Ectomycorrhizal, vesicular-arbuscular and dark septate fungal colonization of bishop pine (Pinus muricata) seedlings in the first 5 months of growth after wildfire

 We followed the colonization frequency of ectomycorrhizal (EM), vesicular-arbuscular mycorrhizal (VAM), and dark septate (DS) fungi in 1- to 5-month-old bishop pine seedlings reestablishing after a wildfire. Seedlings were collected on a monthly basis at either a VAM-dominated chaparral scrub site or an EM-dominated forest site, both of which were burned. In both vegetation types, fully developed EM were observed from the third month after germination. EM fungi observed on the seedlings from the scrub site were limited to Rhizopogon subcaerulescens, R. ochraceorubens and Suillus pungens. Seedlings from the forest were colonized by a greater variety of EM fungi including Amanita spp., Russula brevipes and a member of the Cantharellaceae. VAM structures (vesicles, arbuscules or hyphal coils) were observed in the seedling root systems beginning 1 month after germination at the scrub site and 3 months after germination at the forest site. Seedlings from the scrub site consistently had more frequent VAM fungal colonization than those from the forest site through the fifth month after germination. DS fungi were observed in most seedlings from both the scrub and forest sites beginning in the first month post-germination. We propose that these fungi survived as a resident inoculum in the soils and did not disperse into the sites after the fire. Accepted: 14 February 1998     

Effects of vesicular-arbuscular mycorrhizal inoculation at different soil P availabilities on growth and nutrient uptake of in vitro propagated coffee (Coffea arabica L.) plants

 In a pot experiment, the growth and the nutrient status of in vitro propagated coffee (Coffea arabica L.) microcuttings were investigated for 5 months following vesicular-arbuscular mycorrhizal (VAM) inoculation with either Acaulospora melleae or Glomus clarum at four soil P availabilities. Control plants remained P-deficient even at the highest soil P availability while mycorrhizal plants were P-sufficient at all soil P availabilities. Growth of control plants was only improved at the highest soil P availability. In P-deficient soil, neither of the two VAM species improved plant growth. Plant growth increased by 50% following inoculation with either A. melleae or G. clarum when P availability went from deficient to low. No further plant growth improvement was induced by either VAM species at intermediate and high soil P levels. Nevertheless, growth of plants inoculated with G. clarum was still significantly greater than that of non-mycorrhizal plants at the highest soil P availability. Root colonization by G. clarum increased with increasing soil P availability while root colonization by A. mellea decreased with soil P level increasing above low P availability. Soil P availability also affected Zn nutrition through its influence on VAM symbiosis. With increasing soil P availability, foliar Zn status increased with G. clarum or decreased with A. mellea in parallel to root colonization by VAM. This study demonstrates the beneficial effects of VAM inoculation on in vitro propagated Arabica coffee microcuttings, as shown previously for seedlings. This study also demonstrates differences in tolerance to soil P availability between VAM species, most likely resulting from their differing abilities to enhance coffee foliar P status. Accepted: 14 November 1996     

Genotype dependent variation in mycorrhizal colonization and response to inoculation of pearl millet

Summary Genotypes of pearl millet (Pennisetum americanum L. Leeke) were examined for differences in vesicular-arbuscular mycorrhizal (VAM) colonization and response to inoculation. For thirty genotypes tested across three field locations there was a range of mycorrhizal colonization intensity between 25 and 56%. In another experiment with two male-sterile lines, restorer lines and their derived crosses, grown in pots filled with non-sterilized soil there were significant differences between genotypes for colonization by mycorrhiza. This showed hostgenotype dependence for mycorrhizal colonization.Root growth rates, mycorrhizal root length, percentage root colonization and plant growth and P uptake were studied in ten genotypes. A set of 3 genotypes with similar root lengths varied significantly with regard to mycorrhizal root length and the percentage colonization. This supports the suggestion that VAM colonization and spread is dependent on the host genotype. The growth responses differed significantly between the genotypes and they also differed in their responses to P uptake and VAM inoculation. The utility of host-genotype dependent differences in VAM symbiosis in plant breeding is discussed.Journal Article No. 453     

