Morphological factors of the central whorl leaf associated with leaf surface wetness and resistance in sorghum to shoot fly,Atherigona soccata*

Earlier studies showed that leaf surface water on the central whorl leaf of sorghum seedlings is associated with resistance to shoot fly. In this study, the results of an experiment to determine if leaf surface wetness (LSW) originates from atmospheric condensation or from the plant are described. Morphological structures: trichomes, stomata, leaf cuticle and quantity of surface wax of the central whorl leaf were also examined for their role in LSW production. The results suggest that LSW of the central whorl leaf originates from the plant and is not due to condensation of atmospheric moisture. The presence of trichomes was indirectly associated with LSW and resistance to shoot fly but stomatal density was not associated with LSW production. The amount of wax extracted per 100 mg of fresh weight varied significantly between genotypes and seedling age. It was more in susceptible than in resistant genotypes; however, cuticular thickness was not associated with resistance. It is suggested that LSW could be the result of some form of cuticular movement of water to the leaf surface.     

The dynamics of leaf surface wetness of sorghum seedlings in relation to resistance to the shoot fly, Atherigona soccata

In quantitative measurements of leaf surface wetness (LSW) of the central whorl leaf of sorghum seedlings in August (rainy season) and November (post-rainy season), the highest amount (6.29 mg of water) was recorded in August in the shoot fly Atherigona soccata (Diptera: Muscidae), susceptible sorghum genotype CSH 1, while the lowest (0.07 mg) was recorded in November in the resistant genotype IS 18551. Studies on diurnal fluctuation revealed that LSW was lowest at sunset, was highest between 02.00 and 04.00 h (closely corresponding with hatching of shoot fly eggs) and dropped before sunrise. This fluctuation was associated with the evaporation of water from the plant during the night. More LSW accumulation occurred during the main crop season (June-October) than in the post-rainy season (November-April). Annual fluctuation of LSW followed trends similar to the population dynamics of shoot fly and crop infestation and were correlated with rainfall, temperature and relative humidity. Measurements of leaf temperature and the vapour pressure gradient between the leaf and the air indicated that leaf surface water originates from the plant. This was further supported by the different amounts of LSW on susceptible and resistant cultivars with similar microclimatic conditions.     

Reduction of shoot fly damage in irrigated post-rainy season sorghum by manipulating irrigation

Soil moisture was manipulated in an attempt to control shoot fly (Atherigona soccata Rondani) incidence in irrigated post-rainy season sorghum grown under a rainout shelter (ROS) and in field conditions. After uniform irrigation at sowing, the plants were subjected to water stress at young seedling stage (7–28 days after emergence, DAE) for different lengths of time. Soil water had profound effects on the production of water droplets on the surface of the central whorl leaf of seedlings (leaf surface wetness, LSW) of sorghum genotypes. LSW, which facilitates movement of the larvae, was more drastically affected in susceptible (CSH 5) than in moderately resistant (IS 1054) sorghum genotypes. Shoot fly oviposition (infestation) and deadhearts (crop damage) were much higher in treatments with full irrigation (control) than in treatments to which less water was applied during the first 3 wk after seedling emergence. This resulted in higher plant biomass and overall grain yield in the latter treatments than in the control. Using insecticides to control shoot fly infestation, it was shown that a simple cultural practice of inducing plant stress by reduced soil moisture content during early plant growth gave the same or better control of shoot fly damage and the same or higher grain yield than insecticide-protected plots with full irrigation. Thus the costs associated with irrigation requirement and insecticide can be greatly reduced in the former management option compared with the latter. It is suggested that manipulation of soil water content during the vulnerable early stages of crop growth can reduce shoot fly damage in irrigated post-rainy season sorghum.     

Leaf surface wetness in sorghum and resistance to shoot fly, Atherigona soccata: role of soil and plant water potentials

In experiments with potted plants, the relationships between soil matric potential, plant water potential and production of water droplets (leaf surface wetness) on the folded central whorl leaf of seedlings of sorghum genotypes that are either resistant or susceptible to shoot fly (Atherigona soccata) damage were investigated. Differences in soil matric potentials in the pots affected the plant water status, which in turn had profound effects on the production of water droplets on the central whorl leaf of the sorghum genotype susceptible to shoot fly. There was no consistent variation in the relationship between plant water potential and soil matric potential of resistant and susceptible sorghum genotypes. However, there was very little or practically no water droplets on the central whorl leaf of the resistant genotypes, indicating that the production of water droplets is not solely the result of internal water status of the plant. It is suggested that leaf surface wetness is genetically controlled and that an understanding of the mechanism by which water is transferred to the leaf surface will enhance breeding for resistance to shoot fly.     

