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  • Author or Editor: Roger Young x
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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.

Open Access
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‘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

Open Access
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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.

Open Access
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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.

Open Access
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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.

Open Access
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Abstract

‘Redblush’ grapefruit seedlings, with and without leaves, were sprayed 1 to 5 times with 100 to 1000 ppm abscisic acid or 500 to 3000 ppm cycocel. Plants were subsequently exposed to several day/night temperature regimes which included 70°/50°, 90°/70°, and 95°/95°F. Both abscisic acid and cycocel delayed bud growth of leafy and defoliated seedlings. Abscisic acid was more effective than cycocel, and both compounds were most effective in delaying bud growth at lower temperatures, higher concentrations, and with more than one application. Abscisic acid was more toxic than cycocel, and both compounds were more toxic to defoliated plants than to leafy plants. Gibberellic acid overcame a correlative bud inhibition by the leaves, and abscisic acid decreased the effect of gibberellic acid.

Open Access
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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.

Open Access
Authors: and

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.

Open Access

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.

Open Access
Authors: and

Abstract

Sour orange leaves (Citrus aurantium L.) from cold-hardened and unhardened plants were sectioned and prepared for light and electronmicroscopy examination after freezing at -3.3°C and -6.7°. Severe membrane destruction was visible in both hardened and unhardened cells after freezing. These membranes included the tonoplast, outer chloroplast membrane, and the cristae membrane in the mitochrondria. The thylakoids of the grana, the intergrana lamella, and the outer mitochrondrial membranes remained intact. Membrane destruction resulted in gross disorganization of cell contents. Disruption of cells was evident and included palisade and mesophyll cells, the lower epidermal layer, and xylem, fiber, and pith cells in the vascular system.

Open Access