The effect of mechanical impedance on ethylene evolution and growth of preemergent maize (Zea mays L.) seedlings was investigated by pressurizing the growth medium in triaxial cells in a controlled environment. Pressure increased the bulk density of the medium and thus the resistance to growth. The elongation of maize primary roots and preemergent shoots was severely hindered by applied pressures as low as 10 kilopascals. Following a steep decline in elongation at low pressures, both shoots and roots responded to additional pressure in a linear manner, but shoots were more severely affected than roots at higher pressures. Radial expansion was promoted in both organs by mechanical impedance. Primary roots typically became thinner during the experimental period when grown unimpeded. In contrast, pressures as low as 25 kilopascals caused a 25% increase in root tip diameter. Shoots showed a slight enhancement of radial expansion; however, in contrast to roots, the shoots increased in diameter even when growing unimpeded. Such morphological changes were not evident until at least 3 hours after initiation of treatment. All levels of applied pressure promoted ethylene evolution as early as 1 hour after application of pressure. After 1 hour, ethylene evolution rates had increased 10, 32, 70, and 255% at 25, 50, 75, and 100 kilopascals respectively, and continued to increase linearly for at least 10 hours. When intact corn seedlings were subjected to a series of hourly cycles of pressure, followed by relaxation, ethylene production rates increased or decreased rapidly, illustrating tight coupling between mechanical impedance and tissue response. Seedlings exposed to 1 microliter of ethylene per liter showed symptoms similar to those shown by plants grown under mechanical impedance. Root diameter increased 5 times as much as the shoot diameter. Pretreatment with 10 micromolar aminoethoxyvinyl glycine plus I micromolar silver thiosulfate maintained ethylene production rates of impeded seedlings at basal levels and restored shoot and root extension to 84 and 90% of unimpeded values, respectively. Our results support the hypothesis that ethylene plays a pivotal role in the regulation of plant tissue response to mechanical impedance.

We investigated the metabolism of 1-aminocyclopropane-1-carboxylic acid (ACC) in etiolated maize (Zea mays L.) seedlings subjected to mechanical impedance by applying pressure to the growing medium. Total concentrations of ACC varied little in unimpeded seedlings, but impeded organs accumulated ACC. Roots had consistently higher concentrations of ACC than shoots or seeds, regardless of treatment. The concentration of ACC in the roots increased more than 100% during the first hour of treatment irrespective of the pressure applied; in shoots, total ACC concentration increased 46% at either low or high pressure during the first hour of treatment. The bulk of ACC synthesized under impeded and unimpeded conditions was present in a conjugated form, presumably, 1-(malonylamino)-cyclopropane-1-carboxylic acid. However, 1-(malonylamino)-cyclopropane-1-carboxylic acid increased 73% over controls after 10 hours at 25 kilopascals of pressure. Unimpeded tissue had about 77% ACC as the conjugate and 17% as free ACC, and less than 6% was used in ethylene production. Increased amounts of ACC were converted into ethylene under stress. In vivo ACC synthase activity in roots became six and seven times higher only 1 hour after initiation of treatment at 25 and 100 kilopascals of pressure, respectively, and remained high for at least 6 hours. However, the immediate and massive conjugation of mechanically induced ACC suggests that ACC N-malonyltransferase may play an important role in the regulation of mechanically induced ethylene production. After 8 hours, in vivo activity of the ethylene-forming enzyme complex increased 100 and 50% above normal level at 100 and 25 kilopascals, respectively. Furthermore, ethylene-forming enzyme complex activity was significantly greater at 100 kilopascals than in controls as early as 1 hour after treatment initiation. These data suggest that regulation of ethylene production under mechanical impedance involves the concerted action of ACC synthase, the ethylene-forming enzyme complex, and ACC N-malonyltransferase.Recently, we reexamined the effect of physical impedance on growth and ethylene production of etiolated, maize primary shoots and roots (17). Pressure-simulated impedance caused an inhibition of axial growth and a stimulation of radial expansion of both roots and shoots. We confirmed a strong correlation between mechanical impedance and increased ethylene production; furthermore, we showed that the increase in ethylene production preceded measurable morphological changes by at least 1 h. Effects of mechanical impedance were reproduced by fumigation with ethylene at

Effects of heat on cell membrane permeability, metabolic activity, and glycoalkaloid content were studied in stored potato tubers. Heat-induced alteration of cell membrane permeability was estimated by ion leakage and tissue viability by the (trichlorophenyl)tetrazolium chloride (TTC) reduction test. Solanidine, α-solanine, and α-chaconine contents were determined colorimetrically. Atlantic (a heat-susceptible cultivar) accumulated 74% more total glycoalkaloids (TGA) after 4 h at 35 °C than after 4 h at 22 °C. LT7 (a heat-resistant cultivar) showed a 50% reduction in TGA content after the same treatment. Ion leakage was similar for both cultivars after 90 min at 35 °C, but for the remainder of the incubation period (4 h), it was 15−20% greater in Atlantic than in LT7. At 35 °C, LT7 showed one-third the TTC reduction activity shown by Atlantic. Calcium treatment reduced ion leakage, while it resulted in higher TTC reduction activity in Atlantic than in LT7. Incubation in a solution of glycoalkaloids caused a 50% decrease in TTC reduction activity in Atlantic but did not affect LT7. Damage was aggravated at elevated temperatures, especially in Atlantic. Possible roles of calcium in protection against glycoalkaloid damage are discussed. Keywords: Potato (Solanum); glycoalkaloids; membranes; heat; calcium

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