Efficacy of neem pesticides on whorl larva,stem-borer and panicle insect pests of sorghum in Nigeria

Two field trials were conducted from 2003 to 2005 on the effects of neem pesticides on whorl larva, stem-borer and panicle pests of sorghum. In the first trial (2003–2004) the effect of neem seed granules (NSG) and carbofuran (furadan 3G®) inserted into the sorghum whorl at 30, 40 and 50 days after sowing (DAS) were tested on whorl larva, stem borer and grain yield. Results showed that NSG and carbofuran significantly (P ≤ 0.01) reduced whorl larva feeding, leaf puncturing and chaffy panicle, and significantly (P ≤ 0.01) increased grain yield compared with untreated check. In the second trial, (2004–2005), aqueous neem seed extract (ANSE), neem seed oil (NSO) and deltamethrin (Decis 12 EC®) were sprayed at anthesis and grain filling stages. Results also showed that the three pesticides significantly (P ≤ 0.01) reduced insect damage and also significantly (P ≤ 0.01) increased grain yield by 28.3% (deltamethrin), 19.4% (NSO) and 17.9% (ANSE) above the yield of check. It is suggested that neem pesticides may be suitable as alternatives to synthetic pesticides for the management of sorghum whorl larva, stem borers and panicle insect pests in the Nigerian Sudan savannah.     

高粱、紫苏叶脉密度与光合特性的关系

叶脉是植物叶片光合作用水分输送的重要结构。为阐述叶脉与光合特性之间的关系,以C4植物高粱(Sorghum bicolor)、C3植物紫苏(Perilla frutescens)为实验材料研究了叶脉密度和光合特性之间的关系。结果表明,与紫苏相比,高粱叶片叶脉密度大,导水能力强,蒸腾速率高,但气孔密度小。进一步分析表明,高粱叶片近轴侧气孔密度占总气孔的比例明显高于紫苏。叶脉密度大的高粱具有较高的净光合速率;而紫苏叶脉密度小,净光合速率也较低。由此表明,较高的叶脉密度有利于支持较高的光合速率,但研究表明叶脉密度和气孔密度可能不存在严格的协同变异关系。研究结果对理解植物光合作用适应有重要意义。     

Biochemical Studies on Hydrocyanic Acid Potential and Amino Acid in Sorghums

This research is conducted to study the HCN-p of several types of sorghum, as well as their changes in amino acid. The HCN-p of sorghum seedling reached its maximum in 4–6 days after germination. It was found that the first leaf of the seedling has the highest HCN-p. Among the 14 varieties studied, Atlas had the highest HCN-p (1984ppm), whereas NP-22 had the lowest value (479ppm). Light-grown seedlings had higher HCN-p. There was no obvious difference in protein amino acid in sorghum seedlings with .different HCN-p. However, the free amino acid content did vary in the sorghum varieties. The content of free tyrval, pheleu and ileu in etiolated Atlas seedlings was significantly higher than that in NP-22 seedlings. However, under condition of light, the five free amino acids lowers significantly in content in Atlas when compared be NP-22. This proves that Atlas had a more sensitive enzyme system and under presence of light, its free amino acid content was rapidly converted to cyanide.     

