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Abstract
Seedlings of 18 citrus types were exposed to artificial hardening conditions. ‘Nagami’ kumquat, false hybrid satsuma and ‘Cleopatra’ mandarins were the most cold hardy and ‘Lisbon’ lemon, ‘Mexican’ lime, Rangpur’ lime, and ‘Calamondin’ hybrid kumquat the least cold hardy. Three mandarins, 1 tangelo, 3 oranges, and 4 grapefruit types were intermediate in hardiness. Generally, the most hardy types hardened some at 70° day and 50°F night temperatures, and 60°/40° as well as at lower temperatures while the least hardy types hardened primarily at 50°/30° and 45°/26°. Sugar accumulation was associated with hardening.
Abstract
Although citrus is grown in the southern regions of the United States, it is exposed to periodic freezes. Freezes are a serious production problem, as evidenced by the enormous loss of fruit and trees in recent years in the major citrus-growing areas (8). In 1962 in Florida, approximately 50 million boxes of fruit were lost, and substantial wood damage and loss of bearing surface occurred in approximately two-thirds of the citrus-producing areas (12). The monetary loss was estimated to exceed 500 million dollars. In Texas in 1951, 85,000 acres of trees were killed, and in 1962, 40,000 acres. Surviving trees lost their entire bearing surface. Many trees were killed back to 2-inch wood (27). Severe freezes also occur periodically in California and Arizona. In 1963, for instance, temperatures as low as 20°F occurred in some areas in California for eight consecutive nights. Injury was primarily to the fruit and very young trees (12). Studies of long-term weather records have indicated that freezes of 22°F minimums and lower occur in all major citrus-growing areas in the United States every 8 to 13 years on the average (5). Thus, it is expected that citrus-growing areas will be exposed to damaging freezes periodically.
Abstract
‘Redblush’ grapefruit seedlings were exposed to hardening temperatures in the presence and absence of light. Both reducing and non-reducing sugars increased in leaves and wood of hardened plants and non-reducing sugars increased in roots of hardened plants. The primary sugars involved were glucose, fructose, and sucrose. Plants exposed to hardening temperatures in the dark did not harden, and water soluble proteins did not materially change in the leaves during hardening in the light.
Lower temperatures were required to kill leaves without ice on the surface than with ice, and lower temperatures were required to kill hardened leaves. Hardened leaves developed a small capacity to recover from cell dehydration due to ice nucleation indicating changes in protoplasm stability and membranes during hardening
Abstract
Citrus seedlings sprayed with chemicals which influence the cold hardiness of other plants were hardened in controlled conditions. Maleic hydrazide (MH-30) increased cold hardiness; however, growth retardants (2-chloroethyl)trimethylammoniumchloride (chlormequat) and succinic acid-2,2-dimethylhydrazide (SADH), and growth inhibitor abscisic acid (ABA) did not. ABA at high concns decreased cold hardiness as did gibberellic acid (GA3). Benzyladenine (BA), kinetin (KN), decenylsuccinic acid (DSA), and (2-chloroethyl)phosphonic acid (ethephon) had little or no effect on cold hardiness. These results are consistent with tests on citrus conducted under field conditions.
Abstract
Freeze-injured citrus fruit produced above-normal amounts of ethylene 1 to 4 days after injury. Elevated ethylene levels were often found in fruit 3 weeks after injury. Cellulase activity in the abscission zone increased 4 to 8 days after injury and preceded abscission. Some severely injured fruit that did not abscise were responsive to abscission-inducing chemicals. High internal ethylene content did not correlate as well with abscission as did high rates of abscission-zone cellulase activity.
