Y. Kapulnik

Development and function of Azospirillum-inoculated roots

The surface distribution of Azospirillum on inoculated roots of maize and wheat is generally similar to that of other members of the rhizoplane microflora. During the first three days, colonization takes place mainly on the root elongation zone, on the base of root hairs and, to a lesser extent, on the surface of young root hairs. Azospirillum has been found in cortical tissues, in regions of lateral root emergence, along the inner cortex, inside xylem vessels and between pith cells. Inoculation of several cultivars of wheat, corn, sorghum and setaria with several strains of Azospirillum caused morphological changes in root starting immediately after germination. Root length and surface area were differentially affected according to bacterial age and inoculum level. During the first three weeks after germination, the number of root hairs, root hair branches and lateral roots was increased by inoculation, but there was no change in root weight. Root biomass increased at later stages. Cross-sections of inoculated corn and wheat root showed an irregular arrangement of cells in the outer layers of the cortex. These effects on plant morphology may be due to the production of plant growth-promoting substances by the colonizing bacteria or by the plant as a reaction to colonization. Pectic enzymes may also be involved. Morphological changes had a physiological effect on inoculated roots. Specific activities of oxidative enzymes, and lipid and suberin content, were lower in extracts of inoculated roots than in uninoculated controls. This suggests that inoculated roots have a larger proportion of younger roots. The rate of NO 3 - , K+ and H2PO 4 - uptake was greater in inoculated seedlinds. In the field, dry matter, N, P and K accumulated at faster rates, and water content was higher in Azospirillum-inoculated corn, sorghum, wheat and setaria. The above improvements in root development and function lead in many cases to higher crop yield.

Development and function ofAzospirillum-inoculated roots

SummaryThe surface distribution ofAzospirillum on inoculated roots of maize and wheat is generally similar to that of other members of the rhizoplane microflora. During the first three days, colonization takes place mainly on the root elongation zone, on the base of root hairs and, to a lesser extent, on the surface of young root hairs.Azospirillum has been found in cortical tissues, in regions of lateral root emergence, along the inner cortex, inside xylem vessels and between pith cells. Inoculation of several cultivars of wheat, corn, sorghum and setaria with several strains ofAzospirillum caused morphological changes in root starting immediately after germination. Root length and surface area were differentially affected according to bacterial age and inoculum level. During the first three weeks after germination, the number of root hairs, root hair branches and lateral roots was increased by inoculation, but there was no change in root weight. Root biomass increased at later stages. Cross-sections of inoculated corn and wheat root showed an irregular arrangement of cells in the outer layers of the cortex. These effects on plant morphology may be due to the production of plant growth-promoting substances by the colonizing bacteria or by the plant as a reaction to colonization. Pectic enzymes may also be involved. Morphological changes had a physiological effect on inoculated roots. Specific activities of oxidative enzymes, and lipid and suberin content, were lower in extracts of inoculated roots than in uninoculated controls. This suggests that inoculated roots have a larger proportion of younger roots. The rate of NO−3, K+ and H2PO−4 uptake was greater in inoculated seedlinds. In the field, dry matter, N, P and K accumulated at faster rates, and water content was higher inAzospirillum-inoculated corn, sorghum, wheat and setaria. The above improvements in root development and function lead in many cases to higher crop yield.

Effect of Azospirillum inoculation on some growth parameters and n-content of wheat, sorghum and panicum

The potential of the nitrogen fixing bacteriumAzospirillum brasilense to enhance development and increase growth of several gramineae was investigated. In both sterilized and non-sterilized systems heading and flowering occurred earlier in the inoculated plants as compared to the noninoculated ones. Total shoot and root weights, total-N content, plant height and leaf length were significantly increased by inoculation.

Yield Increases in Summer Cereal Crops in Israeli Fields Inoculated with Azospirillum

Inoculating Zea mays (three cultivars), Sorghum bicolor, Panicum miliaceum and Setaria italica with nitrogen-fixing bacteria of the genus Azospirillum in Northern Negev and Bet Shean Valley field experiments resulted in significant increases in yield of grain and foliage of commercial value. It was concluded that inoculating summer cereal crops in Israel may save valuable nitrogen fertilizer.

Response of Non-irrigated Sorghum bicolor to Azospirillum Inoculation

In two experiments, carried out on non-irrigated fields in the Northern Negev region of Israel, inoculation of Sorghum bicolor with bacteria of the genus Azospirillum resulted in significant increases over controls of 17% in grain yield of cv. 610 and of 19% in the forage yield of cv. 6078. In addition, significant increases over controls in plant dry weight, mineral content (N, P and K) and panicle number were obtained by inoculation of cv. 610.

