Understanding Arbuscular Mycorrhizal Fungi

David Douds, a microbiologist, Philip Pfeffer, a chemist, and Gerald Nagahashi, a cell biologist, are working together at the USDA Eastern Regional Research Center in Wyndmoor, Pennsylvania, to better understand the interaction of arbuscular mycorrhizal (AM) fungi with plants. Living inside and outside root cells, mycorrhizal fungi extend thread-like hyphae into the soil to gather nutrients for the plant. In return, the plant provides the fungus with glucose required for its growth.                

Modern agricultural practices, chemical use in particular, has drastically reduced the populations of AM fungus in the soil. Comparison studies of conventional and low-input, or organic, farming systems showed greater spore populations and greater root colonization in the organic plots. While currently impractical for farmers to repopulate their soil with these beneficial microorganisms, these researchers are examining the feasibility of on-farm mass production of AM fungus inoculum already native to a site, or introduction of nonnative AM fungi through use of colonized host plants.                     

Using advanced scientific techniques, they have established that nitrogen is transported as the amino acid arginine. The external hyphae of the fungal structures in the plant roots take up the nitrogen from the soil. The fungus breaks it down to ammonium and transfers it to the host plant. According to Pfeffer, this discovery implies the role of mycorrhizal fungi may be much more significant in the worldwide nitrogen cycle than previously believed.                             

At this time, researchers cannot cultivate the fungus without a host. The fungus cannot complete its life cycle—producing spores—without the organic nutrients and other stimuli supplied by the hosts roots. They have investigated the effect of several environmental factors including root compounds, blue light from the sun’s spectrum and carbon dioxide. While these factors can independently promote AM fungal growth, they apparently work synergistically when combined.

Core, Jim. 2006. Getting to the Root of the Solution. Agricultural Research. 54:1(12-13).

Botrytis Suppression in Freesia hybrida

Considering the increasing public concern over the human and environmental risks associated with fungicide use, researchers are continually looking for alternative controls for destructive pathogens. Methyl jasmonate (MeJA) has been identified as a potential activator of a plant’s natural defenses. This study, conducted in the United Kingdom, tested the effectiveness of MeJA at suppressing the unattractive specking caused by Botrytis cinerea on the flowers of Freesia hybrida.         

Cut stems of freesia var. Cote d’ Azur were tested at the commercial harvest stage with the oldest bud fully developed but still closed. Treatments varied by MeJA concentration, temperature and time. Flowers in a sealed container were exposed to MeJA vapor for a 24-hour period. After the vapor treatment, half of the flowers were inoculated with a B. cinerea suspension, while the others were not inoculated. The flowers were “incubated,” or held, at 5, 12 and 20C, depending on the experiment.                                

Disease severity was evaluated each day for three days using an arbitrary scale based on the coverage of the lesions over the petal surface. Vase life, based on wilting and fresh weight, was measured in days.                        

Incubation at 20C significantly reduced the disease severity, the number of lesions and the lesion diameter. Compared to untreated controls, flowers treated with 0.1µl MeJa l-1 and incubated at 20°C displayed 58% reduced disease severity, 47% fewer lesions, and 53% smaller lesions.                   
Within the first 12 hours of incubation, severity of the disease symptoms of the MeJA treated flowers were significantly less than the untreated flowers. After the initial 12 hours, the disease appeared to progress at a similar rate while maintaining a significantly lower incidence.                 

Vase life of flowers treated at all concentrations (0.025, 0.05, and 0.1µl MeJa l-1) was slightly longer with the maximum vase life at 11.2 days. The untreated flowers had a vase life of 10.2 days.                       

Infection of B. cinerea occurs when a single conidium germinates, forms a germ tube and penetrates an epidermal cell. Fungal hyphae then colonize adjacent cells resulting in the formation of visible lesions, or specking. This research also looked for a direct in-vitro antimicrobial effect of MeJA on Bcinerea, but found none. Therefore, they suggest the treatment of MeJA reduced the incidence and severity of disease lesions of B. cinerea by activating a natural defense response in freesia.                                                                                                   

Darras, A.I., L.A. Terry, D.C. Joyce. 2005. Methyl jasmonate vapour treatment suppresses specking caused by Botrytis cinerea on cut Freesia hybrida L. flowers. Postharvest Biology and Technology 38(175-182).

