduced the mid-day transpiration rate and total daily water usage of soybean by approximately 25%. Under simulated soil drought in the root misting chamber, 10 and 100 mg 1-⁻¹ Ambiol increased growth of rapeseed and soybean by 25–45%, relative to the 0 mg 1-⁻¹ treatment, yielding plants comparable in size to the fully-irrigated controls. However, Ambiol failed to promote growth of two drought-stressed monocotyledons (corn and winter wheat). At 100 mg P. Ambiol inhibited growth of both well-watered wheat and rapeseed, although this inhibition was mitigated by drought.     

Interactions between drought, ABA application and supplemental UV-B in Populus yunnanensis

To test whether drought and ABA application alter the effects of enhanced UV-B on the growth and biomass allocation of Populus yunnanensis Dode, cuttings were grown in pots at two ABA levels, two watering regimes and two UV-B levels for one growth season. Exposure to enhanced UV-B radiation significantly decreased plant growth and photosynthesis under well-watered conditions, but these effects were obscured by drought, which alone caused growth reduction. Drought may contribute to masking the effects of UV-B radiation. The accumulation of UV-B absorbing compounds and the increase of the ABA content induced by drought could reduce the effectiveness of UV-B radiation. ABA application did not have large direct effects on biomass accumulation and allocation. Evidence for interactions between UV-B and ABA was detected for only a few measured traits. Therefore, there was little evidence to support a pivotal role for ABA in regulating a centralized whole plant response to enhanced UV-B. Yet, we recorded an ABA-induced decrease in stomatal conductance (g(s)) and increase in UV-B absorbing compounds and carbon isotope composition (delta(13)C) in response to enhanced UV-B. The allometric analysis revealed that regression models between root and shoot biomass in response to enhanced UV-B are different for plants under well-watered and drought conditions. Enhanced UV-B led to a significant displacement of the allometric regression line under well-watered condition, while allometric trajectories for both UV-B regimes did not differ significantly under drought condition.     

Cold hardiness and water relations parameters in Rhododendron cv. Catawbiense Boursault subjected to drought episodes

We determined whether increase in cold hardiness of Rhododendron cv. Catawbiense Boursault induced by water stress was correlated with changes in tissue water relations. Water content of the growing medium was either maintained near field capacity for the duration of the study or plants were subjected to drought episodes at different times between 15 July and 19 February. Watering during a drought episode was delayed until soil water content decreased below 0.4 m³ m⁻³ then watering was resumed at a level to maintain soil water content between 0.3 and 0.4 m³ m⁻³. Cold hardiness was evaluated in the laboratory with freeze tolerance tests on detached leaves. Water relations parameters were determined using pressure-volume analysis. Exposure to drought episodes increased cold hardiness during the cold acclimation stage in late summer and fall but not during the winter. When water-stressed plants were re-watered to field capacity, the previous gain in cold hardiness gradually disappeared. Water relations parameters correlating with seasonal changes of cold hardiness included dry matter content (r =−0.67). apoplastic water content (r =−0.60), and water potential at the turgor loss point (r = 0.40). Changes of cold hardiness in water-stressed plants in reference to well-watered plants were correlated with changes of all water relations parameters, except for osmotic potential at full turgor (r = 0.13). It is proposed that water stress reduced the hydration of cell walls, thereby increasing their rigidity. Increased rigidity of cell walls could result in a development of greater negative turgor pressures at subfreezing temperatures and therefore increased resistance to freeze dehydration.     

Relative effectiveness and interaction of ultraviolet-B, red and blue light in anthocyanin synthesis of apple fruit

The effect of light on anthocyanin production in apple ( Malus pumila Mill. cv. Jonathan) skin disks was investigated, with prolonged irradiation from different light sources. High fluence rates of white light provided from a xenon lamp were unable to produce large amounts of anthocyanin, and anthocyanin production became saturated at about 30 W m⁻². When UV-B light, provided by a fluorescent lamp which had an emission peak at 312 nm, was combined with the white light, anthocyanin production was synergistically stimulated and increased up to the highest fluence rates of white light tested (44 W m⁻²). This UV-B light was more effective than red and blue light provided from fluorescent lamps, but anthocyanin production became saturated at about 1.7 W m⁻². However, simultaneous irradiation with red and UV-B light had a synergistic effect. UV-B light was also effective in increasing anthocyanin production in whole fruit. Therefore this synergism seemed to have an important role in the development of the desirable red skin color under field light conditions. The results of aminoethoxyvinylglycine treatment suggested that ethylene was not involved in the stimulative effect of UV-B light.     

