Azospirillum spp. participation in dry matter partitioning in grasses at the whole plant level

Plant growth-promoting rhizobacteria, particularly those from the genus Azospirillum spp., may affect root functions such as growth and nutrient/water uptake, which in turn may affect shoot growth. Calculations based on data from literature on shoot and root mass of crop grasses (79 plant/bacteria associations were analyzed) revealed that inoculation with Azospirillum spp. increased the shoot-to-root (S/R) ratio in about half of reported cases and decreased the S/R ratio in the other half. In 11 of 35 cases, the S/R ratio increased when the shoot mass increased more than the root mass. In 23 of 35 cases, the root mass did not increase, yet the S/R ratio still increased. Thus, the increase in the S/R ratio indicated that shoot growth responds to inoculation more than root growth. A decrease in the S/R ratio occurred when (a) root growth dominated shoot growth even though both increased (16 of 36 cases), or (b) root growth either increased or remained unchanged, and shoot growth was either unaffected or even decreased (19 of 36 cases). This analysis suggests that: (a) Azospirillum spp. participates in the partitioning of dry matter (both carbon compounds and minerals) at the whole plant level by affecting root functions, and (b) the bacteria affect crop grass through multiple mechanisms operating during plant development.     

Cadmium in Fagus sylvatica L. trees and seedlings: Leaching,uptake and interconnection with transpiration

Most of the airborne Cd-polluted particles which eventually precipitate in forest regions remain on the surface of the tree leaves and do not penetrate into the plants' live tissues. Such pollutions can be removed from the leaves by cation exchange or can be washed off with water of low pH. Acid rains and acid soils have contributed very much to the solubilization of Cd and to its transformation into an available ionic form which is later absorbed by tree roots. ^115mCd uptake by young beech trees (Fagus sylvatica L.) seems to be positively correlated with the concentrations of the applied solutions as well as with the duration of the exposure. Low environmental pH increases the rate of ^115mCd uptake. High or low transpiration had no apparent effect on root absorption of Cd, but exposure of beech trees to a Cd(NO₃)₂ solution reduced their rates of transpiration after very few days of treatment.     

Enhanced establishment and growth of giant cardon cactus in an eroded field in the Sonoran Desert using native legume trees as nurse plants aided by plant growth-promoting microorganisms and compost

To evaluate the feasibility of long-term desert reforestation technology of mixed vegetation, cardon cactus (Pachycereus pringlei) seedlings from indoor and outdoor nurseries were planted in the field adjacent to one seedling of potential legume nurse trees: mesquite amargo (Prosopis articulata), yellow palo verde (Parkinsonia microphylla), and blue palo verde (Parkinsonia florida). Some of the planting holes were also supplemented with common dairy compost. Additionally, the combinations of legume tree–cactus were inoculated with either a consortium of desert arbuscular mycorrhizal (AM) fungi, plant growth promoting bacteria (PGPB; the diazotroph Azospirillum brasilense Cd, and the phosphate solubilizer Paenibacillus sp.), or a mixture of all. The field experiments were evaluated periodically during 30 months for survival and growth. Cardons reared in an outdoor screen house survived better in the field than those reared in a controlled growth chamber and hardened later outdoors. Association with any legume nurse tree increased survival and enhanced growth of untreated cardons. For cardons growing alone, application of either compost, AM fungi, and all the treatments combined increased survival. For these plants, no treatment affected plant growth during the first 3 months after transplanting. Later, all treatments, except for AM fungi, enhanced plant growth. However, only 2 years after transplanting the enhanced growth effect of AM fungi was also significant. In the presence of the legume nurse trees, transient positive effects on cardon growth were recorded. General evaluation after 30 months of cultivation showed that the treatments positively affected cardon growth when growing alone or in combination only with mesquite amargo but not with the other two legume trees. This study proposes that young legume trees have the capacity to enhance survival and growth of cardon cactus, depending on the legume cactus combination. Additional treatments such as compost or PGPB can either amplify the effect or else attenuate it.     