Effect of Azospirillum inoculation on nitrogen fixation and growth of several winter legumes

Inoculation of naturally nodulated Pisum sativum L. (garden pea) with Azospirillum in the greenhouse caused a significant increase in nodule numbers above controls. Field inoculation of garden peas in the winter 1981–1982 and Cicer arietinum L. (chick pea), in winter 1982–1983, with Azospirillum one week after plant emergence, produced a significant increase in seed yield, but did not affect plant dry matter yield. For Vicia sativa L. (vetch) grown in soil in the greenhouse and in the field for forage, winter 1980–1981, inoculation significantly increased dry matter yield, %N, N-content, and acetylene reduction (nitrogen fixation) activity. In Hedysarum coronarium L. (sulla clover), winter 1981–1982, inoculated with both its specific Rhizobium (by the slurry method) and Azospirillum, 7 days after emergence, there was an increase in acetylene reduction above controls inoculated with Rhizobium alone. These results suggest that it is possible, under conditions tested in this work, to increase nodulation, nitrogen fixation, and crop yields of winter legumes by inoculation with Azospirillum.

Adsorption of Azospirillum brasilense to corn roots

Abstract The adsorption of Azospirillum brasilense strain Cd to corn roots was measured using 32 P-labeled Azospirillum cells. Binding kinetics showed that adherence of bacteria to roots increased during the first 90 min and attained a maximum level within 4.5 h. Maximum binding occurred at pH 6.1. The presence of Ca 2+ , Mg 2+ , Na + or poly- l -lysine (a polycation) reduced binding. Bacterial adherence to roots increased with increasing bacterial concentration up to 10 9 cells ml −1 of binding mixture. At higher concentrations, binding approached saturation. The bacteria attached to the roots and the bacteria in the solution around the roots were found to be in a dynamic equilibrium. Killing bacteria or roots with heat or glutaraldehyde inhibited the attachment. High concentrations of bacteria in the binding mixture (10 9 −10 11 cells ml −1 ) resulted in binding of bacteria to the roots in aggregates. The presence of several sugars, amino acids and organic acids inhibited adsorption. Chemotaxis of Azospirillum towards corn root exudates was observed. Treating the extract with trypsin eliminated this effect. At pH 6.1, Azospirillum brasilense strain Cd adsorbed in higher numbers than Klebsiella pneumoniae, Pseudomonas flourescens and Escherichia coli . Differences in adsorption levels were also found among several strains and mutants of Azospirillum .

Contribution of nitrogen fixed by Azospirillum to the n nutrition of spring wheat in Israel

Acetylene reduction activity (ARA) was measured in cores containing roots of various Israeli wild and cultivated wheat lines colonized by Azospirillum. The inoculated plants were grown under greenhouse or field conditions. Although, no measurable ARA was detected during earlier stages of wheat development, 50–600 nmol C2H4 g−1 dry root h−1 was measured during heading and flowering stages. By using N yield balance and 15N dilution techniques, it was found that Triticum aestivum cv. Miriam inoculated with Azospirillum accumulated 20% more N (14N and 15N) at the booting stage than did the uninoculated control. This difference in N content became less apparent in grains. No significant 15N dilution could be found and the contribution of atmospheric N2 to the N content of grains of inoculated plants was negligible. It was concluded that the potential contribution of biological N2 fixation to spring wheat cultivation in Israel is very low.

Yield response of spring wheat cultivars (Triticum aestivum and T. turgidum) to inoculation with Azospirillum brasilense under field conditions

SummaryEight commercial Israeli spring wheat cultivars (six Triticum aestivum and two T. turgidum) grown with 40 and 120 kg N/ha were tested for responses to inoculation with Azospirillum brasilense. At the low level of N fertilization (40 kg/ha), five cultivars showed significant increases in plant dry weight measured at the milky ripe stage; however, by maturation only the cultivar “Miriam” showed a significant increase in grain yield. Two cultivars, which had shown a positive inoculation effect at the earlier stages, had a significant decrease in grain yield. No significant effect of inoculation was found at the high N level. To confirm those results, four wheat (T. aestivum) cultivars were tested separately over 4 years in 4 different locations under varying N levels. Only Miriam showed a consistently positive effect of Azospirillum inoculation on grain yield. Inoculation increased the number of roots per plant on Miriam compared with uninoculated plants. This effect was found at all N levels. Nutrient (N, P and K) accumulation and number of fertile tillers per unit area were also enhanced by Azospirillum, but these parameters were greatly affected by the level of applied N. It is suggested that the positive response of the spring wheat cultivar “Miriam” to Azospirillum inoculation is due to its capacity to escape water stresses at the end of the growth season.

THE EFFECT OF AZOSPIRILLUM INOCULATION ON GROWTH AND YIELD OF CORN

ABSTRACT Commercial yields of sweet corn cv. Jubilee were increased by field inoculation with Azospirillum. This bacterium was found in association with grass roots in different soils all over the world. In field experiments carried out in several parts of Israel Azospirillum was shown to affect both vegetative and reproductive growth of corn at different levels of nitrogen fertilization. Effects on vegetative growth induded increase in plant height and fresh and dry weight of plant foliage. The effects on reproductive structures were expressed mainly via increase in ear number per plant. It was concluded that it may be possible partially to replace combined N fertilizer in corn by Azospirillum.