Oxidative stress and antioxidant content in Chlorella vulgaris after exposure to ultraviolet-B radiation

Growth of Chlorella vulgaris was measured in cultures irradiated with 0, 0.8, 2.0 and 4.4 kJ m² UV-B. Growth expressed as chlorophyll content, declined significantly with increased UV-B dose. Ultraviolet-B irradiated cultures in log phase of growth showed a 284% increase in oxygen radical generation and a 145% increase in lipid peroxidation compared with unirradiated cultures, whereas cultures in the stationary growth phase showed no significant changes in these parameters. The activities of superoxide dismutase and catalase increased by 40 and 500%, respectively, after exposure to a UV-B dose of 4.4 kJ m⁻². Contents of the lipophilic antioxidants α-tocopherol and β-carotene increased by 180 and 63 amol cell⁻¹ respectively, between log and stationary phases in unirradiated cultures; but in UV-B-irradiated cultures these increases were significantly depressed. Photoreducing capacities of chloroplasts were decreased following UV-B irradiation of both isolated chloroplasts and those isolated from irradiated algae. Cells exposed to UV-B exhibited increased size and starch accumulation. These results suggest that oxidative stress conditions related to UV-B exposure trigger an antioxidant response that includes an increase in the activity of the antioxidant enzymes (superoxide dismutase and catalase).     

CO2 enrichment, drought stress and growth of Alaska pea plants (Pisum sativum)

The interaction of CO₂ enrichment and drought on water status and growth of pea plants was investigated. Pisum sativum L. (cv. Alaska) plants were grown from seeds in growth chambers using 350 and 675 μl I1 CO₂, a photon flux density of 600 μmol M-2 S-1, a 16 h photoperiod and a temperature regime of 20/14°C. The drought treatment was started at the beginning of branch initiation and lasted for 9 or 11 days. The water status of the plants was monitored daily by measuring total leaf water potential and stomatal conductance. The total leaf water potential of well-watered plants was not affected by the CO₂ level. Under draughting conditions total leaf water potential decreased, with a slower decrease under the high CO₂ regime, due, at least in part, to reduced stomatal conductance. Upon rewatering, total leaf water potential and stomatal conductance recovered within one day. High CO₂ counteracted the reduction in height and, to some extent, leaf area that developed in low CO₂ unwatered plants. Additional CO₂ had no effect on branch number and did not prevent the complete inhibition of branch development that resulted from drought stress. Removing the drought conditions resulted in a rapid recovery of the internal water status and also a rapid recovery of most, but not all, plant growth parameters.     

Influence of UV-B radiation on developmental changes, ethylene, CO2 flux and polyamines in cv. Doyenne d'Hiver pear shoots grown in vitro

In vitro shoots of cv. Doyenne ďHiver pear ( Pyrus communis L.) were irradiated under controlled environments for 6 h per day at 5 different levels of biologically effective UV-B radiation (UV-B_BE). UV-B exposure caused a progressive increase in apical necrosis above background levels and stimulated leaf abscission. Shoots grown for 2 weeks at 7. 8 mol m⁻² day ⁻¹ of photosynthetic photon flux (PPF) and treated with 8. 4 or 12. 0 kJ m⁻² day ⁻¹ UV-B_BE produced up to 4 times more ethylene than those given 2. 2 or 5. 1 kJ m⁻² day⁻¹ UV-B_BE or untreated controls. Exposure of shoots to 12 kJ m⁻² day ⁻¹ of UV-B_BE caused an increase in free putreseine content after 4 to 14 days of irradiation. Shoots showed a decrease in CO₂ uptake after 3 days of UV-B: thereafter, they appeared to recover their photosynthetic capacity. Under typical PPF conditions used in micropropagation (90 μmol m⁻² S⁻¹). 8. 4 kJ m⁻² day ⁻¹ of UV-B radiation was injurious to realatively tender tissues of in vitro pear shoots: increasing the level of UV-B_BE to 12 kJ m⁻² day⁻¹ produced even more adverse effects.     

Photobiological properties of the inhibition of etiolated Arabidopsis seedling growth by ultraviolet-B irradiation

Alteration of 'normal' levels of ultraviolet-B light (UV-B, 280–320 nm) can affect plant chemical composition as well as growth; however, little is known about how plants perceive UV-B light. We have carried out fluence response curves, and demonstrated that the growth inhibition of etiolated Arabidopsis thaliana seedlings by low fluence UV light is specific to UV-B and not UV-A (320–390 nm). The response shows reciprocity between duration and intensity, at least over a limited range, and thus depends only on photon fluence and not on photon flux. The action spectrum for this response indicates a peak of maximum effectiveness at 290 nm, and response spectra at different fluences indicate that the most effective wavelength at 30 000  µ mol m^–2 is 290 nm, whereas 300 nm light was the most effective at 100 000  µ mol m^–2. This response occurs in mutant seedlings deficient in cryptochrome, phytochrome or phototropin, suggesting that none of the known photoreceptors is the major UV-B photoreceptor. Some null mutants in DNA repair enzymes show hypersensitivity to UV-B, suggesting that even at low fluence rates, direct damage to DNA may be one component of the response to UV-B.     