Abstract
(2-chloroethyl)phosphonic acid (ethephon) applied as a preharvest spray at rates of 200, 300, and 500 ppm induced significant on-the-tree degreening of fruit of ‘Robinson’, ‘Lee’ ‘Nova’, and ‘Dancy’ tangerines and ‘Hamlin’ oranges. Greatest degreening occurred 2 to 6 days following application and subsequent to peak-ethylene evolution. Fruit which were partially or totally degreened on the tree required less postharvest degreening and showed less decay in storage than untreated fruit. Ethephon applied at 200 to 500 ppm induced varying degrees of fruit loosening and, often, fruit drop. Generally, less than 10% ofthe leaves abscised on all cultivars with rates under 200 ppm and on ‘Nova’ and ‘Dancy’ tangerines and ‘Hamlin’ oranges with rates under 500 ppm. Considerable leaf abscission occurred on ‘Robinson’ and ‘Lee’ tangerines treated with 300 and 500 ppm ethephon.
Abstract
Citrus trees were sprayed with ethephon when the fruit were mature but still green or partially degreened. Harvested fruit that were mature and partially or fully degreened were held in an atmosphere containing 10 ppm ethylene for 5 to 9 days. One preharvest ethephon spray hastened carotenoid accumulation in rinds of green ‘Nova’ tangerines, partially degreened ‘Robinson’ and ‘Dancy’ tangerines, and fully degreened ‘Robinson’ tangerines, but was not effective on green ‘Bearss’ lemons and partially degreened ‘Hamlin’ oranges. Effective concn varied between 50 and 500 ppm and with the cultivar evaluated. Two 500-ppm ethephon sprays, applied 2 weeks apart, hastened carotenoid accumulation in rinds of partially degreened ‘Hamlin’ oranges and ‘Robinson’ tangerines. Postharvest-ethylene treatment induced carotenoid accumulation in rinds of partially degreened ‘Bearss’ lemons and ‘Lee’ and ‘Dancy’ tangerines and degreened ‘Robinson’ and ‘Dancy’ tangerines. Tangerines showed greater ethylene-induced increases in rind carotenoids than did ‘Hamlin’ oranges and ‘Bearss’ lemons. Fruit which had higher rind carotenoid contents as a result of ethylene or ethephon application had better visible external color.
Abstract
Tree and fruit losses from cold injury are important problems in growing citrus. Severe losses from the freezes of 1894-95, 1957-58, 1962, and 1970-71 in Florida; 1949, 1950, and 1962 in Texas; and 1913, 1937, 1949, and 1950 in California, have stimulated research on cold hardiness of citrus. One method of reducing losses from freezes is the production of cold hardy cultivars by breeding and selection. Citrus physiologists and breeders with the USDA at Orlando, Florida; Indio, California; and Weslaco, Texas, have coordinated their research to develop more cold hardy citrus cultivars (2, 3). This paper summarizes some recent efforts to develop methods for screening citrus hybrids for cold hardiness. The glossary (Table 1) of the citrus types and names used here include cold hardiness ratings. Common names or a designated number will be used for simplicity of discussion.
Abstract
Light-reflectance measurements at 648-740 and 674-740 nm decreased as chlorophyll was lost during the maturation and degreening of citrus fruits. The difference between these measurements changed as the chlorophyll level declined. This change was shown as an initial decrease followed by an increase in 648-674 nm measurements. Analyses of rind samples revealed changes in the relative concentration of chlorophyll a and b and consequent decreases in the a/b ratio as total chlorophyll levels decreased. Formulas were developed to convert light-reflectance readings at 674-740 and 648-740 nm to concentration of chlorophyll a and b in the tissue. The greater resistance of chlorophyll b to degradation during color development may explain the difficulty of satisfactorily degreening some fruit and may serve as a basis in selecting for improved coloring characteristics.
Abstract
Tests of citrus seedlings exposed to a series of hardening temperatures showed that kumquat, Fortunella hindsii (Champ.) Swing., acquired more hardiness at 21°/10°C than did ‘Redblush’ grapefruit, Citrus paradisi Macf., or citron, C. medica L. After 8 weeks’ hardening kumquat was the most cold hardy; citron, the least. Leaf photosynthetic CO2 uptake decreased, and leaf diffusion resistance (sec/cm) increased with hardening in all cultivars, but did not reflect the degree of hardening attained. Stomatal closure during hardening was not caused by moisture stress. Ethylene evolution from leaves did not change during hardening of kumquat, mandarin, C. reticulata Blanco, or grapefruit, but did increase from hardened citron leaves.