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Greenhouse-grown l-year-old sweet orange trees [Citrus sinensis (L.) Osbeck cv. Valencia] on cold-hardy trifoliate orange [Poncirus trifoliata (L.) Raf.] and cold-sensitive citron (C. medica L.) rootstocks were exposed to cold-acclimation conditions and freeze-tested at -6.7C for 4 hours in a temperature-programed walk-in freezer room. Nonhardened trees generally did not survive the freeze, whereas cold-hardened trees survived with no wood kill on either rootstock. Essentially, all leaves died or abscised during the subsequent 5 weeks in the greenhouse. Freeze survival did not separate rootstocks nor did supercooling in separate trials where Yalencia' wood reached –8.8C before apparent nucleation. Increases in concentration of carbohydrates and proline and decreases in water content in Yalencia' leaves during cold hardening were generally associated with increased freeze tolerance. Other tests, that matched 9-month-old seedlings of citron with trifoliate orange rootstock, showed clear differences in the superior cold acclimation of trifoliate orange over citron, which, however, performed better than expected.
Abstract
CO2 exchange rate (CER), stomatal conductance (Cs), and transpiration in mature attached leaves of ‘Valencia’ orange [Citrus sinensis (L.) Osbeck] were determined outdoors from predawn to 1100 EDT. The maximum values of CER and Cs, which were about 7 μmol CO2·s−1m−2 and 0.27 cm·s−1, respectively, at solar photosynthetic photon flux density (PPFD) of 500-700 μmol·s−1m−2, remained at these levels as PPFD, temperature, and vapor pressure deficit (VPD) between leaf and air continued to increase. Transpiration rates, with maximum values ranging from 1 to 4 mmol H2O·s−1m−2, increased throughout the measurement periods of the morning as leaf-air VPD increased. Thus, photosynthetic water use efficiency decreased with increasing VPD.