Search Results
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
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
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
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
Early in the season, postharvest applications of 2-(4-chlorophenylthio)-triethylamine (CPTA) had little effect on carotenoid synthesis in ‘Bearss’ lemon (Citrus limon Burm. f.), ‘Robinson’ tangerine (C. reticulata Blanco × (C. paradisi Macf. × C. reticulata)), ‘Marsh’ grapefruit (C. paradisi Macf.), and ‘Hamlin’ orange [C. sinensis (L.) Osbeck]. The responses increased as the fruit matured, but greater CPTA responses were induced by storage of the fruit at 16°C before treatment or by exposing treated fruit to ethylene. Observations suggested that cultivars with low natural carotenoid levels (lemon and grapefruit) are more responsive to CPTA applications than are those with higher levels (tangerine). Improved color of ‘Hamlin’ orange was obtained with CPTA applications made before or after a 3-day degreening treatment. This response did not appear to be prevented by waxing. However, the practical use of CPTA to improve the color of oranges appears limited, although it may be useful in research on carotenoid synthesis.
Abstract
Preharvest applications of (2-chloroethyl)phosphonic acid (ethephon) on ‘Bearss’ lemons were relatively ineffective for inducing degreening or abscission. This was not due to lack of absorption or ethylene production. Similar rates of application as a postharvest dip induced degreening, suggesting that a factor from the tree inhibited the response to ethylene. This possibility was supported by data from further tests on ‘Bearss’ lemons and on ‘Robinson’, ‘Lee’, and ‘Dancy’ tangerines and ‘Hamlin’ oranges. Degreening and abscission responses to ethephon in detached fruit or fruit on which the stem was girdled were greater than in fruit on the tree. Applications of gibberellic acid retarded these responses. The results varied among cultivars and between the degreening and abscission responses. However, the general response pattern suggests that the tree provides a factor (or factors) which, on translocation to the fruit, inhibits degreening and abscission. This inhibitory factor may be a growth promoter such as auxin, gibberellin, or cytokinin.
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.