Investigations on Net CO2 Assimilation, Transpiration and Root Growth of Fagus sylvatica Infested with Four Different Phytophthora Species

Abstract: In this contribution, we compare the influence of four different Phytophthora species on root development, net CO₂ assimilation and transpiration of beech seedlings and saplings. Some few days after inoculation, photosynthesis and transpiration of seedlings infected with either P. citricola or P. cambivora were strongly reduced. In parallel, about 60 % of their root systems was destroyed compared to control plants. Three weeks after infection, all seedlings were dead, showing severe wilt symptoms on leaves. Remarkably, P. syringae and P. undulata infected seedlings and older beech plants did not differ from controls regarding photosynthesis and transpiration, although the root systems were damaged. However, a significant influence on net CO₂ assimilation and transpiration of P. citricola infected beech saplings was visible after bud break in the following year. Some days before plants started to wilt, photosynthesis and transpiration were reduced to almost zero. Water use efficiency data (WUE) clearly indicated that infected plants suffered from severe drought.     

Anatomical Structures of the VA Mycorrhiza in the Apocynaceae (Gentianales)

Roots of 19 Apocynaceae species were studied anatomically with respect to their symbiosis with VAM-fungi. In plants collected from the field, VAM-fungi were established in the root cortex. Also, inoculations with different Glomus species on the cultured plants are very successful in the infection and colonization of the root cortex. After penetration of the rhizodermis, the special exodermal short cells become colonized by winding hyphae. Then, in the root cortex of many Apocynaceae species, the VAM-fungi produce intercellular running hyphae which leads to extensive colonization of the root. Arbuscules develop on intracellular running hyphae, whereas vesicles develop mainly on intercellular hyphae. Except for some special details, this is the most common type of colonization of VAM fungi in flowering plants. But in Amsonia tabernaemontana, Nerium oleander, and Thevetia peruviana, another type of colonization could be observed. In these species, the colonization of the hyphae within the root cortex is only possible by intracellular growth. Intercellular running hyphae in the root are lacking. Therefore, after penetration the colonization in the cortex is cluster-like and strictly limited. Only by additional penetrations from hyphae in the soil, will roots show heavy infestations. This type of growth of the VAM fungi in the root is well known from the Gentianaceae and was explained as a structural incompatibility. In Catharanthus roseus, Pachypodium lamerei, and Trachelospermum jasminoides, intermediate stages of both types of colonization could be described. The results are discussed in the search for possible stimulants for structural incompatibility.     

Influence of temperature on colonization of spring barleys by vesicular arbuscular mycorrhizal fungi

The effects of three soil temperatures on growth of spring barleys (Hordeum vulgare L.) and on their root colonization by vesicular arbuscular mycorrhizal (VAM) fungi from agricultural soils in Montana (USA) or Syria at different inoculum concentrations were tested in soil incubators in the greenhouse. The number of mycorrhizal plants as well as the proportion and intensity of roots colonized increased with higher soil temperatures. VAM fungi from Montana, primarily Glomus macrocarpum, were cold tolerant at 11°C while those from Syria, primarily G. hoi, were heat tolerant at 26°C. Inoculum potential of Montana VAM fungi was higher than Syrian VAM fungi in cool soils. Harmal, selected from Syrian barley land races, had the highest colonization by mycorrhizal fungi of the cultivars tested.Journal Series Paper: J-2532 Montana Agricultural Experiment Station.     

The mycorrhizal status of vascular epiphytes in Bale Mountains National Park,Ethiopia

Roots of 28 species of epiphytic vascular plants were collected on tree trunks and branches at six afromontane forest sites between 1700 and 3300 m above sea level in Bale Mountains National Park, Ethiopia. Seven of the 28 epiphyte species were colonized by vesicular-arbuscular mycorrhizal fungi (VAM). Mycorrhizal colonization only occurred at two of the six sites examined, at 2900 m and 3300 m, but more than one type of VAM endophyte was present in each case. Three facultative epiphytic species were all highly colonized by VAM on the forest floor, whereas roots from epiphytic habitats were weakly colonized. No correlations were found between VAM colonization, fine root diameter and root hair length, but VAM colonization and root hair abundance were negatively correlated. The lack of VAM colonization of potential, epiphytic host species at the majority of the sites examined points to the dispersal of VAM propagules as the factor limiting mycorrhizal colonization of epiphytic habitats. It is suggested that root systems of hemiepiphytic tree species serve as corridors between forest floor and tree trunks through which VAM may spread via hyphal growth.     

