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  • Author or Editor: Albert H. Markhart x
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Reduction of floral number in Capsicum annuum has been observed during growth at high temperature. To determine whether decreased flower production or increased flower abscission is a direct response to high temperatures or a response to water stress induced by high temperatures, we compared flowers and fruit produced and flowers aborted to leaf growth rate, osmotic potential, stomatal conductance, and chlorophyll fluorescence of two cultivars. To determine the stage(s) of floral development that are most sensitive to high temperatures, flower buds were wax-embedded and examined at each stage of development during heat treatment. Rate of floral development also was examined. At first visible floral bud initiation, plants were transferred to each of three controlled environment growth chambers with set temperatures and vapor pressure deficits (VPD) of 25°C, 1.1 kPa; 33°C, 1.1 kPa; and 33°C, 2.1 kPa. Flower bud production and leaf growth rate were not significantly affected by high temperatures. Pepper fruit set, however, was inhibited at 33°C at either VPD. Preliminary water relations data suggested that water potentials were more negative under high temperature conditions. Differences in leaf fluorescence were statistically significant for temperature treatments, but not for VPD. Temperature is the primary factor in the decrease of fruit production in pepper. Decreased production is due to flower abortion and not to decreased flower initiation or plant growth.

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Abstract

I wish to point out a discrepancy in the conclusion on the value of the J-14 hydraulic press by A.H. Markhart, III, and B. Smit-Spinks [HortScience 19(l):52–54, Feb. 1984] that the J-14 readings prevent reliable predictions of the Scholander pressure bomb for leaf water potential measurements.

Open Access

Leaves on cut stems of commercially grown Rosa hybrida cv. Kardinal placed in preservative solutions containing sucrose developed necrotic dry patches that began interveinally and progressed toward the major veins until the entire leaf was dehydrated. Ultrastructural observations of initial damage showed disorganized protoplasm and plasmolyzed cells. Leaves on cut stems pretreated with abscisic acid for 24 hours and transferred to preservative solution containing sucrose remained healthy. We propose that sucrose accumulates in the mesophyll cell wall, thus decreasing apoplastic osmotic potential, leading to cell collapse and tissue death.

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Tepary beans (Phaseolus acutifolius Gray) are more drought tolerant and have stomata that are more sensitive to low leaf water potentials (ψ w) than common beans (P. vulgaris L.). This study was designed to examine the role of ABA in controlling stomatal behaviour in these species. Comparison of the bulk leaf ABA content does not explain why tepary stomata are more sensitive to low leaf ψ w compared to common bean (at -1.4 MPa ABA content increased 40-fold in common bean and 25-fold in tepary). We hypothesize that the greater sensitivity of tepary stomata to low leaf ψ w is related to a higher concentration of ABA in the xylem sap, and/or to a greater sensitivity of tepary stomata to ABA. Xylem sap of well-watered and water stressed plants is analyzed to determine the concentration of ABA, and whether ABA is a putative candidate serving as a chemical root signal in response to water stress in Phaseolus. To test stomatal sensitivity to ABA, epidermal strips and detached leaves are exposed to a range of ABA concentrations. The relationship between stomatal aperture and different ABA concentrations is discussed.

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Abstract

Three- to 4-month-old seedlings of an improved selection of Asparagus officinalis L. cv. Mary Washington were artificially hardened and crowns subjected to controlled freezing tests. Two low-temperature acclimation regimes were used. The first was 3C for 0, 1, or 2 weeks before freezing at 0, −5, or − 10C; the second, 3C for 0, 1.5, or 3 weeks, followed by freezing at 0, −2.5, −4.5, −6.5, or −8.5C. Regrowth tests showed that hardiness increased with 2 and 3 weeks of acclimation, with tolerance to −5 and −6.5C, respectively. Water-stressed seedlings (relative water content at 57%) withstood exposure to −5C, but not to −6.5C; rehydrated crowns and well-watered controls were hardy to −3.5C.

Open Access

Weed control in organic vegetable production is a major challenge. During Summer 2004, we conducted field trials to manage weeds in organic sweet corn, carrots and onions. In sweet corn, we evaluated the efficacy of transplanting greenhouse-grown sweet corn seedlings. In carrots and onions, we tested vinegar and several concentrations of acetic acid. Studies were conducted in southwestern Minnesota at the Lamberton Research and Outreach Center and in eastern Minnesota at Foxtail Farm in Shaefer. Ten-day-old corn transplants were effective at both locations. Stand establishment was greater, less tillage was needed, and yield was greater than in the seeded plots. Straight vinegar was not very effective in controlling weed populations. Although there was greater damage to broadleaf weeds than grasses, straight vinegar did not reduce the need for tillage. Although 10% to 20% acetic acid did provide better weed control, it significantly damaged carrot and onion seedlings. These results suggest that using sweet corn transplants is time and cost effective for small acreage sweet corn production such as CSAs. Vinegar and acetic acid are problematic. Nonselectivity, potential danger in handling, and poor control at low concentrations were all considered significant disadvantages.

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Abstract

The relationship of foliar injury in Pinus sylvestris (Scots pine) to low temperature and irradiance levels was studied in growth chambers at –l°C(warm) and –7°(cold) under 3 light treatments: 1) nonshaded (1000 watt, Metalarc), 2) 47% shade cloth, and 3) 4-mil white polyethylene. During the 9-week treatment period, plants were tested for hardiness, electrolyte leakage, chlorophyll content, and relative water content. Foliar injury was observed in plants under the cold treatment but the degree of injury was not related to irradiance level. After 6 weeks, electrolyte leakage levels were greater than 50% for all cold-treated plants. No foliar injury occurred on warm-treated plants regardless of light treatment. Minimizing desiccation and rapid temperature fluctuations did not prevent injury. Needle water content of plants from both temperature regimes decreased about 6% over the duration of the experiment. Maximum foliar temperature fluctuations were 5.5° (l.l°/minute) and 4° (0.8°/minute) in the cold and warm chambers, respectively. Although warm-treated plants showed no injury during the treatment period, they deteriorated after several weeks in the greenhouse. Severe root injury to the warm-treated plants may have caused this deterioration.

Open Access

Experiments were conducted to determine if the fungicide imazalil infused into shrunken-2 sweet corn (Zea mays L. var. rugosa Bonaf.) seed via acetone could protect against soil- and seedborne fungi enough to improve germination and vigor. `Florida Staysweet' and `Crisp-n-Sweet 710' seeds were infused for 0.25 hours with 1% or 2% (w/w) imazalil-acetone (LA) solutions, air-dried, and subjected to a modified laboratory seedling growth cold test using sterile soil or soil inoculated with Fusarium moniliforme Sheldon. Both IA concentrations significantly reduced the incidence of diseased seedlings in soil inoculated with F. moniliforme when compared to nontreated controls. Neither treatment significantly reduced the incidence of seedborne fungi. Chemical name used: 1-[2-(2,4-dichlorophenyl)-2-(2-propenyloxy)ethyl-H-imidazole (imazalil).

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