SYMMETRY IN EQUISETUM

Bierhorst , David W. (Cornell U., Ithaca, N. Y.) Symmetry in Equisetum. Amer. Jour. Bot. 46(3) : 170-179. Illus. 1959.—A total of 118 leaf whorls and corresponding nodes from a total of 9 species of Equisetum were studied in serial cross-section. The number of whorls having the same leaf number as the 2 adjacent ones was 67. The size of the arc measured in degrees of circumference occupied by each leaf, as well as its position relative to leaves of adjacent whorls were measured. The disposition of the internodal and trans-nodal vascular strands was determined for each of the nodes. The average per cent deviations from theoretical leaf size within whorls were: whorls not involved in change in leaf number, 3.9; those with fewer leaves than the one below, 9.5; those with more leaves than one above, 7.1; those with more leaves than one below, 9.4; those with different leaf number from the 2 adjacent ones, 12.5. In Equisetum, it is a general rule that (1) a leaf which falls directly above a leaf in the next whorl where the younger whorl possesses fewer leaves than the older one is usually the largest leaf within its whorl, (2) a leaf which falls directly below a leaf in the next younger whorl where the younger whorl possesses fewer leaves than the older one is usually the smallest leaf within its whorl, (3) a leaf which falls directly above a leaf in the next older whorl where the younger whorl possesses a greater number of leaves than the older one is usually the smallest leaf in its whorl, and (4) a leaf which falls directly below a leaf in the next younger whorl where the younger whorl possesses a greater number of leaves than the older one is usually the largest leaf within its whorl. The numerous variations in the disposition of vascular tissue associated with changes in leaf number are described. Double leaf traces which originate in various ways are common, as well as vertical strands traversing the nodes. The leaf trace system is considered to be determined by the number, position, and relative sizes of leaf primordia. The disposition of the trans-nodal protoxylem seems to be determined to a large extent by the proximity of the leaf traces above and below the node.     

Plant quality vs. risk of parasitism: within-plant distribution and performance of the corn leaf aphid, Rhopalosiphum maidis

1 Colonies of the aphid Rhopalosiphum maidis on Johnsongrass, Sorghum halepense, usually occur inside the whorl. The present work assessed the role of two biotic factors, plant quality and parasitism by Lysiphlebus testaceipes, in determining within‐plant distribution and performance of R. maidis. The mean relative growth rate of aphids inside the whorl and on a mature leaf was compared, and the concentration of hydroxamic acids in those tissues determined as an indicator of plant quality. Parasitism effectiveness and parasitoid behaviour were evaluated using three treatments: aphid colonies placed (1) wholly inside the whorl, (2) on the inner and outer surfaces of the whorl, and (3) on a mature leaf. 2 The mean relative growth rate of aphids was lower on the whorl than on the mature leaf, and hydroxamic acid concentration in the whorl was higher than in the mature leaf. 3 The number of parasitized dead aphids (mummies) inside the whorl was considerably lower than in the other two treatments. Mummies were present in 80% of the replicates in the whorl, whereas mummies were present in all replicates of the other treatments. 4 Host location time of the parasitoid was increased on the mature leaf compared with the other treatments. No significant differences in the residence time of the parasitoid in the aphid colony occurred between treatments. 5 Host location time showed a negative correlation, and residence time of the parasitoid a positive correlation with the number of aphids on the outer surface of the whorl.     

Effects of DDT on carbohydrate metabolism and translocation in secale species

An inhibition of photosynthetic electron transport in susceptible rye following treatment with DDT is accompanied by an increase in dry weight of leaves contacting the pesticide due to an accumulation of fructose, glucose, and to lesser extent, sucrose. Several days after treatment over 40% of the dry weight is due to these sugars. The assimilation of ¹⁴CO₂ by leaf segments was decreased as a consequence of DDT treatment, but labelling patterns were similar to those for leaf segments from untreated plants. However, if given a prolonged period in darkness before extraction of assimilates the leaf segments from treated seedlings retained ¹⁴C in sugars and did not show the substantial decrease in extractable soluble material which was characteristic of untreated controls. In DDT-treated seedlings the translocation of metabolites from leaves to roots was severely impaired.     

Behavior of Corn and Sorghum under Water Stress and during Recovery

Corn (Zea mays L.) and sorghum (Sorghum vulgare, Pers.) plants were grown in a vermiculite-gravel mixture in controlled environment chambers until they were 40 days old. Water was withheld until they were severely wilted, and they were then rewatered. During drying and after rewatering stomatal resistance was measured with a diffusion porometer each morning, and water saturation deficit and water potential were measured on leaf samples. The average resistance of the lower epidermis of well watered plants was lower for corn than for sorghum. When water stress developed, the stomata began to close at a higher water potential in corn than in sorghum. The stomata of both species began to reopen normally soon after the wilted plants were rewatered, and on the 2nd day the leaf resistances were nearly as low as those of the controls. The average leaf water potential of well watered corn was −4.5 bars; that of sorghum, −6.4 bars. The lowest leaf water potential in stressed corn was −12.8 bars at a water saturation deficit of 45%. The lowest leaf water potential in stressed sorghum was −15.7 bars, but the water saturation deficit was only 29%. At these values the leaves of both species were tightly rolled or folded and some injury was apparent. Thus, although the average leaf resistance of corn is little lower than that of sorghum, corn loses much more of its water before the stomata are fully closed than does sorghum. The smaller reduction in water content of sorghum for a given reduction in leaf water potential is characteristic of drought-resistant species.     

