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  • Author or Editor: George Yelenosky x
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Potted greenhouse-grown, l-year-old `Hamlin' orange [Citrus sinensis (L.) Osbeck] trees on 1.5-year-old rough lemon (C. jambhiri Lush.) rootstock were temperature-conditioned for 6 consecutive weeks in a controlled-environment room to test cold-hardening ability. Holding at 15.6 ± 0.6C during 12-hr days [425 μmol·s-1·m-2 photosynthetic photon flux (PPF) at top of trees] and 4.4C during nights resulted in 100% tree survival and no leaf loss “after 4 hr of – 6.7C in a dark freeze test room. Unhardened greenhouse trees were killed to rootstock. Solute efflux (dS·m-1) from unhardened frozen leaves was > 20-fold that from frozen leaves on hardened trees and nonfrozen leaves on unhardened trees. Oxygen uptake was not significantly impaired in frozen hardened leaves. No 02 uptake was evident for frozen unhardened leaves.

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One- to 4-year-old sweet orange trees, Citrus sinensis (L.) Osbeck cv. Valencia on rough lemon (C. jambhiri Lush.) rootstock, were used in a series of tests on the depth and stability of supercooling in various parts of greenhouse-grown trees held in pots during controlled freezes. Thermocouples were attached to flowers, fruit, leaves, and wood. Supercooling levels were inconsistent, ranging from – 3C to – 7C. Nucleation was spontaneous and well defined by sharp exotherms. Rapid progression of crystallization (≈ 60 cm·min–1) indicated no major obstacles to ice propagation throughout the tree above soil level. The site of initial freezing was variable, with a tendency for trees to freeze from the base of the stem toward the top. The location of tissue damage did not necessarily correspond to the location of initial freeze event. Freezing in the wood often preceded freezing of flowers.

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Abstract

A low-density, semirigid insulating wrap of L-shaped Styrofoam halves with plastic bag inner liners partially filled with water protected young citrus trees adequately in controlled freezes and showed no physical deterioration after 2 continuous years in field trials. Protection was provided by a lag in temperature decrease under the wraps relative to that of ambient air and by the release of heat as water froze in the plastic bags which pressed against the tree trunk. Crystals of either phenazine or silver iodide prevented supercooling of the water below −2.8°C. Citrus plants small enough to fit entirely within the wrap, even succulent new growth, remained uninjured during −6.7°C freezes.

Open Access

Abstract

Leaves of different citrus selections (Citrus sp.) developed visible bleaching (low chlorophyll content) within 14 days during constant 1.7°C and less than 500 μE/m2per sec (PAR) continuous light regimes in controlled temperature facilities. Leaves did not bleach at 21.1° and 10° in continuous light and/or in the dark at 1.7°. Larger amounts of amino acids leaked from bleached than nonbleached leaves and leaf disks from bleached leaves had lower rates of O2 uptake during respirometry at 30°. Plants at 1.7° for 14 days in continuous light were injured more than plants conditioned at 10° during 4-hr freeze tests at −6.7°.

Open Access

Abstract

Shoot organogenesis from detached leaves is not characteristic of all plants, but, when found, the phenomenon is a valuable tool for research and plant propagation. Some of the recent observations of whole-plant development from leaf cuttings include lily (Lilium longiflorum Thunb.) (3) and sweet potato [Ipomoea batatas (L.) Lam] (1).

Open Access

Abstract

Water-stressed young, potted citrus seedlings and budded trees on different rootstocks were tested for freeze avoidance and tolerance. Water stress increased the supercooling of cold-sensitive citrus seedlings in freezeavoidance tests and increased the cold hardiness of grapefruit and orange trees in freeze-tolerance tests. Water stress was most effective in reducing injury during freezes above −6.7°C. Temperatures below −6.7° resulted in complete kill regardless of −25 bars water stress in the leaves. Tissue analyses showed increased in proline accompanying increases in sugars during forced dehydration of citrus leaves.

Open Access

Abstract

Cultivars of citrus, having a wide range in cold tolerance, were conditioned in controlled-environment rooms with and without light: Plants conditioned in the light were injured less than plants conditioned in the dark during subsequent freeze tests. The effect of light was more pronounced at a conditioning temperature of 15°C than at 5°C.

Open Access

Abstract

Colorimetric determination of hydroxyproline in acid hydrolysates of ‘Valencia’ orange (Citrus sinensis [L.] Osbeck), ‘Carrizo’ citrange (C. sinensis × Poncirus trifoliata [L.] Raf.), ‘Cleopatra’ mandarin (C. reticulata Blanco), and rough lemon (C limon [L.] Burm. f.) indicated 14 to 71 μg of hydroxyproline per g of oven-dried leaf tissue. Cold-hardening and growth regulator treatments of different citrus cultivars changed the concentration of hydroxyproline in the leaves. Cultivars with greater concentrations of hydroxyproline before cold-hardening were also more tolerant of freeze conditions after cold-hardening.

Open Access

Abstract

Seedlings of ‘Pineapple’ sweet orange [Citrus sinensis (L.) Osbeck] (Swt), Cleopatra mandarin (C. reshni Hort ex Tan) (Cleo)], and trifoliate orange [Poncirus trifoliata (L.) Raf.] (Tri) were grown from seed for 10 months in 2-liter containers of native Candler fine sand in a glasshouse, watered two times per week, and fertilized weekly with a complete nutrient solution. NaCI at 0, 15, 30, or 60 mM was added to the watering solution for 2 additional months. Increases in salinity decreased hydraulic conductivity of roots, transpiration rate, leaf water potential, and root growth. The effect of salinity on mineral composition of tissues was rootstock-dependent. High salinity leaves of Tri had the highest N, K+, and Cl but the lowest Na+, whereas Tri roots had the highest Na+ at the highest salinity. High-salinity Cleo leaves had the lowest Cl and K. All seedlings survived −4°C for 6 hr in a controlled freeze test. Salinity decreased leaf loss, except in the deciduous Tri, in which 60 mM NaCI may have been excessive. Thus, moderate salinity treatment can reduce growth and modify water and mineral nutrient relations so as to increase cold hardiness of certain Citrus species.

Open Access

Potted Marsh grapefruit [Citrus paradisi (Mad.)] trees with mature fruit were subjected to successive 7-day exposures to 20, 15, 10, and 5C, followed by 7 days at 25C in controlled-environment walk-in rooms. Circular depressions (pits), characteristic visible symptoms of chilling injury, developed in the peel of the fruit. To the best of our knowledge, this is the first report of inducible chilling injury of grapefruit on trees under artificial environments. The ability to induce chilling injury in the peel of grapefruit on trees enhances opportunities to study the mechanism of resistance to chilling injury that can be developed by various preharvest strategies.

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