GRAFTING AND ROOTING OF LEAVES OF GUAREA (MELIACEAE): EXPERIMENTAL STUDIES ON LEAF AUTONOMY

The pinnately compound, indeterminate leaves of G. glabra and G. guidonia were air layered, detached from their original shoots, and grown on their own adventitious root systems for up to 58 mo and 26 mo, respectively. The detached leaves grew in the same indeterminate manner and reached sizes similar to attached leaves. Although detached leaves grew autonomously, they never produced shoot buds. Leaves of both species were grafted onto their own stems and cut free of their original leaf bases. Leaf scions survived and grew for up to 29 mo and 20 mo, respectively, similar to ungrafted leaves. Axillary branches were grafted onto subtending leaves. Branch scions grew on their leaf stocks for over 30 mo and 24 mo, respectively, after being cut free from the branch bases. Secondary growth of the leaf axis (petiole) was promoted, and vascular tissues of leaf and branch axes were continuous. However, the unlignified basal region of the leaf, including the abscission zone, remained unchanged after grafting. The results indicate that proximity of roots and bypassing the abscission zone did not enhance leaf longevity or pinna production. The presence of a growing branch on a leaf did not modify the structure of the abscission zone, which suggests that the zone is strongly committed or developmentally fixed.     

Influence of edaphic and climatic factors on dynamics of root colonization and spore density of vesicular-arbuscular mycorrhizal fungi in Acacia farnesiana Willd. and A. planifrons W.et.A.

 A study was conducted to assess the dynamics of vesicular-arbuscular mycorrhizal (VAM) fungi associated with Acacia farnesiana and A. planifrons in moderately fertile alkaline soils. The intensity of root colonization by VAM fungi and the distribution of VAM fungal structures varied with host species over a period of time. The occurrence of vesicles with varied morphology in the mycorrhizal roots indicates infection by different VAM fungal species. This was further confirmed from the presence of spores belonging to different VAM fungal species in the rhizosphere soils. Root colonization and spore number ranged from 56% – 72% and 5 – 14 g^{–  1}soil in A. farnesiana and from 60% – 73% and 5 – 15 g^{–  1} soil in A. planifrons. Per cent root colonization and VAM spore number in the rhizosphere soil were inversely related to each other in both the Acacia species. However, patterns of the occurrence of VAM fungal structures were erratic. Spores of Acaulospora foveata, Gigaspora albida, Glomus fasciculatum, G. geosporum and Sclerocystis sinuosa were isolated from the rhizosphere of A. farnesiana whereas A. scrobiculata, G. pustulatum, G. fasciculatum, G. geosporum and G. microcarpum were isolated from that of A. planifrons. The response of VAM status to fluctuating edaphic factors varied with host species. In A. farnesiana though soil nitrogen (N) was positively correlated with root colonization, soil moisture, potassium and air temperature were negatively correlated to both root colonization and spore number. Per cent root colonization and spore number in A. planifrons were negatively related to each other. Further, in A. planifrons as the soil phosphorus and N were negatively correlated with the density of VAM fungal spores, the same edaphic factors along with soil moisture negatively influenced root colonization. Received: 16 May 1995 / Accepted: 7 February 1996     

Influence of phosphorus and formononetin on isozyme expression in the Zea mays‐Glomus intraradices symbiosis