Water droplets in intercellular spaces of barley leaves examined by low-temperature scanning electron microscopy

Low-temperature scanning electron microscopy was used to examine fracture faces in leaf blades taken from well-watered or drought-stressed barley (Hordeum vulgare L. cv. Mazurka) seedlings. The leaf blades were freeze-fixed while hydrated and were examined with or without gold-coating. There were droplets (with a smooth surface at the resolution achieved) on the surface of cell walls in leaf blades (0.91 g^-1 water content) from well-watered seedlings grown in an environment of 67% relative humidity. These were mainly on the vascular bundle sheath, the guard and subsidiary cells, and on some mesophyll cells around the substomatal cavity and between the stoma and vascular bundle. The droplets occurred, more abundantly, in the same places in seedlings from 100% relative humidity. They occurred on a few guard cells from wilting leaf blades (0.81 g·g^-1 water content) and were absent from severely drought-stressed leaf blades (0.15 g·g^-1 water content). The droplets sublimed at the same moment as both water which was in leaf cells and water which was allowed to condense (after freeze-fixation) on the wall surface. It is suggested that the droplets are aqueous. Their possible origin and importance is discussed.     

In vivo labelling of the proteins in the first leaf of intact oat plants (Avena sativa)

In order to label the primary leaves of intact oat plants ( Avena sativa L. cv. Victory) for studying protein turnover using two-dimensional polyacrylamide gel electrophoresis, the following methods were tested: growth of seedlings on ³⁵S-sulfate-containing Knop medium, labelling with ³⁵S-methionine by vacuum infiltration of the leaf, injection into the leaf base or into the seed near the embryo, or wiping the surface of the leaf with ethanol followed by incubation in the labelled solution. A specific activity of 1.6 kBq per 20 μg of leaf protein applied was minimally necessary to obtain a well-resolved fluorogram of the gels. This level of labelling was reached only upon the treatment with ethanol, which did not require more than 0.55 MBq of ³⁵S-methionine per leaf. The method may be useful locally to apply compounds to intact plants.     

干旱胁迫对持绿性高粱叶片渗透调节及叶绿体超微结构的影响

在盆栽条件下,研究了开花期和灌浆期干旱胁迫(土壤含水量为田间最大持水量的45%~50%)对持绿性高粱(B35)和非持绿性高粱(三尺三)叶片水分、渗透调节物质以及叶绿体超微结构的影响.结果表明: 干旱胁迫下,两高粱品系叶片自由水含量下降,束缚水含量增加,相对含水量降低,水分饱和亏缺增加,相对电导率增大,但三尺三各指标的变化幅度均大于B35.对于渗透调节物质,干旱胁迫下,三尺三可溶性糖含量的增幅大于B35,脯氨酸含量的增幅小于B35,可溶性蛋白含量的降幅大于B35.干旱胁迫下,B35与三尺三的叶绿体超微结构均受到一定程度的破坏,但B35叶绿体结构保持相对完好,受损程度明显小于三尺三.在干旱胁迫下,持绿性高粱通过较强的渗透调节表现出更好的干旱适应能力.
     

A new technique for measurement of water permeability of stomatous cuticular membranes isolated from Hedera helix leaves

Transpiration of cuticular membranes isolated from the lower stomatous surface of Hedera helix (ivy) leaves was measured using a novel approach which allowed a distinction to be made between gas phase diffusion (through stomatal pores) and solid phase diffusion (transport through the polymer matrix membrane and cuticular waxes) of water molecules. This approach is based on the principle that the diffusivity of water vapour in the gas phase can be manipulated by using different gases (helium, nitrogen, or carbon dioxide) while diffusivity of water in the solid phase is not affected. This approach allowed the flow of water across stomatal pores ('stomatal transpiration') to be calculated separately from the flow across the cuticle (cuticular transpiration) on the stomatous leaf surface. As expected, water flux across the cuticle isolated from the astomatous leaf surface was not affected by the gas composition since there are no gas-filled pores. Resistance to flux of water through the solid cuticle on the stomatous leaf surface was about 11 times lower than cuticular resistance on the astomatous leaf surface, indicating pronounced differences in barrier properties between cuticles isolated from both leaf surfaces. In order to check whether this difference in resistance was due to different barrier properties of cuticular waxes on both leaf sides, mobility of 14C-labelled 2,4-dichlorophenoxy-butyric acid 14C-2,4-DB) in reconstituted cuticular wax isolated from both leaf surfaces was measured separately. However, mobility of 14C-2,4-DB in reconstituted wax isolated from the lower leaf surface was 2.6 times lower compared with the upper leaf side. The significantly higher permeability of the ivy cuticle on the lower stomatous leaf surface compared with the astomatous surface might result from lateral heterogeneity in permeability of the cuticle covering normal epidermal cells compared with the cuticle covering the stomatal cell surface.     