Involvement of a phytotoxic peptide in the development of the Northern leaf blight of corn

Non pathogenic isolates ofExserohilum turcicum successfully infect corn plants in the presence of syntheticE. turcicum toxin during inoculation. The toxin significantly increased the number of appressoria and the ramification of germinating conidia both on host leaves and on artificial media. These findings indicate that this toxin plays an important role in infection of Northern leaf blight.This article is dedicated to the memory of Yehouda Levy who passed away recently.     

Supplementation of tryptophan and lysine in Diplodus sargus larval diet: effects on growth and skeletal deformities

Amino acids are the building blocks for growth and the major energy source during fish larval stages. Deficient amino acids can be supplemented in the diets, overcoming problems such as low growth rates and skeletal deformities. In this study, three experimental diets were tested: a balance diet supplemented with lysine, a balance diet supplemented with tryptophan and a control with no supplementation. Trials were conducted with Diplodus sargus larvae from 1 to 25 days after hatching (DAH). A microencapsulated diet was introduced at 15 DAH in co-feeding with live feed and from 20 DAH larvae were fed only this diet. The effect of the supplemented diets was assessed in terms of survival, growth rate, skeletal deformities, ammonia excretion and activity of amino acid catabolism enzymes. The results showed a similar survival in all treatments. However, larvae given tryptophan supplementation had a lower weight on 25 DAH. No significant differences were found in ammonia excretion, frequency or type of deformities or enzymatic activity. Tryptophan and lysine supplementation failed to improve larval growth, survival or larval quality.     

Growth promotion of the seawater-irrigated oilseed halophyte Salicornia bigelovii inoculated with mangrove rhizosphere bacteria and halotolerant Azospirillum spp.

 Inoculation of the oilseed halophyte Salicornia bigelovii Torr. with eight species of halotolerant bacteria, grown in seawater-irrigated pots under environmental conditions native to the plant's habitat, resulted in significant plant growth promotion by the end of the growing season, 8–11 months later. Statistical analysis demonstrated that inoculation with Azospirillum halopraeferens, a mixture of two Azospirillum brasilense strains, a mixture of Vibrio aestuarianus and Vibrio proteolyticus, or a mixture of Bacillus licheniformis and Phyllobacterium sp. significantly increased plant height and dry weight at the end of the season. Some of the bacterial strains also increased the number of side branches and the size of the spikes. The bacteria did not affect the number of seeds or their weight. Inoculation with the mangrove cyanobacterium Microcoleus chthonoplastes had no effect on plant foliage variables. At the end of the growing season, the N and protein content of the plant foliage was significantly reduced by bacterial inoculation; however, the N and protein content of seeds significantly increased. The P content in foliage increased significantly in plants treated with all the bacteria except M. chthonoplastes, whereas the total lipid content of foliage increased significantly only when plants were inoculated with a mixture of A. brasilense strains or with M. chthonoplastes. In three inoculation treatments palmitic acid in seeds significantly increased and linoleic acid significantly decreased. This study demonstrates the feasibility of using bacteria to promote the growth of halotolerant plants cultivated for forage and seed production in proposed seawater-irrigated agriculture. Received: 3 January 2000     

Interactions of Azospirillum spp. in soils: a review

 This review summarizes and discusses the current knowledge and the, as yet, unanswered questions on the interactions of Azospirillum spp. in bulk soil (but not in the rhizosphere). It contains sections on the isolation of these bacteria from tropical to temperate soils, and on their short- and long-term persistence in bulk soil. The interactions of these bacteria with soil particles and minerals such as clay, sand and Ca, and the effect of soil pH, soil redox potential, and the cation exchange capacity of the soil on them is demonstrated. Data is presented on the distribution of Azospirillum spp. in soils, on their production of fibrillar material essential for anchoring the cells to soil particles, on the effects of soil irrigation, and of external soil treatments, and on the effect of soil C and C used in bacterial inoculants on the cells. It shows that root exudates possibly govern bacterial motility in the soil. Finally, the effect of pesticide applications, the relationships with other soil microorganisms such as Bdelovibrio spp., Bradyrhizobium spp., and phages, and the potential use of a community-control model of Azospirillum spp. in soil and in the rhizosphere is suggested. Received: 11 November 1998     