Maize (Zea mays L. cv. Great Lakes 586) plants were either inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus intraradices Schenck and Smith, or grown in the presence of the isoflavone formononetin or were provided with both G. intraradices and formononetin. All plants were grown in soil containing one of five levels of inorganic P (between 8 and 110 µg g^?1 soil). By 3 weeks there were significant differences in a number of enzyme activities and in the pattern of isoenzymes in roots colonized by the VAM fungus or treated with formononetin. One NAD-malate dehydrogenase (MDH) isozyme was expressed only in mycorrhizal roots, whether treated or not with formononetin. Despite differences in the soil P level, the expression of this isozyme was not observed in non-mycorrhizal roots, indicating specific expression in the mycorrhizae. We suggest that MDH isozyme could serve as a specific, early indicator of the Zea-Glomus symbiosis. Differences in the esterase (EST) isozyme pattern were not detectable between VAM and non-VAM roots, suggesting that this enzyme system is not a good parameter for the evaluation of mycorrhizal colonization. As available P in the soil increased, total EST activity appeared to increase as well. Interestingly, total peroxidase (POX) activity increased along with P suggesting that as plant P nutrition improved, both cell wall ramification and the quantity of defense peroxidases increased as well. Total POX activity from mycorrhizal roots was inversely correlated with root colonization, indicating that there was suppression of POX activity by the host under low soil P. Most interestingly, formononetin further decreased POX activity regardless of the level of P or mycorrhizal status. This may suggest one mechanism by which formononetin enhances root VAM colonization. The presence of this isoflavone suppressed POX activity in mycorrhizal roots allowing a rapid penetration and spread of the fungus in the root cortex. The interplay between host root, soil P levels, secondary metabolites and endogenous host enzyme activities and a particular VAM fungus has a profound effect on the efficiency, duration and functioning of an endomycorrhizal symbiosis.     

Vesicular-arbuscular mycorrhizae in a wastewater-irrigated oldfield ecosystem in Michigan

The incidence of vesicular-arbuscular mycorrhizae (VAM) in wastewater irrigated and non-irrigated oldfield soils in Michigan was studied. Soil and root samples were taken monthly from field plots on the second and third years of consecutive irrigation with municipal wastewater at rates of 0, 5 and 10 cm wk^–1. The oldfield ecosystem contained a high VAM fungal spore population density, but low species diversity. The most common VAM fungal species were Glomus mosseae and G. fasciculatum. Both spore density and root colonization were higher in irrigated than in non-irrigated plots. Irrigation effects were largest early in the growing season. In addition to increasing VAM incidence, wastewater irrigation shifted VAM fungal species composition. Irrigation favored G. mosseae over G. fasciculatum. Bioassays using either Sorghum vulgare or Daucus carota, an oldfield native species, indicated that the VAM systems were still functioning after the third year of consecutive wastewater irrigation. The data from experiments using nutrient solutions at wastewater concentrations suggest that the effects of wastewater irrigation on VAM are due to the effects of both water and nutrients. Since VAM are a very important component of the plant's water and nutrient uptake system and equally important in structuring plant communities under limiting growth conditions, it is suggested that the stimulatory effect of wastewater irrigation on VAM in an oldfield ecosystem enhances the ecosystem's ability to function as a living filter for wastewater clean up.Michigan Agricultural Experimental Station Journal Article No 13137.On sabbatical leave from ESAL, Lavras-MG, Brazil     

Effects of a vesicular-arbuscular mycorrhizal fungus and other soil microorganisms on growth,mineral nutrient acquisition and root exudation of soil-grown maize plants

Maize (Zea mays L. cv. Alize) plants were grown in a calcareous soil in pots divided by 30-m nylon nets into three compartments, the central one for root growth and the outer ones for hyphal growth. Sterle soil was inoculated with either (1) rhizosphere microorganisms other than vesicular-arbuscular mycorrhizal (VAM) fungi, (2) rhizosphere microorganisms together with a VAM fungus [Glomus mosseae (Nicol. and Gerd.) Gerdemann and Trappel], or (3) with a gamma-irradiated inoculum as control. Plants were grown under controlled-climate conditions and harvested after 3 or 6 weeks. VAM plants had higher shootroot ratios than non-VAM plants. After 6 weeks, the concentrations of P, Zn and Cu in roots and shoots had significantly increased with VAM colonization, whereas Mn concentrations had significantly decreased. Root exudates were collected on agar sheets placed on the interface between root and hyphal compartments. Six-week-old VAM and non-VAM plants had similar root exudate compositions of 72–73% reducing sugars, 17–18% phenolics, 7% organic acids and 3% amino acids. In another experiment in which root exudates were collected on agar sheets with or without antibiotics, the amounts of amino acids and carbohydrates recovered were similar in VAM and non-VAM plants. However, threeto sixfold higher amounts of carbohydrates, amino acids and phenolics were recovered when antibiotics were added to the agar sheets. Thus, the high microbial activity in the rhizosphere and on the rhizoplane limits the exudates recovered from roots.     