Leaf surface chemistry of sorghum seedlings influencing expression of resistance to sorghum shoot fly, Atherigona soccata

Sorghum shoot fly, Atherigona soccata is one of the serious constraints to sorghum production, and host plant resistance is an important component for controlling this pest. We studied the expression of resistance to A. soccata in a diverse array of sorghum genotypes in relation to composition of leaf surface chemicals during the seedling stage. The sorghum genotypes IS 1054, IS 1057, IS 2146, IS 4664, IS 2312, IS 2205, SFCR 125, SFCR 151, ICSV 700, and IS 18551 exhibited antixenosis for oviposition, and suffered less deadhearts due to sorghum shoot fly, A. soccata. Compounds undecane 5- methyl, decane 4- methyl, hexane 2, 4- methyl, pentadecane 8- hexyl, and dodecane 2, 6, 11- trimethyl, present on the leaf surface of sorghum seedlings, were associated with susceptibility to shoot fly; while 4, 4- dimethyl cyclooctene was associated with resistance to shoot fly. The compounds associated with resistance/susceptibility to shoot fly, can be used as marker traits to select for resistance as well as for diversifying and increasing the levels of resistance to this pest. The role of biochemical compounds for developing sorghum varieties with resistance to shoot fly, A. soccata has been discussed.     

The role of leaf surface wetness in larval behaviour of the sorghum shoot fly, Atherigona soccata

The susceptibility of sorghum to the shoot fly Atherigona soccata Rondani, (Diptera: Muscidae) is affected by seedling age and is highest when seedlings are 8–12 days old. This corresponds with high moisture accumulation on the central leaf which is the path of newly hatched larva as it moves downwards from the oviposition site, towards the growing apex. Studies showed that leaf surface wetness (LSW) of the central shoot leaf was higher in 10-day old seedlings than in seedlings of other ages. Similarly, LSW was much higher in the susceptible sorghum genotype CSH 1 than in the resistant genotype IS 2146. Larvae moved faster towards the growing point and produced deadhearts much earlier in CSH 1 than in IS 2146. They also moved faster in 10-day old seedlings than in seedlings of other ages. It was also shown that the leaf surface wetness of the central shoot leaf is a more reliable parameter of resistance than the glossy leaf trait or trichome density.

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L'influence de la humidité de la surface foliaire sur le comportement de la mouche des pousses du sorgho