Tyrosine and phenylalanine supplementation on Diplodus sargus larvae: effect on growth and quality

Phenylalanine is the precursor of tyrosine, which is involved in the synthesis of several molecules with key roles in the regulation of metabolism and growth, stress response and pigmentation. In this study, three experimental diets were tested: an amino acid (AA) balanced diet supplemented with phenylalanine, another supplemented with phenylalanine and tyrosine and a non‐supplemented AA balanced diet. Rotifers were enriched with liposomes encapsulating free AA in order to obtain a balanced AA profile. The experimental diets resulted in similar larval survival, growth, enzyme activities of AA catabolism and nitrogen excretion in all treatments. High levels of skeletal deformities were registered and significant differences were found between the control and the phenylalanine treatment for the percentage of vertebral compressions in the trunk region of the vertebral column (30% in the control and 5% in the phenylalanine group). A significantly higher survival to a temperature stress test was found for larvae fed the diet supplemented with phenylalanine and tyrosine. The results suggest that supplementation of phenylalanine/tyrosine in fish diets may be useful in order to reduce skeletal deformities and mortalities caused by stress. The present study confirms that AA requirements may be sufficient for covering growth and survival but insufficient to cover other metabolic processes.     

Reduction of Bacterial Speck (Pseudomonas syringae pv. tomato) of Tomato by Combined Treatments of Plant Growth-promoting Bacterium, Azospirillum brasilense, Streptomycin Sulfate, and Chemo-thermal Seed Treatment

Inoculation of tomato seeds with the plant growth-promoting bacterium Azospirillum brasilense, or spraying tomato foliage with A. brasilense, streptomycin sulfate, or commercial copper bactericides, separately, before or after inoculation with Pseudomonas syringae pv. tomato, the casual agent of bacterial speck of tomato, had no lasting effect on disease severity or on plant height and dry weight. Seed inoculation with A. brasilense combined with a single streptomycin foliar treatment and two foliar bactericide applications at 5-day intervals (a third or less of the recommended commercial dose) reduced disease severity in tomato seedlings by over 90% after 4 weeks, and significantly slowed disease development under mist conditions. A. brasilense did not induce significant systemic resistance against the pathogen although the level of salicylic acid increased in inoculated plants. Treatment of tomato seeds that were artificially inoculated with P. syringae pv. tomato, with a combination of mild chemo-thermal treatment, A. brasilense seed inoculation, and later, a single foliar application of a copper bactericide, nearly eliminated bacterial leaf speck even when the plants were grown under mist for 6 weeks. This study shows that a combination of otherwise ineffective disease management tactics, when applied in concert, can reduce bacterial speck intensity in tomatoes under mist conditions.     

Mechanisms of symptom production by foliar bacterial pathogens

Most foliar bacterial pathogens are pathovars ofPseudomonas syringae andXanthomonas campestris. Many of them live during the greater part of their life-cycle on the upper parts of the plant. In order to survive and develop in this environment, the pathogens have elaborated an array of mechanisms which enable them to penetrate the plant foliage, produce a substantial endophytic population, and use the host tissue as a nutrient source. These activities result in the formation of different categories of symptoms,e.g. necrotic and chlorotic lesions, wilting, gall formation, leaf abscission and inhibition of plant growth. The operating mechanisms of foliar phytopathogenic bacteria can be divided into four major groups: (i) production of toxins (mostly non-specific), mainly byP. syringae pathovars; (ii) excessive production of plant growth hormones; (iii) enzymatic hydrolyzing activity; and (iv) inhibition of seedling growth by unknown mechanism(s). More than one pathogenic mechanism may operate in a given leaf disease or by a single pathogen. Despite the large amount of literature describing visual, biochemical and biophysical aspects of any leaf disease, basic knowledge is fragmental and comprehensive understanding of the mode of action of any disease is lacking. It is proposed that in order to close up these gaps in our knowledge other approaches, such as molecular biology technology and more intensive use of both bacterial and plant mutants, should be incorporated in pathological studies of foliar bacterial diseases.