Effects of host plant drought stress on the performance of the bird cherry-oat aphid, Rhopalosiphum padi (L.): a mechanistic analysis

Abstract.  1. The growth (increase in height and leaf number) of four grass species was reduced by a −0.5 MPa drought stress, but the performance of an associated herbivore, Rhopalosiphum padi (L.), was not affected consistently. The intrinsic rate of increase of R. padi was reduced by drought stress on three grass species, including Dactylis glomerata (L.), but was unaffected on Arrhenatherum elatius (L.). Therefore, there is no general relationship in the effect of plant drought on an insect herbivore, even among closely related host plant species.
2. Drought stress increased the quality of plant phloem sap, as indicated by increased sieve element osmotic pressure and essential amino acid concentrations. Thus, diet quality could not account for the reduced performance of R. padi under drought stress. The concentration of essential amino acids in the phloem of well-watered A. elatius was, however, lower than that of well-watered D. glomerata , correlating with the decreased performance of aphids on well-watered A. elatius .
3. There were no differences in aphid feeding duration between watering treatments or plant species but sap ingestion rates were reduced significantly under drought stress.
4. Using the measure of dietary amino acid concentrations and the estimate of sap ingestion, the essential amino acid flux through aphids was calculated. Compared with the flux through aphids feeding on well-watered D. glomerata , there was a reduction in aphids feeding on drought-stressed D. glomerata and drought-stressed A. elatius due to lower sap ingestion rates. The flux through aphids on well-watered A. elatius was also reduced due to low phloem essential amino acid concentrations. Thus, the performance of an aphid is correlated with the availability and accessibility of essential amino acids.     

Response of 19 cultivars of soybeans to ultraviolet-B irradiance

Nineteen soybean cultivars were grown for four weeks in controlled environmental chambers with artificial daylight supplemented by five UV-B irradiance regimes to determine the range of growth and development responses of seedlings. Data from nine plant characteristics were assessed: leaf area, dry weight of leaves, stems and roots, total plant dry weight, height, ratio of roots to shoots and leaf area to weight and rating of leaves for damage. Significant differences were observed in the responses noted. Stunting, leaf chlorosis and loss of apical dominance were three general symptoms apparent on all cultivars which received UV-B irradiance. Varying degrees of reduced leaf area and dry weight of the plants and altered ratios of weights of leaves per unit area and weight of roots to shoots were also found. It was concluded that different soybean cultivars demonstrate a marked difference in sensitivity to UV-B radiation under the artificial conditions of controlled environmental growth chambers and this may indicate a genetic basis for variability in sensitivity of soybean cultivars to this waveband. However, the sensitivity to UV-B radiation was increased by the lower than normal photon fluence of photosynthetically active radiation (225 μE m⁻² s⁻¹).     

Differential response of Trifolium species to 4-(2,4-dichlorophenoxy)butyric acid treatments

The differential response of white clover ( Trifolium repens L. cv. Regal Ladino) and berseem clover ( Trifolium alexandrinum L. cv. Mississippi ecotype) was investigated by treating greenhouse cultured plants with 4-(2,4-dichlorophenoxy)butyric acid (2,4-DB). Berseem clover plants were significantly injured by a treatment concentration of 0.6 kg ha^-1 of 2,4-DB, whereas white clover plants were not injured by treatment levels below 2.4 kg ha^-1. The metabolism of 2,4-DB in cell suspension cultures of white clover and berseem clover was investigated using [ring-¹⁴C]-2,4-DB and non-labeled 2,4-DB. White clover cell cultures metabolized ca 4-fold more 2,4-DB than berseem cultures over a 44-h treatment period. The decrease in berseem cell population was 4-fold greater than the decrease in white clover cell population in response to the 8 μ M 2,4-DB treatment. The herbicide and its [ring-¹⁴C]-labeled metabolites were isolated from treated cells and medium after 44 h by partition and thin-layer chromatography. White clover cells metabolized 90% of the [¹⁴C]-2,4-DB and berseem clover cells metabolized 22% of the herbicide. The major portion of the radiolabel was in the glycoside fractions from extracts of both species. The differential response of Trifolium species to 2,4-DB is implied to be due to the differential rate of 2,4-DB metabolism to a glycoside by the clover plants.