Effects of acifluorfen on auxin translocation in bean seedlings

The effect of 5-(2-chloro-4-(trifluoromethyl)phenoxy)-2-nitrobenzoic acid (acifluorfen) on the translocation of the¹⁴C-labeled auxins 2,4,5-trichlorophenoxyacetic acid (2,4,5-T-1-¹⁴C) and indole-3-acetic acid (IAA-1-¹⁴C) was determined. The auxins and acifluorfen were injected into the stem at the cotyledonary node of 9-day-old bean (Phaseolus vulgaris L. cv Tenderpod) seedlings. The plants were harvested 4 h after treatment and analyses of¹⁴C were made of various plant parts. Acifluorfen increased 2,4,5-T,-1-¹⁴C translocation out of the treated area and especially into the large primary leaves. This translocation pattern is indicative of apoplastic translocation and suggests that acifluorfen inhibited vein loading of the auxins. Acifluorfen affected auxin translocation in the dark as effectively as in the light even though the herbicidal effects of acifluorfen are known to be expressed only after light treatment.Journal article no. 4403 of the Agric. Exp. Stn., Oklahoma State Univ.     

The effect ofAcremonium coenophialum on the growth and nematode infestation of tall fescue

The presence of the endophytic fungusAcremonium coenophialum Morgan-Jones et Gams in tall fescue (Festuca arundinacea Schreb.) induces toxicity when this grass is grazed by cattle; however, there is evidence that removing the endophyte reduces the stand vigor and longevity of fescue. A field trial was conducted to determine the effects of water supply and the presence of the endophytic fungus on plant growth, drought tolerance, and soil nematode populations in Kentucky 31 tall fescue. The design included two factors, level of endophyte infection (0 and 75%) and irrigation regime (none, low, and high). Where water deficits occurred, herbage yield and leaf area were lower, and percentage dead tissue and canopy minus air temperature were greater in endophyte-free compared with endophyte-infected fescue. Soil populations ofPratylenchus scribneri andTylenchorhynchus acutus were substantially higher in the noninfected than in the endophyte-infected plots. The endophyte apparently confers drought tolerance to Kentucky 31 tall fescue, and this effect may be at least partially mediated through enhanced resistance to soil-borne nematodes.Published with the approval of the Director of the Ark. Agric. Exp. Stn.     

Ferredoxin-NADP+ reductase,a nuclearly-coded enzyme unaffected by tentoxin treatment

Previous studies in our laboratory have shown that tentoxin prevents the incorporation of polyphenol oxidase (PPO), a nuclearly-coded protein, into the chloroplasts of sensitive species. In this study, we show, by comparison of electrophoretically separated isozymes, that ferredoxin-NADP⁺ reductase (FNR) is nuclearly coded in Nicotiana. Electrophoresis of FNR isozymes from tentoxin treated seedlings of a sensitive and a resistant species demonstrated that, unlike PPO, ferredoxin-NADP⁺ reductase was unaffected by tentoxin treatment. These data indicate that tentoxin selectively inhibits transport of cytoplasmically synthesized proteins into the chloroplast, and does not produce a generalized disruption of cellular integration.This research was supported, in part, by funding under cooperative agreement number 58-7B30-3-548, and is published with the approval of the Director of Arkansas Agr. Exp. Stn. Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the US Dep. Agric. or cooperating agencies and does not imply its approval to the exclusion of other products or vendors that may also be suitable.     

Response of wheatgrasses and wheat × wheatgrass hybrids to barley yellow dwarf virus

Summary Resistance to barley yellow dwarf virus (BYDV), manifested by low enzyme-linked immunosorbent assay (ELISA) values in plants exposed to viruliferous aphids, was identified in several wheatgrasses (Agropyron spp.). ELISA results were similar for root and leaf extracts of infested plants. No difference in reaction to BYDV was found between plants grown in the field and those in the growth chamber. Interspecific hybrids were generated using pollen from single resistant plants of Agropyron spp. to pollinate soft red winter wheat spikes. Resistance in hybrids appeared to be at the level of virus replication rather than at the level of vector inoculation. The hybrids varied in their reaction to BYDV. Expression of BYDV resistance in hybrids was influenced not only by wheat genotype and Agropyron species but, in some cases, reaction varied even among hybrids between the same wheat genotype and Agropyron plant. Implications of these results are discussed.Contribution from the Purdue Univ. Agric. Exp. Stn., West Lafayette, IN 47907, and USDA-ARS. The research was supported in part by Public Varieties of Indiana. Purdue Univ. Agric. Exp. Stn. Journal No. 11656     

Effect of nitrogen and phosphorus status on the translocation of three herbicides in field bindweed (Convolvulus arvensis L.)

