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  • Author or Editor: George Yelenosky x
  • Journal of the American Society for Horticultural Science x
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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

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

The combined effects of O3 and acid rain on freeze resistance, growth, and mineral nutrition were studied using broadleaf-evergreen citrus and avocado trees. Using a factorial design, `Ruby red' grapefruit (Citrus paradisi L.) trees on either Volkamer lemon (Citrus volkameriana Ten. & Pasq.) or sour orange (Citrus aurantium L.) rootstock and `Pancho' avocado trees (Persea americana Mill.) on `Waldin' rootstock were exposed to O3 and acid rain for 8 months in open-top chambers under field conditions. The O3 treatments were one-third ambient (0.3X), ambient (1X), twice ambient (2X), or thrice ambient (3X). Ambient O3 concentrations averaged 39.1 nl·liter-3 over a 12-hour day. The acid rain treatments had a pH of 3.3, 4.3, or 5.3 and were applied to simulate long-term rainfall averages. In general, the effects of acid rain on growth and freeze resistance were small. Rain of high acidity (pH = 3.3) offset the negative effects of O3 on growth (total leaf mass) in avocado and grapefruit/Volkamer lemon trees. In contrast, rain of high acidity magnified the detrimental effects of O3 on electrolyte leakage of leaf disks at subzero temperatures, especially for citrus. Freeze resistance, determined by stem and whole-plant survival following freezing temperatures, was lower in the most rapidly growing trees. Consequently, for trees exposed to a combination of O3 and acidic rain, leaf electrolyte leakage did not correlate significantly with stem survival of freezing temperatures. We conclude that the danger of acid rain to citrus and avocado in Florida is rather slight and would only present a potential problem in the presence of extremely high O3.

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