Seasonal dynamics of the association between sweet potato and vesicular-arbuscular mycorrhizal fungi

To better understand the behavior of selected vesicular-arbuscular mycorrhizal (VAM) isolates in the field, we documented the growth of roots, root hairs, and VAM colonization of inoculated and noninoculated sweet potato plants (Ipomea batatas (L.) Lam. cv White Star) over a growing season. We also determined the seasonal dynamics of P and Zn uptake, and shoot and storage-root growth. Shoot cuttings were inoculated with an isolate of either Glomus etunicatum Becker and Gerdemann or Acaulospora rugosa Mortan, or were not inoculated, and were harvested 2, 4, 8, 13, 20, and 27 weeks after planting (WAP). At each harvest, roots were sampled at 0 to 30, 30 to 60, and 60 to 90 cm depths and at 0, 23, 83, and 116 cm from the base of the shoot. At the end of the study, the roots of three non-inoculated plants were sampled by soil horizon. Inoculation had no affect on shoot growth or total shoot uptake of P and Zn; shoot dry mass and P and Z content increased rapidly up to 20 WAP, while shoot length continued to increase through 27 WAP. Shoot-P concentration of plants inoculated with A. rugosa at 2 and 8 WAP were higher than the noninoculated plants, while shoot-Zn concentration was not affected by inoculation. Storage-root yields of inoculated plants were higher than yields for noninoculated plants. Root length density, and percentage of root length with root hairs and VAM colonization were highest and most dynamic near the base of the plant. Percentage of root length colonization by VAM fungi was highest in the E2 horizon, intermediate in the Bh horizon, and lowest in the Ap horizon. Percentage of root length with root hairs had the opposite pattern. Intensive measurements of root characteristics close to the base of the plant, and shoot P-content and concentration during the period of rapid yield production, provided the most useful data for evaluating the activity of effective isolates.Published as Florida Agricultural Experimental Station Journal Series No. R-02576     

The modes of action of two benzimidazoles and thiophanate-methyl used for the control of Verticillium wilt of strawberry

Benomyl, thiabendazole and thiophanate-methyl were found to be fungistatic against Verticillium dahliae in vitro. All materials suppressed wilt symptoms on artificially inoculated strawberry plants when applied as a root dip, soil drench or a foliar application. Quantitative bioassays of the amount of toxic residues and fungal colonization in tissues of treated plants after root treatment revealed that these materials or their toxic breakdown products were readily translocated to crown and leaves: they suppressed the internal spread of the pathogen throughout the plant. After foliar application there was no marked basipetal movement of material within the plant: fungal colonization was greatly reduced in treated leaves, but the roots and crowns were colonized to a similar extent to those of untreated plants.     

Salinity and flooding stress effects on mycorrhizal and non-mycorrhizal citrus rootstock seedlings

Seedlings of the rootstocks Pineapple sweet orange (SwO), Carrizo citrange (CC), and sour orange (SO) were grown in low phosphorus (P) sandy soil and either inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungus,Glomus intraradices, or were non-mycorrhizal (NM) and fertilized with P. VAM and NM seedings of similar shoot size and adequate P-status were selected for study of salinity and flooding stress. One-third of each of the VAM and NM plants were given 150 mM NaCl for a period of 24 days. One-third of the plants were placed into plastic bags and flooded for 21 days while the remaining third were non-stressed controls. In general, neither stress treatment affected mycorrhizal colonization. Salinity stress reduced the hydraulic conductivity of roots, leaf water potential, stomatal conductance and net assimilation of CO₂ (ACO₂) of mycorrhizal and non-mycorrhizal seedlings to a similar extent. VAM plants of CC and SO accumulated more Cl in leaves than NM plants. Cl was higher in non-mycorrhizal roots of SwO and CC than in mycorrhizal roots. Flooding the root zone for 3 weeks did not produce visible symptoms in the shoot but did influence plant water relations and reduce ACO₂ of all 3 rootstocks. VAM and NM plants of each rootstock were affected similarly by flooding. Comparable reduction in nitrogen and P content of both mycorrhizal and non-mycorrhizal plants suggested that flooding stress was primarily affecting root rather than hyphal nutrient uptake. Florida Agricultural Experimental Station Journal Series No. 7773.     