Twenty-eight day old field bindweed plants grown in culture solutions deficient in nitrogen (N) or phosphorus (P) for the last seven days of growth translocated significantly less foliarly applied dicamba (3,6-dichloro-o-anisic acid) and 2,4-D [(2,4-dichlorophenoxy) acetic acid] to their roots than did plants grown in complete nutrient solutions. In contrast, N deficiency stimulated basipetal translocation of glyphosate [N-(phosphonomethyl) glycine] and inhibited its acropetal translocation in field bindweed. Deficiencies of both N and P decreased translocation of dicamba from the treated area, but had no influence on translocation of glyphosate or 2,4-D from the treated area.Journal Article No. 4406 of the Agric. Exp. Stn., Oklahoma State University.     

Drought resistance of wheat plants inoculated with vesicular-arbuscular mycorrhizae

Summary Drought resistance of wheat (Triticum aestivum L.) as influenced by two vesiculararbuscular mycorrhizal (VAM) fungi,Glomus fasciculatum ¹⁰ andGlomus deserticola ¹⁹, was evaluated. Soil columns 0.15 m diam. by 1.20 m length were used to reduce the influence of limited rooting space. With initial soil water at 0.5 MPa (0.145 kg kg^–1), plants were subjected to low-level water stress throughout the experiment and severe water stress for 24 h at one (55 days after transplanting, Feekes scale 10.1) two (55 and 63 days, Feekes 10.1 and 10.2), or three (55, 63, and 70 days, Feekes 10.1, 10.1, and 10.2) periods. After each stress period, one set of plants was watered and grown to maturity without subsequent water stress. A second set of plants was harvested 1 week after stress.G. fasciculatum-inoculated plants harvested 7 days after stress at 55 days had greater leaf area and leaf, total plant, and root weight than non-VAM plants.G. deserticola-inoculated plants had greater leaf area and leaf weight than non-VAM plants. After stress at 55 and 63 days, leaf area, and leaf and total dry weight were again greater for VAM than for non-VAM plants. However, after stress at 55, 63, and 70 days, differences in aboveground biomass between VAM and non-VAM plants were not significant at P=0.05. Aboveground biomass was not affected by VAM species in plants stressed at 55 or 55 and 63 days, butG. fasciculatum-inoculated plants produced more tillers atter stress at 55 days. When grown to maturity, VAM plants which had undergone three stress periods had twice the biomass and grain yield as non-VAM plants subjected to the same stress. The three stress periods reduced number of heads and kernel numbers of weight of non-VAM plants compared to VAM plants.G. fasciculatum-inoculated plants consistently had increased root weight and rooting depth.Contribution from the Agricultural Research Service, USDA, in cooperation with the Nebr. Agric. Exp. Stn., Univ. Nebr.-Lincoln, Lincoln, Nebr. Published as Paper No. 7571 Journal Series, Nebr. Agric. Exp. Stn.     

Nitrogen fertilization effects on dinitrogen fixation as influenced by legume species and proportion in legume-grass mixtures in Uruguay

