A Closer Look at Stem Blockage: Air Bubble or Wound Response?

Researchers in the Netherlands have performed followup research to previous studies in an effort to determine the cause of inhibited water uptake in dry-stored cut flowers. It has long been accepted that an air embolism (or air bubble) is created when air is sucked into the xylem when the stem is cut. In addition to an air embolism, water inhibition can be caused by a physiological stem plugging that may be part of a wound response.
Chrysanthemum x morifolium cv. Cassa plants were grown in a greenhouse until commercial maturity. At harvest, the stem length ranged from 0.7 to 1.0 m (27.5 to 39 in.). Stems were cut underwater at the root-shoot junction prior to treatment. Treatments included various periods out of water followed by recovery periods in water. A pulse treatment of tropolone, a metal chelator, was also tested.
After a dry period as short as 30 minutes, the fresh weight of the chrysanthemum stem did not recover to the original fresh weight. The decreased hydraulic capacity of the stem, a result of the air embolism, caused the permanent decrease in fresh weight. A decreased hydraulic capacity was observed after only 5 minutes of dry storage.
However, after a dry period of two to four hours, the water blockage was removed in chrysanthemum stems that were placed in degassed water or underwent vacuum infiltration. Removal of the blockage resulted in increased hydraulic capacity and fresh weight.
After a second dry period and recovery period, the researchers observed that removing the air embolism from stems that were dry for more than two hours failed to remove the blockage, as previously demonstrated, indicating the presence of a second type of xylem blockage. Interestingly, stems that had been treated with the tropolone pulse did not exhibit this second blockage even in stems that were dry for 24 hours. After consideration of several possibilities, the researchers concluded that tropolone acts as an enzyme inhibitor in the xylem.
In another experiment, after two hours of dry storage, a 4-cm (1.6-in) stem segment was removed from the base of the stem in an effort to physically remove the blockage. While this was effective for periods of dry storage less than two hours, after a dry period of 24 hours, the removal of a 4-cm (1.6-in) stem segment did not remove the blockage.
Prior hypotheses have suggested a bacterial blockage in the xylem. Since bacterial plugging is limited to the xylem conduits that are opened by the cutting of the stem and bacteria can are not able to pass through the physical membrane in the xylem, a blockage 4 cm (1.6 in) from the base of the stem that is present after a 4- to 24-hour period of dry storage, would be highly unlikely. This further supports the suggestion that the blockage that developed after four hours of dry storage is an enzyme-mediated wounding response.
For Chrysanthemum ‘Cassa’, an air embolism is the sole obstruction for water uptake during the first one to two hours of dry storage. However, longer periods of dry storage will likely result in water inhibition by a plant-induced xylem blockage.
A key finding as they compared the results of this study to a previous study that used Chrysanthemum cv. Vyking, is that the genotype affects the xylem vessel diameter and therefore effects the emboli removal. Another study that used Zinnia elegans also found that recovery of hydraulic conductance was considerably better in plants grown in a water-stressed environment compared to those that were well-irrigated. It seems that genetics and pre-harvest environmental conditions play an additional role in the degree of water inhibition and recovery ability for dry stored cut flowers.

Van Meeteren, U., L. Arevalo-Galarza, W.G. van Doorn, 2006. Inhibition of water uptake after dry storage of cut flowers: Role of aspired air and wound-induced processes in Chrysanthemum. Postharvest Biology and Technology, Vol. 41. pp. 70-77.

Male and Female Responses to Flowers and Floral Fragrance

Doctoral research at Kansas State University analyzed the visual and olfactory effects of flowers and floral fragrance on males and female psychophysiological, emotional and cognitive responses. A cut flower arrangement was used to test the visual effect of flowers while lavender fragrance was used to assess the olfactory effect.  Additional treatments included a combination of the two treatments and a control with neither treatment.
Psychophysiological (P) responses were measured by recording brainwave activity, electrodermal activity and skin temperature. Emotional (E) responses were determined using a self-rated inventory of personal reactions. Cognitive performance (C) was assessed using a mental arithmetic task. Some of the results are outlined in the following table.

Lavender Fragrance (Olfactory)Less Relaxation More Physical Arousal (P) Lowered sadness and anger/aggression (E) Enhanced cognitive performance in calculating speed and accuracy (C)
Flower arrangement (Visual) Enhanced cognitive performance in calculating accuracy (C) Lowered fear arousal (E) Improved cognitive performance in calculating speed (C)
Flower arrangement & Lav. Fragrance (V&O) Increased attentive coping (E) More relaxation Less Physical arousal (P)

Liu, M. 2002. Visual and olfactory effects of flowers and floral fragrance on human psychophysiological, emotional and cognitive responses. Dissertation Abstracts International: Section B: The Sciences and Engineering, Vol 62. pp. 5459.

Fluorescing for Fun and Function

Researchers at a commercial company in Holland and a university in Singapore have generated glow-in-the-dark flowers. The Dutch company has tested their patented dye on roses and chrysanthemums. The fresh cut stems are sprayed with the dye, which glows after a short exposure to daylight or black light. The company, floraholland, claims the treatment has no effect on the flower’s vase life. The first batches sold at auction brought 50% more than regular, non-glowing flowers. Of course the purpose is to create a marketing trend and offer customers a new, fun flower feature.
The student project at Singapore Polytechnic is based on advanced biology with an eye toward production efficiency. Plants were genetically modified with a green fluorescent marker gene from a jellyfish. The gene was introduced in a way that the plant “lights up” when under dehydration stress. A special optical sensor is used to detect the green glow. They consider the gene-marking technology to hold potential for other plant stresses such as heat or nutrient deficiency. The grower could identify the stress by color-coded marker genes. In the case of water stress, the students see potential to increase irrigation efficiency in arid regions by assessing the entire field’s water needs using the fluorescent marker.

News brief. 2006. A rose by any other…color? and Plants aglow? Add water! Chemical and Engineering News, Vol. 84. p. 64.

Funding for this column is provided by the ASCFG Research Committee.

Megan Bame

Megan Bame is a freelance writer in Salisbury, North Carolina. Contact her at [email protected]