The interaction of rockphosphate,Bradyrhizobium, vesicular-arbuscular mycorrhizae and phosphate-solubilizing microbes on soybean grown in a sub-Himalayan mollisol

A factorial design 2³ × 4 with two levels of Mussorie rockphosphate (RP) with or without vesicular-arbuscular mycorrhizal (VAM) fungi and Bradyrhizobium japonicum, and four treatments of phosphate-solubilizing microbes (PSM) Pseudomonas striata, Bacillus polymyxa, Aspergillus awamori was employed using Patharchatta sandy loam soil (Typic Hapludoll). The observations included mycorrhization, nodulation, grain and straw yield, N and P uptake, available soil P and the PSM population in the soil after crop harvest. Inoculation with endophytes alone caused about 70% root colonization. Addition of rockphosphate or inoculation with PSM, except B. polymyxa, stimulated root infection of native as well as introduced VAM endophytes. Application of RP or inoculation with Bradyrhizobium japonicum, mycorrhizal fungi or phosphate-solubilizing microorganisms significantly increased nodulation, N uptake, available soil P and the PSM population in the soil after the crop harvest. The grain and straw yields did not increase following RP addition or mycorrhizal inoculation but increased significantly after inoculation wit Bradyrhizobium or PSM. In general, the application of RP, Bradyrhizobium, VAM and PSM in combinations of any two or three resulted in significant increases in nodulation, plant growth, grain yield and uptake of N and P. Among the four factor interactions, rockphosphate, Bradyrhizobium and P. striata in the absence of VAM resulted in maximal nodulation, grain and straw yields and N uptake by soybean. The highest P uptake by soybean grain was recorded with Bradyrhizobium and A. awamori in the absence of rockphosphate and VAM. Generally, available soil P and PSM population after crop harvest were not significantly increased by the treatment combinations giving the maximal uptake of nutrients. However, they increased significantly in response to PSM, which produced no significant increase in total uptake of nutrients.Research paper no. 7498     

A fungal root symbiont modifies plant resistance to an insect herbivore

Vesicular-arbuscular mycorrhizal (VAM) fungi are common root-colonizing symbionts that affect nutrient uptake by plants and can alter plant susceptibility to herbivores. I conducted a factorial experiment to test the hypotheses that colonization by VAM fungi (1) improves soybean (Glycine max) tolerance to grazing by folivorous Mexican bean beetle (Epilachna varivestis), and (2) indirectly affects herbivores by increasing host resistance. Soybean seedlings were inoculated with the VAM fungus Glomus etunicatum or VAM-free filtrate and fertilized with high-[P] or low-[P] fertilizer. After plants had grown for 7 weeks first-instar beetle larvae were placed on bagged leaves. Growth of soybean was little affected by grazing larvae, and no effects of treatments on tolerance of soybeans to herbivores were evident. Colonization by VAM fungus doubled the size of phosphorus-stressed plants but these plants were still half the size of plants given adequate phosphorus. High-[P] fertilizer increased levels of phosphorus and soluble carbohydrates, and decreased levels of soluble proteins in leaves of grazed plants. Colonization of grazed plants by VAM fungus had no significant effect on plant soluble carbohydrates, but increased concentration of phosphorus and decreased levels of proteins in phosphorus-stressed plants to concentrations similar to those of plants given adequate phosphorus. Mexican bean beetle mass at pupation, pupation rate, and survival to eclosion were greatest for beetles reared on phosphorus-stressed, VAM-colonized plants, refuting the hypothesis that VAM colonization improves host plant resistance. VAM colonization indirectly affected performance of Mexician bean beetle larvae by improving growth and nutrition of the host plant. Received: 28 February 1997 / Accepted: 23 June 1997     