Grasses grown in mixture with nodulated legumes often are N-limited, but N fertilization may result in reductions of N₂ fixation and legume stands. We studied N-fertilizer effects on N₂ fixation for three binary legume-grass mixtures in Uruguay. Replicated swards of white clover (Trifolium repens L.) (WC), red clover (Trifolium pratense L.) (RC), or birdsfoot trefoil (Lotus corniculatus L.) (BT), each in combination with tall fescue (Festuca arundinacea Schreb) (TF) at two legume proportions were sown in 1983 (Exp. 1) and 1984 (Exp. 2). In the fall of 1984, N treatments at 100 kg ha⁻¹ and controls were randomly assigned to subplots in Exp. 1 (established swards) and in Exp. 2 (at seeding). The soil for both experiments was a fine, montmorillonitic, mesic, Typic Argiudolls. Herbage fixed-N was estimated by ¹⁵N isotope-dilution with pure stands of TF as reference. In both experiments, N fertilization reduced the proportion of legume N derived from air (% Ndfa) and increased herbage yield only during the first 18 to 20 weeks after application. Fertilizer-N reduced annual fixed-N yield from 178 to 148 kg ha⁻¹ in Exp. 1 and from 65 to 29 kg ha⁻¹ in Exp. 2 Fixed-N yield for BT was markedly reduced by N in both experiments (33 to 53%), whereas for the clovers reduction was lesser in Exp. 1 (9 to 13%) than in Exp. 2 (46 to 64%). Negative effects of N on % Ndfa were more evident for the high legume proportion. We conclude that fertilization with 100 kg N ha⁻¹ reduced % Ndfa only for the immediate 18 to 20 weeks after application. Fertilizer-induced reduction of fixed-N yields lasted longer because of a more prolonged depression of legume proportion, especially for BT and for newly seeded swards. Journal Paper no. J.-13327 of the Iowa Agric. and Home Econ. Exp. Stn., Ames, U.S.A. (Project 2281). Supported in part by the Facultad de Agronomía, Montevideo, Uruguay; and the International Atomic Energy Agency, Vienna, Austria (Project URU/5/012). Journal Paper no. J.-13327 of the Iowa Agric. and Home Econ. Exp. Stn., Ames, U.S.A. (Project 2281). Supported in part by the Facultad de Agronomía, Montevideo, Uruguay; and the International Atomic Energy Agency, Vienna, Austria (Project URU/5/012).     

Seasonal distribution of topsoil ammonium and nitrate under legume-grass and grass swards

Losses of soil N through leaching and N₂ fixation by legumes often are related to soil nitrate concentration. The seasonal distribution of soil ammonium and nitrate concentrations under ungrazed legume-grass and grass swards were evaluated on two experiments that were established in 1983 (Exp. 1) and in 1984 (Exp. 2). Treatments were white clover (Trifolium repens L.) (WC), red clover (Trifolium pratense L.) (RC), and birdsfoot trefoil (Lotus corniculatus L.) (BT), each grown with tall fescue (Festuca arundicacea Schreb.) (TF) at two legume proportions, and a pure stand of TF. The concentrations of both forms of N were measured in the top 20-cm layer during 2 years in Exp. 1 and for 1 year in Exp. 2. The concentrations of nitrate and ammonium were least in winter and spring, and greatest in summer. The concentration of nitrate for the mixtures decreased in the order WC-TF, RC-TF, and BT-TF in both summers of Exp. 1 but there were no mixture differences in Exp. 2. The concentration of soil ammonium was not affected by the treatments applied. We conclude that the concentration of soil nitrate usually was small for these swards but became greater and often dependent on species and legume proportion during summer. The concentration of soil ammonium also was greater in summer but was not affected by species or legume proportion. Journal of Paper no. J.-13359 of the Iowa Agric. and Home Econ. Exp Stn., Ames. Project 2281. Supported in part by the Facultad de Agronomía, Montevideo, Uruguay. Journal of Paper no. J.-13359 of the Iowa Agric. and Home Econ. Exp Stn., Ames. Project 2281. Supported in part by the Facultad de Agronomía, Montevideo, Uruguay.     

Varietal differences in potassium uptake by excised roots ofNicotiana tabacum

Summary The rate of potassium uptake by two varieties of burley tobacco (Nicotiana tabacum) differed by as much as two-fold when excised roots were absorbing K from solutions of 2.5 × 10^–4 M KCl. With increasing substrate levels of KCl the difference between varieties decreased and no difference was observed at a KCl concentration of 1.6 × 10^–2 M. The K-absorption by excised roots of several varieties from solutions of 5 × 10^–4 M KCl was compared.Contribution of the Department of Agronomy, Kentucky Agr. Exp. Sta., Lexington, and published with the approval of the Experiment Station Director.     

