You are looking at 1 - 10 of 18 items for
- Author or Editor: Albert H. Markhart x
Chilling injury has been defined as injury at temperatures low enough to cause damage but not cause freezing of water (17). Common usage refers to temperatures between 0° and 15°C, which cause irreversible damage to tropical and subtropical species. Except for the obvious postharvest loss of quality in many fruits and poor stand establishment in some field crops, the economic importance of chilling injury is difficult to quantify. Chilling injury often results in plants being “set back” so that maturation is delayed and yield reduced. A better understanding of the physiological and biochemical causes of injury and mechanisms of resistance will enable the horticulturist to design better cultural practices and the breeder to develop more resistant cultivars.
Although the number of students in conventional production agriculture is declining, there is increased interest and opportunity in growing organic fruits and vegetables. Land grant universities need to invest in resources to develop curricula and hands-on opportunities to attract students from varied backgrounds who may currently be enrolled in a number of non-agricultural majors. At the University of Minnesota the student organic farm Cornercopia has successfully attracted students from 12 different majors to plan, plant, harvest, and market organic produce. The enthusiasm, interest, experiential learning, and public relations were well worth the land, faculty, and staff time.
Measurements of leaf water potential for 5 plant species were made with the J-14 leaf press and a Scholander-type pressure bomb. Although a significant linear relationship was found between the readings from both instruments, the variability in the J-14 readings at a particular pressure bomb value prevents reliable predictions of the pressure bomb valve from the J-14 measurements.
Roses are grown in Minnesota in the winter in closed greenhouses with the aid of HID lamps, and carbon dioxide enrichment. Although productivity is good, consumers often complain of a rapid dehydration or crisping of the leaves. Through a series of experiments using controlled environment chambers and known vase solutions we have determined that the crisping is due to the deposition of high levels of sucrose in the leaf cell walls due to transpiration from the leaves. The sucrose dehydrates the cell protoplast causing cell collapse and tissue death. Crisping is reduced by lowering the sucrose in the vase solution or reducing transpiration from the leaves. Abscisic acid added to the vase solution effectively reduced transpiration and crisping.
Large lectures continue to challenge teaching and learning. Our plant propagation course attracts a large number of non-majors seeking to fulfill their science requirement. Although the laboratory is quite successful in maintaining interest, the lecture is plagued by poor attendance and lack of commitment. To deal with these issues, I have incorporated an audience response system (as used in America's Funniest Home Videos) and a multiple-choice exam that uses a scratch-off answer system similar to the instant-win lottery tickets. The audience response system facilitates attendance, and both systems provide immediate feedback to questions. Student and faculty assessment will be presented. Technological and pedagogical challenges will be discussed.
High temperature reduces fruit set in bell pepper [Capsicum annuum L. var. annuum (Grossum Group)], and reduction of pepper productivity, resulting from high temperature, may be a direct effect of temperature or an indirect effect of water stress induced by increased vapor pressure deficits (VPDs) at high temperature. We evaluated responses of plant growth, reproduction, net photosynthesis (PN), chlorophyll fluorescence, predawn respiration, leaf water potential, and stomatal conductance of `Ace' and `Bell Boy' bell pepper to elevated temperature (33 °C) with increased VPD (2.1 kPa) or elevated temperature with no increase in VPD (1.1 kPa). VPD had no effect on flower number or fruit set and did not adversely influence the physiological processes measured. Therefore, deleterious effects of high temperature on pepper fruit set does not appear to be temperature induced water stress, but is more likely a direct temperature response. Elevated temperature decreased fruit set but not flower production. Gas exchange measurements suggest failure to set fruit was not due to reduced leaf photosynthesis.
A major limitation to plant growth in spring is low night temperatures. A variety of plant protection systems have been developed to keep the temperatures around the plant warmer than the ambient air. One system that has been developed for use with individual plants is a double walled stiff plastic tent. The space between the walls can be either filled with water or air. The top of the tent can be either open or closed. The objective of this investigation is to quantify the effect of these protection systems under controlled environmental conditions. Two wash-tubs filled with wet soil were placed in a controlled environment growth chamber. One tent was placed on the soil surface of each tub. The chamber was programmed to simulate a cold night. Temperatures started at 20 °C and then decreased to –5 °C at a rate of about 4 °C/h. During this time, ambient air temperature, jacket temperature, soil temperature, and air inside the tent was measured continuously with self-contained data loggers. Water filled tents delayed the time it took for the inside temperature to reach the outside temperature by 2 hours. There was not apparent effect on soil temperature. The effect of water vs. air-filled jackets and the effect of capping the top will also be presented.
The concentration of active ingredient in a given dry weight of a medicinal herb is important to the consumer and producer of herbal remedies. Feverfew is a commonly used medicinal herb where the active compound has been identified. There is considerable variability in the amount of the active ingredients in different genotypes of feverfew. Important secondary plant compounds are often produced in the trichomes of leaves. The objective of this investigation is to determine if there is a correlation between the number of leaf trichomes and the level of active ingredient in several feverfew genotypes. Rooted cuttings of feverfew (Chrysanthemum parthenium) genotypes previously characterized for parthenolide content were grown under identical conditions in an environmentally controlled greenhouse. Light and scanning electron microscopy were used to describe and quantify the number and type of trichomes on the youngest fully expanded leaf of each plant from each genotype. The relationship between trichome number and parthenolide content will be presented.
Environmental conditions are known to affect the growth and quality of culinary and medicinal herbs. Hydroponic growing conditions often produces greater yields for many leafy crops compared to growth in more-traditional media. The objective of this investigation was to compare the yield and quality of sweet basil grown in continuous flow solution culture or well-irrigated Universal Mix. Sweet basil plants were germinated under mist and then transplanted to a continous-flow hydroponics system or to 6-inch pots containing Universal Mix. Rows of pots alternated with a row of hydroponic plants in a temperature-controlled greenhouse. Temperatures were maintained between 20 and 25 °C, the relative humidity was not controlled, pot-grown plants were irrigated as needed. HID lights added sublimentry irradiation and maintained a photoperiod of 18 h. Cohorts of plants were harvested at five time points between transplanting and maturity. Plants were divided into leaves, stems, and roots, dried, and the data subjected to mathmatical growth analysis. Several leaves from each plant were harvested and analyzed by gas chromotograpth for essential oils. Plants grown in hyroponics grew faster and produced more harvestable leaf material than the media-grown plants. Details of the plant growth analysis and the essential oil composition will be presented.