Résumé La sensibilité du sorgho à la mouche des pousses du sorgho, Atherigona soccata Rondani, est liée à l'âge de la plantule. Elle est plus forte lorsque la plantule est âgée de 8 à 12 jours et la sensibilité est maximale à 10 jours. A ce stade de croissance on observe une forte accumulation d'humidité sur la feuille centrale de la tige. Les jeunes larves traversent cette zone humide lorsqu'elles descendent vers la zone de croissance à partir des pontes déposées sur la face ventrale des feuilles déroulées.Des études ont été menées à l'ICRISAT (Inde) sur la relation entre l'humidité de la feuille centrale de la tige des plantules du sorgho et les dégâts provoqués par la mouche des pousses. L'humidité de la surface des feuilles (HSF) a été estimée grâce à une échelle visuelle graduée 1 à 5 où, 1 = pas d'humidité apparente et 5 = surface de la feuille recouverte de gouttes d'eau. La HSF est plus élevée sur des pousses de sorgho âgées de 10 j que sur les pousses appartenant à d'autres classes d'âge. Les valeurs observées sont également plus fortes pour les variétés non résistantes à ce ravageur (CSH 1,4.8) que pour les variétés résistantes (IS 2146, (2)). La vitesse du déplacement larvaire entre le cornet et la zone de la croissance varie en fonction de l'âge de la plante et des cultivars. Les larves migrent plus rapidement vers la zone de croissance et provoquent la mort du coeur du sorgho plus tôt dans la variété CSH 1 que dans IS 2146. Les larves se déplacent plus rapidement dans les pousses âgées de 10 j que dans les pousses appartenant à d'autres classes d'âge.Des études ont également démontré que la HSF n'est pas directement liée au caractère feuille lisse où à la densité des trichomes. La HSF est faible pour les génotypes résistants présentent où non le caractère feuille lisse. Par contre la HSF est élevée pour les génotypes non résistants présentant le caractère feuille lisse ou non. Aucune relation directe entre la densité des trichomes et les dégâts provoqués par la mouche des pousses n'a pu être mise en évidence. L'analyse des correlations établie pour les caractères de surface des feuilles avec la mort du cur des sorghos indique que les correlations sont faibles et non-significatives pour le caractère feuille lisse (0.49) et la densité des trichomes (0.39 et 0.2). Par contre les correlations sont fortes et significatives pour la HSF (0.82).On conclue que la HSF de la feuille centrale de la tige est un facteur important dans le déterminisme de la résistance du sorgho vis à vis de la mouche des pousses. Les relations entre les processus physiologiques de la plante et les facteurs impliquées dans l'accumulation d'eau sur la surface des feuilles font actuellement l'objet d'études détaillées.

     

Spatiotemporal transfer of carbon-14-labelled photosynthate from ectomycorrhizal Pinus densiflora seedlings to extraradical mycelia

Seedlings of Pinus densiflora colonized by an unidentified ectomycorrhizal fungus (T01) were labelled photosynthetically with 14C. Movement of 14C-labelled photosynthates within the underground part of the seedlings was investigated by temporal autoradiography using an imaging plate. Within 1 day, 14C was transferred from the shoot to the underground part that included roots, mycorrhizae, and the extraradical mycelium; within 3 days, the 14C in the underground part reached its maximum density. Mycorrhizae and actively growing root tips were large C sinks. Three days after 14C labelling, counts of 14C radioactivity in the underground part of the mycorrhizal seedlings were 2.6 times those of nonmycorrhizal seedlings. The mycorrhizae of mycorrhizal plants accumulated 5.2 times the 14C counts in the short-root tips of nonmycorrhizal plants. 14C counts in various areas of the extraradical mycelium demonstrated that all 14C-photosynthate transfer from the host root to the extraradical mycelium occurred within 3 days after 14C labelling, and that there was only a short lag of < 1 day between 14C accumulation in the basal and distal parts of the mycelium. Although more 14C accumulated in the distal than in the basal parts, 14C counts per unit hyphal biomass were similar between the two. These results suggest that 14C spread rapidly throughout the entire mycelium. Thirteen days after 14C labelling, we estimated 14C allocation to extraradical mycelia by taking autoradiographs after removing host roots. About 24% of 14C counts in the underground part of the mycorrhizal seedlings had been allocated to extraradical mycelia in this system, indicating that the fugal mycelium is an important sink for photosynthates.     

Cavitation induced by a surfactant leads to a transient release of water stress and subsequent 'run away' embolism in Scots pine (Pinus sylvestris) seedlings

Cavitation decreases the hydraulic conductance of the xylem and has, therefore, detrimental effects on plant water balance. However, cavitation is also hypothesized to relieve water stress temporarily by releasing water from embolizing conduits to the transpiration stream. Stomatal closure in response to decreasing water potentials in order to avoid excessive cavitation has been well documented in numerous previous studies. However, it has remained unclear whether the stomata sense cavitation events themselves or whether they act in response to a decrease in leaf water potential to a level at which cavitation is initiated. The effects of massive cavitation on leaf water potential, transpiration, and stomatal behaviour were studied by feeding a surfactant into the transpiration stream of Scots pine (Pinus sylvestris) seedlings. The stomatal response to cavitation in connection with the capacitive effect was also studied. A major transient increase in leaf water potential was found due to cavitation in the seedlings. As cavitation was induced by lowering the surface tension, the two mechanisms could be uncoupled, as the usual relation between xylem water potential and the onset of cavitation did not hold. Our results indicate that the seedlings responded more to leaf water potential and less to cavitation itself, as stomatal closure was insufficient to prevent the seedlings from being driven to 'run-away' cavitation in a manner of hours.