Association and heritability of sugarcane smut resistance to races A and B in Hawaii

Summary This study was conducted to estimate the degree of association of smut (Ustilago scitaminea Syd.) resistance in sugarcane (Saccharum spp.) between races A and B in Hawaii and to estimate the heritability of resistance to both races. The estimated degree of association was 0.18. Although statistically significant, the degree of association of resistance between races A and B was near zero and therefore of no practical importance. Selection for resistance to one race would have little effect on the frequency of resistance to the other race in the following sexual generation. Heritabilities in the broad sense estimated from the parent population on a plot mean basis were 0.96 and 0.91 for races A and B, respectively. Selection for smut resistance should be very effective between populations of asexual generations. Heritabilities in the narrow sense estimated from parent-progeny regression analysis on a family mean basis were 0.51 and 0.47 for races A and B, respectively. Selecting and breeding for resistance should result in a fairly rapid increase in the frequency of resistance in the progeny population.Contribution from Department of Genetics and Pathology, Hawaiian Sugar Planters' Assn. Exp. Stn., P.O. Box 1057, Aiea, HI 96701. Published with the approval of the Director as paper no. 652 in the Journal Series of the Exp. Stn., Hawaiian Sugar Planters' Assn. Research funded in part by USDA/ARS Cooperative Agreement No. 58-9AHZ-0-492     

The effect of ribonuclease on the binding of calcium by mitochondria

Summary Mitochondria, isolated from potato tuber, pretreated with ribonuclease (RNase) showed an increase in calcium binding at low enzyme concentration. The same dosage-response pattern was obtained whether the enzyme treatment was conducted for 10, 30 or 120 min. However, when the enzyme treatment was carried out at 0° instead of at 30°, no increase in Ca-binding was obtained, suggesting that no interaction occurred between the enzyme and the divalent cation. Oxygen consumption under the same conditions was not affected. Microsomes treated with RNase did not show a change in their Ca-binding capacity, although untreated microsomes showed about the same increase in Ca-binding with increase in temperature as did the mitochondria. When mitochondria prelabeled with ⁴⁵Ca, after extracting their inorganic calcium phosphate, were treated with RNase a liberation of Ca, ribonucleotide, and phosphate was observed. It is suggested that Ca ions form a bridge like bond between ribonucleic acid and either phospholipids or phospholipoproteins, because RNase liberated more phosphate than nucleotides and the extra phosphate cannot be inorganic calcium phosphate, since the calcium phosphate was extracted before addition of RNAse.The investigation reported in this paper (No. 68-3-71) is in connection with a project of the Kentucky Experiment Station and is published with approval of the Director.     

The relationship of oxalic acid to the translocation and utilization of calcium inNicotiana tabacum

Summary Burley tobacco varieties (Nicotiana tabacum L.) which differed in degrees of susceptibility to calcium deficiency were analyzed for calcium and oxalic acid content. Calcium accumulated by the total plant top was not related to susceptibility to calcium deficiency. Upper leaves and buds of plants of susceptible varieties contained less calcium per unit weight than did upper leaves and buds of non-susceptible varieties.The upper half of stalks of susceptible varieties contained more oxalic acid per unit weight than did stalks of non-susceptible varieties. The calcium content minus the oxalic acid content in buds and young leaves of the plants of the susceptible varieties was considerably less than that of buds and young leaves of the plants of the non-susceptible varieties. High levels of oxalic acid in the upper stalks and young leaves of varieties susceptible to calcium deficiency apparently induce calcium deficiency by interfering with the translocation and utilization of absorbed calcium.Contribution of the Department of Agronomy, Kentucky Agr. Exp. Sta., Lexington, and published with the approval of the Experiment Station Director. This paper is based on a portion of a thesis submitted by the senior author to the Graduate School, University of Kentucky, in partial satisfaction of the requirements for the Master's degree, 1963.     

Soil temperature effects on competitiveness and growth ofRhizobium japonicum and on Rhizobium-induced chlorosis of soybeans

Summary The effects of temperature on growth in broth and soil and on competition for nodule formation betweenRhizobium japonicum serotypes USDA 76 and 94 compared to 6 and 110 were studied. Increasing root temperatures of Lee soybean from 20 to 35°C increased the competitiveness of 76 and 94 relative to 6 and 110 for all inoculum ratios such that at 30 and 35°C symptoms ofRhizobium-induced chlorosis appeared. Tolerance to elevated temperatures was exhibited by 76 and 110, but not 94 and 6 in broth and soil which suggested that increased competitiveness of 76 and 94 at high soil temperatures was not dependent upon growth at elevated temperatures. Nodulation and vegetative growth of Lee soybeans were at a minimum at 20°C and optimum at 30°C. Differences in competitiveness of 6 to previous studies indicated the need to standardize temperatures of assays. Differences in growth responses of 76 and 94 to temperature from a previous study suggested a confounding effect on different carbon sources in growth media. Scientific Article No. A-3721 Contribution No. 6697 of the Maryland Agric Exp Sta, Dept of Agronomy, College Park, MD 20742 and the USDA, ARS, Beltsville, MD 20705. Part of a thesis submitted by the senior author in partial fulfillment of the requirements for the M.S. Degree.     

