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Aaron E. Walworth and Ryan M. Warner

Freezing tolerance of many plant species increases after exposure to low, nonfreezing temperatures, a process termed cold acclimation. In some species, shortened photoperiods also bring about an increase in freezing tolerance. Within the plant family Solanaceae, species vary widely in cold acclimation ability. The objectives of this work were to examine the effects of low temperature and photoperiod on cold acclimation of Petunia hybrida (Hook.) Vilm. ‘Mitchell’ and to evaluate cold acclimation of several Petunia species by measuring freezing tolerance using an electrolyte leakage assay on leaf tissue discs. Temperature, but not photoperiod, influenced cold acclimation of P. hybrida. Whether grown under long days or short days, nonacclimated plants had an EL50 value (temperature at which 50% of cellular electrolytes are lost) of ≈–2 °C. Plants acclimated by gradual cooling at temperatures of 15 °C, 10 °C, and 3 °C for 7 days each reached an EL50 of ≈–5 °C regardless of photoperiod. Exposure to 3 °C under short days for 1 or 3 weeks resulted in EL50 temperatures of –3.9 and –4.9 °C, respectively. Freezing tolerance of petunia species P. exserta Stehmann, P. integrifolia (Hook.) Schinz & Thell., P. axillaris (Lam.) Britton et al. (USDA accessions 28546 and 28548), and P. hybrida ‘Mitchell’ was similar before cold acclimation, but varied from –5 °C for P. exserta to –8 °C for P. axillaris (accession 28548) after cold acclimation. Our results demonstrate the cold acclimation ability of Petunia spp. and identify wild germplasm sources with potential usefulness for improving freezing tolerance of cultivated petunia.

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Guo-qing Song, Aaron E. Walworth, and Wayne H. Loescher

Grafting is a well-established agricultural practice, and it now has implications for the commercialization of transgenic plants. In transgrafted plants, only one part (scion or rootstock) is transgenic with the other part untransformed. However, transgenes may affect both mobile and immobile endogenous metabolites (e.g., RNAs, proteins, and phytohormones) and mobility has implications for transgrafting. In the phloem, long-distance transport of mobile metabolites can play important roles in plant development and signaling. In a transgrafted plant, an immobile transgene product (ITP) is not likely to be translocated across the graft union. In contrast, mobile transgene products (MTP) may be translocated across the graft. Regardless of the mobility of transgene products (TP), interaction of transgenic and nontransgenic parts in transgrafted plants through either the MTP or ITP has been demonstrated to be effective in facilitating changes in nontransgenic portions of the plant. Consequently, and particularly in fruit crops, transgrafting provides the potential for improving products from their nontransgenic parts with the possibility of minimizing the controversy over transgenic crops. This review focuses mainly on the mobility of TP and effects on the whole transgrafted plant.