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Abstract
During the last decade intensive research has been conducted on the phenomenon of organ abscission in plants, as evidenced by several reviews (5, 9, 10, 40) and this symposium. Abscission is of interest because, among other things, a knowledge of the processes involved gives insight into the metabolism of cell walls, the ramifications of which are well known. Horticulturists have an additional interest, for a basic knowledge of this process has application in fruit production programs.
Abstract
‘Hamlin’ and ‘Valencia’ oranges [Citrus sinensis (L.) Osbeck] were treated by stem uptake or dipping in 5-chloro-3-methyl-4nitro-1H, pyrazole (Release) and (2-chloro-ethyl) phosphonic acid (ethephon) of varying concentrations. Depending upon chemical concentration and fruit maturity, fruit removal force (FRF) was reduced within 24 or 48 hours. More buffer and salt extractable cellulase activity were produced after ethephon than after Release treatment, both exceeding the control. Buffer soluble cellulase activity was treatment concentration dependent while salt released cellulase activity was chemical (Release, ethephon, or control) dependent, increasing by the same amount regardless of treatment concentration. Two ethylene peaks, resulting in an M-shaped curve, occurred after dipping oranges in Release. The peak due to chemical treatment (chemical peak) appeared first and was concentration dependent, being of greater magnitude and earlier in time with the higher concentrations. Measurable ethylene production after stem uptake of the chemicals was erratic and represents an inadequate and erroneous procedure for determining effectiveness of a potential abscission chemical.
Abstract
Studies with enzymatically isolated cuticles revealed that the abaxial cuticle was the primary site of ethylene diffusion from the leaf. With fruits, the flavedo and cuticle of the rind were found to offer considerable resistance to ethylene diffusion. The albedo was quite permeable. Measurement at various positions on the fruit show that the point of stem attachment was apparently the path of least resistance for ethylene diffusion from the fruit cavity.
Abstract
Using leaf expiants of Citrus madurensis Lour. cv. ‘Calamondin’, it was demonstrated that many chemicals will accelerate the abscission processes at concentrations of 10−2 to 10−1M. Representative of classes of chemicals tested were mineral acids, bases, and salts; organic acids and salts; non-ionic organic compounds. Several observations were made: Low (< 3) or high (> 13) pH will accelerate the abscission processes; cations and anions do differ in the capacity to accelerate the abscission processes but those that were most effective were active in the concentration range of 10−2 to 10−1M; inorganic PO4 −3 was as effective on abscission as the organic anions tested; acceleration of abscission by these various agents at 10−2M was accomplished without visible tissue damage.
Abstract
2-Chloroethane-phosphonic acid (CEPA), applied as a spray to 5-year-old ‘Maygold’ peach trees in the 80% to full bloom stage, effectively thinned blossoms and young fruit and was accomplished without severe gumming or dwarfing of subsequent vegetative growth. The reaction time required to obtain abscission (9 days) indicated that CEPA interfered with processes leading to sexual fertilization. Laboratory studies of the influence of CEPA and ethylene on pollen germination and pollen tube growth indicated that these 2 processes were not inhibited, except from exposure to high concentrations (100 to 1000 ppm) for several hours. At low concentrations (.01 to 10 ppm) both processes were stimulated.
Abstract
The concept of cellulolytic enzymes being involved in abscission has been confirmed and extended in this study to include the fruits of ‘Calamondin’ (Citrus madurensis Lour.) and ‘Valencia’ sweet orange [C. sinensis (L.) Osbeck]. Data have been presented which demonstrate that an increase in cellulase activity is associated with natural and chemically accelerated abscission of citrus fruits. Ethylene, the most effective accelerant of abscission studied, shortened the time for attainment of measurable increases in cellulase activity preceding separation and shortened the time between detectable increases in cellulase activity and attainment of high cellulase activity. Pectinase did not seem to be a major factor associated with abscission of ‘Calamondin’ and ‘Valencia’ fruit.
The effect of inhibitors of protein and RNA synthesis on abscission of ‘Calamondin’ and ‘Valencia’ fruit was determined and cycloheximide was found to inhibit both abscission and increased cellulase activity. Cellulase-containing proteins extracted from tissues immediately surrounding and including abscission zones were separated electrophoretically and the appearance of a new protein band with carboxymethyl-cellulase activity was noted in this tissue during abscission.
Abstract
Foliar application of Fe and Mn to Fe-deficient ‘Pineapple’ orange seedlings (Citrus sinensis (L.) Osbeck) substantially increased the rate of photosynthesis and the Hill activity over Fe alone; Zn plus Fe and Mn had little effect. The Hill reaction was impaired more by a deficiency of Mn than of Fe. The disappearance of chlorotic symptoms or increased Fe and chlorophyll contents in the leaves may not indicate complete recovery of the leaves from photosynthetic impairment. Photosynthetic rate and the Hill reaction could be used as additional indicators for diagnosing mineral element deficiencies in citrus plants.
Abstract
Effects of UV-B radiation (280-310 nm) and Zn deficiency on photosynthesis and carbonic anhydrase (CA) activity in peach seedlings (Prunus persica (L.) Batsch) were studied. Low Zn markedly reduced CA activity; however, the reduction was not proportional to the reduction in leaf Zn content. Zn-deficient leaves irradiated with UV-B had lower CA activity, chlorophyll content, spectral reflectance and rates of photosynthesis. The reduction was more pronounced when leaves were distinctly chlorotic. Higher CA activity was not necessary for maintaining high rates of photosynthesis. Target areas in the leaf for UV-B absorption are discussed.
Abstract
Discs (2cm diameter) from freshly-harvested leaves of ’Pineapple’ orange (Citrus sinensis (L.) Osbeck) were incubated in darkness at 26 ± 2° for 72 hours in a solution containing of 4% pectinase, 0.1% cellulase, 0.1% hemicellulase and 0.2M ethylenediamine tetracetic acid (EDTA) in 2N acetate buffer, pH3.8, to isolate cuticles. Each cuticle was used as a partition between two smaU chambers, with 59Fe in one chamber and distlled water in the other. Samples were taken from the water side to determine 59Fe penetration through isolated cuticle. Greater penetration of 59Fe into cuticles and intact citrus leaves was obtained with 59FeEDTA and through adaxial surfaces than any other 59Fe formulation tested.
Abstract
The cambium of both ‘Early Amber’ peach and ‘Sungold’ nectarine, Prunus persica L. Batsch gained cold hardiness from November through January even though day and night temperatures were above 15°C. Cultivars attained the greatest hardiness in January, surviving temperatures of -12°C. ‘Early Amber’ maintained its hardiness until just prior to bud break (February 8) and decreased thereafter. ‘Sungold’ remained somewhat resistant to cold until February 15 and gradually lost hardiness until March 8. Characteristically the low chilling cultivars reached an acclimation base higher than reported for cultivars adapted to higher latitudes.