Comparison of convective and diffusive absorption of nutrients by growing roots

With the roots assumed growing at an exponential rate the effects of the diffusive and convective components of flux and nutrient uptake are examined in a non-dimensional setting. Two cases considered are root-root on no root-root competition. Several examples are presented to illustrate the general effects of interroot competition.Contribution from the Purdue Agric. Exp. Stn., West Lafayette, IN, 47907. Journal Paper Number7656.     

Expressed resistance to black shank among tobacco callus cultures

Summary Quantitatively inherited resistance to the black shank pathogen (Phytophthora parasitica var. nicotianae) was expressed among callus tissue cultures of tobacco (Nicotiana). Tissue cultures of genotypes known to posses polygenic mechanisms for black shank resistance expressed that resistance in vitro when challenged by the viable pathogen. Callus of a susceptible cultivar was readily parasitized in culture. Furthermore, single gene resistance to the common pathogen race was also shown to operate in vitro. Nongenetic factors examined did not contribute significantly to the observed differences. Disease expression in vitro appeared to be highly correlated with its expression at the whole plant level.Screening for quantitative disease resistance can be complicated at the whole plant level by variable hostpathogen reactions and by significant genotype × environment interactions. Since quantitatively inherited mechanisms of black shank resistance are expressed in tobacco callus cultures, an in vitro host-pathogen system may be useful in screening tobacco lines for black shank resistance.The research reported in this paper (No. 82-3-6) is in connection with a project of the Kentucky Agr. Exp. Stn., and the paper is published with the approval of the director     

Effects of glyphosate on the movement of assimilates of two Lolium perenne L. populations with differential herbicide sensitivity

Glyphosate applications trigger the depletion of aromatic amino acid pools and the decrease of photosynthesis that results in changes in carbon metabolism. The aim of this work was to determine the effect of glyphosate on the export of ¹⁴C from ¹⁴C-glucose to the main sinks, by comparing a glyphosate-resistant Lolium perenne population with a susceptible one. Untreated plants of the two populations grown in hydroponics were labeled with ¹⁴C-glucose applied at the youngest expanded leaf at the tillering stage. Similar ¹⁴C-glucose absorption and ¹⁴C distribution patterns were recorded in both populations. In another experiment, half of the plants of each population were treated with glyphosate, whereas the other half was sprayed with water (controls). Glucose absorption did not vary under glyphosate treatment, regardless of the sensitivity of each population to the herbicide. However, the translocation of ¹⁴C and its distribution patterns were significantly affected by glyphosate within 1 day in the susceptible population. The treated susceptible plants showed 57% higher ¹⁴C retention at the labeled area than their controls. The lower ¹⁴C movement significantly affected the unexpanded leaves and the apical meristem on the labeled tiller. Moreover, the ¹⁴C released from roots was significantly decreased by glyphosate only in the susceptible plants. Glyphosate did not influence leaf absorption, translocation, or release of ¹⁴C-labeled glucose plus radiolabeled metabolites in the resistant population.     

The Effect of Sodium and Calcium on the Translocation of 14C-sucrose in Excised Cotton Roots

The effect of sodium and calcium on the translocation of ¹⁴C-sucrose in excised cotton roots (Gossypium birsutum) was studied. The roots were allowed to elongate in a modified Guinn's medium that was very low in calcium (6.25 × 10^?2 mM) and sodium (8.70 × 10^?3 mM). After a period of six days the roots were transferred to 20 × 100 mm Petri dishes that contained 10 × 40 mm Petri dishes as center wells. The roots were draped over the edge of the center well and extended into the outer dish. The outer Petri dish and its center well contained the same solution except that sucrose was supplied only in the center well. The sucrose used was spiked with uniformally labeled ¹⁴C-sucrose. Four treatments were started which varied in their Na and Ca content. Three and six day harvests were taken and the amount of ¹⁴C that had moved from the distal (in the center well) to the apical root section (in the outer dish) was determined. Increasing substrate Na or Ca caused an increase in ¹⁴C-sucrose translocation and the effects of both ions were additive by the final harvest. These results were found to be independent of treatment effects on growth and respiration of the excised roots. These data support the conclusion that Na may partially substitute for Ca in carbohydrate translocation. Thus, roots supplied the Low Ca-High Na and High Ca-Low Na treatments had equal translocation rates over a six day period. The highest translocation rate was obtained with the High Ca-High Na treatment. Data from the High Ca-High Na treatment on two successive three day periods indicate that Na may have a role in translocation other than that associated with substitution for Ca, or maintenance of tissue hydration.     

