HortScience 33(4):744-748. 1998
Freezing Tolerance and Growth Characteristics of USDA Intergeneric Citrus Hybrids US 119 and Selection 17-11
Milton E. Tignor, Frederick S. Davies, and Wayne B. Sherman
Department of Horticultural Sciences, University of Florida, Gainesville, FL, 32611-0690
Abstract. Two USDA intergeneric, hybrid citrus scions, US 119 {(Citrus paradisi Mac. 'Duncan' x Poncirus trifoliata (L.) Rad.] x C. sinensis Osb. 'Succory'} and selection 17-11 {C. grandis US 145 x [Citrus paradisi Mac. 'Duncan' x P. trifoliata (L.) Raf.]} on 'Swingle' citrumelo (C. paradasi x P. trifoliata) rootstocks were examined for freeze hardiness traits (4 years) and general growth characteristics (2 years). Hardiness was compared with that of 'Hamilin' orange and Satsuman mandarin from Fall 1993 to Spring 1997. As expected, US 119 and 17-11 were both hardier than 'Hamilin' orange as determined by leaf disc electrolyte leakage (EL). Both showed freezing tolerance similar to that of satsuma mandarin, but 17-11 was significantly hardier than satsuma or US 119 at several times during the 4-year study. Trunk diameter and tree height were similar for US 119 and selection 17-11.
A U.S Dept. of Agriculture (USDA) breeding program in Orlando, Fla., has been developing more freeze-hardy citrus scions and rootstocks for many years.
US 119 survived freezes with -12.2 °C minimum air temperature in 1981 (Barrett, 1990). The trees were completely defoliated at this temperature, but bloomed and set fruit during the following year. We observed similar defoliation in both US 119 and 17-11 following two freezes when air temperature reached -10 °C in 1996. In addition, defoliation occurred to a lesser extent when air temperatures reached -9 °C in 1995. In both years defoliation occurred via abscission at the petiole-leaf interface with the petiole remaining on the tree until the spring flush occurred. This indicates less severe freezing injury than dead leaves persisting on a tree following a severe freeze.
Selection 17-11, in addition to being hardier than 'Hamlin' orange, may be potentially hardier than US 119 and satsuma, and therefore is also a potential parent for citrus breeding programs.
The accompanying picture shows, from 17-11, a single leaf, a bifoliate, and a trifoliate leaf, all three have a small winged petiole at the base of the leaf.
Apparently 17-11 displays a mix of mono and trifoliate leaves.
I tried to copy just the most important parts from the article.