Nitrogen Form and Quantity Affect Postharvest Quality of Ranunculus

Researchers at The Volcani Center in Israel investigated effects of nitrogen form and quantity on the development of Ranunculus asiaticus plants with a special interest in parameters associated with cut flower production and quality. Long known that nitrogen(N) is one of the most important macronutrients plants need, most research conducted to observe the differences in morphological development and physiological processes as affected by the nitrogen source (nitrate or ammonium ions) has focused on general plant growth rather than flower quality. This study recognizes that N nutrition affects various factors that may influence cut flower quality such as osmoregulation, water content in the plant, leaf water potential, accumulation of sugars and organic acids, ion uptake and chlorophyll content.                          

The treatment groups for greenhouse grown R. asiaticus ‘White Friandin’ were three levels of ammonium (10, 20 and 30%, under 100 ppm total N application) and two levels of N (50 and 100 ppm, under 20% ammonium application). Macro and micronutrients were also applied at an appropriate rate to all treatments for a complete fertility program. Flowers were harvested daily at the acceptable harvest stage—a closed bud.                                     

Vase life evaluation conditions were 20°C, 60-70% RH, and 12 hours of cool white fluorescent light. Leaf yellowing, flower senescence, wilting and stem topple were factors observed to determine vase life.                        

The cumulative number of flowers produced throughout the season was not significantly affected by the N fertilization treatments. However, fewer short (0-25cm), unmarketable flowers were produced in the 50 ppm N treatment. Flower bud diameter, an important quality parameter, was greater on stems from plants receiving 50 ppm N compared to those receiving 100ppm N. The largest shoots, based on shoot biomass, were found in the 50 ppm N treatment group. Root biomass was not affected by the N fertilization treatments.                

Ranunculus stems from the treatment group receiving 50 ppm N had a significantly increased vase life compared to the other treatments, however they also displayed increased leaf yellowing. The highest N (100 ppm) and ammonium (30%) concentrations displayed the lowest percentage of leaf yellowing.         

The fewest number of toppled stems occurred in the 50ppm N treatment group. Stem topple is a common problem in some geophytes including ranunculus and tulip. In tulips, calcium deficiency is thought to be the cause of stem topple. Since it is known that a high supply of ammonium interferes with calcium uptake, the incidence of stem topple would be expected increase as the amount of ammonium supplied increases. However, in this study, the lowest ammonium concentration produced the highest incidence of stem topple. Calcium content in the leaves of the ranunculus was unaffected by the increasing rate of ammonium suggesting the stem toppling disorder in ranunculus is the result of a different physiological mechanism than that which is associated with tulip.

Bernstein, N., M. Ioffe, M. Bruner, Y. Nishri, G. Luria, I. Dori, E. Matan, S. Philosoph-Hadas, N. Umiel, A. Hagiladi. 2005.

Effects of supplied nitrogen form and quantity on growth and postharvest quality of Ranunculus asiaticus flowers. HortScience 40(6):1879-1886.

Calcium Sulfate Affects Botrytis Severity and Vase Life of Roses

Researchers in Brazil are investigating environmentally friendly alternatives to the use of harmful fungicides for the control of Botrytis cinerea (gray mold) in roses. Not only does calcium elicit a natural defense response, it also appears to affect ethylene action on cell membranes reducing the effect of naturally produced ethylene on senescence. Delayed senescence is also desirable in the effort to combat B. cinerea as the pathogen prefers to infect senescent tissue.             

Calcium sulfate solutions of 2.5, 5.0 10.0 and 20.0 mM concentrations were sprayed to run-off on rose plants a) 1 day before harvest, b) 1 and 3 days before harvest, and c) 1, 3, and 5 days before harvest. The experiment only tested the variety ‘Kiss’ grafted on root stock Rosa indica Major.                 

As the flowers were harvested, half were sprayed with a B. cinerea spore suspension. Normal procedure from the greenhouse to packing house was followed with the inoculated flowers covered with a plastic bag. The flowers were held at 10°C, 90% RH for 36-48 hours. To observe vase life, the flowers were set in vases with tap water. The vases were constantly illuminated, in a room set to 20°C, 80% RH.                

Increased calcium sulfate concentration resulted in significantly increased vase life for flowers that were not inoculated. The control group had a vase life of 7-8 days while the highest concentration of calcium resulted in a vase life a 12-13 days. In the inoculated treatments, vase life also increased as calcium sulfate concentration increased, though the vase life compared to the non-inoulated treatments was reduced. The control for the inoculated treatment group averaged an eight day vase life while the highest calcium concentration only prolonged the vase life two days.

De Capdeville, G., L.A. Maffia, F.L. Finger, U.G. Batista. 2005. Pre-harvest calcium sulfate applications affect vase life and severity of gray mold in cut roses. Scientia Horticulturae 103(329-338).