Genetic variation in tissue cultures of red clover

Summary Design II matings were made among randomly selected clones of Arlington red clover (Trifolium pratense L.). Progeny were evaluated in vitro on two regeneration media for callus growth and differentiation. Additive genetic variance was a significant source of variability for nearly all traits evaluated, including somatic embryogenesis. In vitro traits, such as rapid callus growth, colony vascularization, root initiation, chlorophyll production and embryogenesis were highly heritable and should respond to breeding and selection. Dominance genetic variance was significant for only a few in vitro characters. Maternal and cytoplasmic factors were significant primarily in the early subcultures. Highly significant additive genetic correlation of performance on two regeneration media was found. A population selected on one of the regeneration media for such characteristics as improved plantlet regeneration, rapid callus growth, long term colony viability or the frequency of root initiation should show correlated improvement on the other medium. No significant differences for embryogenesis were attributable to differences in the regeneration media used. Furthermore, no interaction of additive genetic effects with regeneration media were observed. These data indicate that improvement in the frequency of plantlet regeneration from callus of red clover could effectively be achieved by breeding and selection for embryogenic types.The research reported in this paper (No. 80-3-152) is in connection with a project of the Kentucky Agric. Exp. Stn. and the paper is published with the approval of the director. Part of a thesis submitted by the senior author in partial fulfillment of the requirements for the M.S. degree     

Numerical study of some age-dependent parameters in root nutrient uptake

Computer simulation of root nutrient uptake has become a very powerfull tool in the analysis of plant and soil characteristics. One of the shortcomings of earlier numerical models is the lack of a proper accounting for age-dependent root parameters. In this article we present an algorithm that not only allows for time-and/or space-varying root growth rates, convective moisture uptake, root density, initial distribution of nutrient, effective diffusion coefficients, and buffer power; but also accounts for time-varying root efflux, root absorption power and maximum nutrient influxive rate.Several elementary numerical examples of NH ₄ ⁺ , NO ₃ ^– , P and K uptake for roots with temporally varying characteristics are presented.Contribution from the Purdue Agric. Exp. Stn., W. Lafayette, IN. Journal Paper No. 9476.     

The influence of externally supplied sucrose on phloem transport in the maize leaf strip

Sucrose (2,5–1000 mmol l^–1), labeled with [¹⁴C]sucrose, was taken up by the xylem when supplied to one end of a 30-cm-long leaf strip of Zea mays L. cv. Prior. The sugar was loaded into the phloem and transported to the opposite end, which was immersed in diluted Hoagland's nutrient solution. When the Hoagland's solution at the opposite end was replaced by unlabeled sucrose solution of the same molarity as the labeled one, the two solutions met near the middle of the leaf strip, as indicated by radioautographs. In the dark, translocation of ¹⁴C-labeled assimilates was always directed away from the site of sucrose application, its distance depending on sugar concentration and translocation time. When sucrose was applied to both ends of the leaf strip, translocation of ¹⁴C-labeled assimilates was directed toward the lower sugar concentration. In the light, transport of ¹⁴-C-labeled assimilates can be directed (1) toward the morphological base of the leaf strip only (light effect), (2) toward the base and away from the site of sucrose application (light and sucrose effect), or (3) away from the site of sucrose application independent of the (basipetal or acropetal) direction (sucrose effect). The strength of a sink, represented by the darkened half of a leaf strip, can be reduced by applying sucrose (at least 25 mmol l^–1) to the darkened end of the leaf strip. However, equimolar sucrose solutions applied to both ends do not affect the strength of the dark sink. Only above 75 mmol l^–1 sucrose was the sink effect of the darnened part of the leaf strip reduced. Presumably, increasing the sucrose concentration replenishes the leaf tissue more rapidly, and photosynthates from the illuminated part of the leaf strip are imported to a lesser extent by the dark sink.Supported by Deutsche Forschungsgemeinschaft