The Effect of Photoperiod on Levels of Endogenous Cytokinins in Xanthium strumarium

The cytokinin content of Xanthium strumarium L. plants decreased markedly when they were exposed to short days (SD). There was a significant decrease in the content of the butanol-soluble cytokinins of the mature leaves after only 5 SD cycles, and after 10 SD there was no significant cytokinin activity in butanol extracts; the changes in the young leaves were less marked. Most of the cytokinin activity in mature leaves appears to be present in the aqueous fraction, whereas in young leaves most activity occurs in the butanol-soluble fraction. SD treated plants produced less root exudate than LD plants, but there were no significant differences in the amounts of cytokinin in the root exudates from LD and SD plants collected over an equivalent time period. The cytokinin levels of SD-induced leaves remained low even when transferred back to LD. The observed differences in cytokinin levels did not appear to be the result of photosynthetic differences. Exposure of detached leaves to LD or SD did not result in differences in cytokinin content. It is not clear whether the observed changes in cytokinin levels in the leaves under SD are involved in the flowering response, but they may be causally related to a reduced chlorophyll content observed in SD-induced leaves.     

Hyphal contribution to water uptake in mycorrhizal plants as affected by the fungal species and water status

Vesicular-arbuscular mycorrhizae may increase resistance of plants to drought by a number of mechanisms, such as increased root hydraulic conductivity, stomatal regulation, hyphal water uptake and osmotic adjustment. However, a substantial contribution of vesicular-arbuscular mycorrhizal (VAM) hyphae to water uptake has not been demonstrated unequivocally. The objective of this investigation was to examine the contribution of hyphae from two VAM fungi to water uptake and transport by the host plant. Lettuce (Lactuca sativa L.) plants were grown in a container divided by a screen into two compartments. One was occupied by roots, the other only by VAM hyphae, which the screen permitted to pass. Roots were colonized by the VAM fungi Glomus deserticola or Glomus fasciculatum, or were left uninoculated but P-supplemented. Water was supplied to the hyphal compartment at a distance of 10 cm from the screen (root). CO₂ exchange rate, water-use efficiency, transpiration, stomatal conductance and photosynthetic phosphorus-use efficiency of VAM or P-amended control plants were evaluated at three levels of water application in the hyphal compartment. Results indicate that much of the water was taken up by the hyphae in VAM plants. VAM plants, which had access to the hyphal compartment, had higher water and nutrient contents. G. deserticola functioned efficiently under water limitation and mycelium from G. fasciculatum-colonized plants was very sensitive to water in the medium. This discrepancy in VAM behaviour reflects the various abilities of each fungus according to soil water levels. Different abilities of specific mycelia were also expressed in terms of nutritional and leaf gas-exchange parameters. G. fasciculatum caused a significant increase in net photosynthesis and rate of water use efficiency compared to G. deserticola and P-fertilized plants. In contrast, the G. deserticola treatment was the most efficient affecting N, P and K nutrition, leaf conductance and transpiration. Since no differences in the intra- and extra-radical hyphal extension of the two endophytes were found, the results demonstrate that mycorrhizal hyphae can take up water and that there are considerable variations in both the behaviour of these two VAM fungi and in the mechanisms involved in their effects on plant water relations.     

Response of mycorrhizal periwinkle plants to aster yellows phytoplasma infection

The objective of our research was to assess if arbuscular mycorrhizal (AM) fungal colonization can modify the effect of infection by two aster yellows phytoplasma strains (AY1, AYSim) in Catharanthus roseus plants. Both phytoplasma strains had a negative effect on the root fresh weight, but they differed in symptoms appearance and in their influence on photosynthetic and transpiration rates of the periwinkle plants. AM plants showed significantly reduced shoot fresh weight, while the transpiration rate was significantly increased. AM fungal colonization significantly affected shoot height and fresh weight of the plants infected by each phytoplasma strains as well as the root system of plants infected with the more aggressive AYSim phytoplasma strain. Double inoculation did not reduce the negative effects induced with phytoplasma alone on the photosynthetic activity of phytoplasma-infected plants.