Effect of rootstocks in inducing cold hardiness in scions varied somewhat with the part 

 of the country in which the citrus was grown. In California and Florida, P. trijoliata induced 

 cold hardiness in satsuma mandarin scions but failed to do so at Weslaco, Tex. Researchers 

 conducting the Texas experiment concluded that the difference in performance of P. trijoliata 

 as a cold-hardy rootstock there and in the other areas might be explained by the fact that the 

 soil at the Texas location is generally warmer than in California and Florida. 



Further emphasizing the complexity of the factors affecting hardiness, recent studies have 

 yielded evidence that cold tolerance in citrus is also affected by soil nutrient conditions, such 

 as salinity. Although normally rather cold resistant, grapefruit grafted onto sweet orange or 

 Rusk citrange became susceptible to cold injury when grown on saline soil. 



Severe freezes of recent years have intensified the interest of citrus growers in cold hardi- 

 ness. Present-day citrus research gives a high priority to the breeding, selection, and in- 

 troduction of new and improved citrus varieties and rootstock-scion combinations that are 

 tolerant to cold. 



The need for combining cold hardiness with quality fruit, high yield, resistance to pests 

 and plant diseases, and other specific requirements such as suitability for canning, freezing, or 

 fresh fruit sales indicates that the job ahead is a big one. However, improved techniques for 

 breeding and testing citrus varieties for cold hardiness should help speed up the work. As 

 mentioned earlier in this report, the increased use of cold-hardy monoembryonic seed parents 

 for the production of true hybrids enables plant breeders to accelerate the production and 

 selection of new citrus varieties combining cold hardiness and other needed characteristics. 



Recently developed techniques for testing the cold hardiness of citrus trees grown under 

 field conditions have expedited the selection of hardy experimental field plants for breeding. 

 A portable freezer of sufficient dimensions to surround several trees was used for this purpose 

 in experiments conducted by ARS in cooperation with the Texas Agricultural Experiment 

 Station at Weslaco. By means of fight and temperature controls, researchers were able to re- 

 duce or break dormancy in trees before they were enclosed by the freezer unit and to subject 

 the plants to freezing at times related to dormancy. In one experiment, 7 citrus species were 

 exposed to night temperatures that were gradually reduced from 60° F. to 48° F. over a 38- 

 week preconditioning period. The plants were then exposed to below freezing temperatures 

 (21° F.) for 4 hours. When the plants were examined for freeze injury, citrus varieties that 

 required rather low night temperatures to become dormant were found to be less cold hardy 

 than those that become dormant in response to higher night temperatures. As a result of 

 these findings, investigators hope they may soon be able to rate the cold hardiness of young 

 citrus seedlings quickly and accurately by inducing dormancy and relating them, by means 

 of the night temperature required to go into dormancy, to varieties of known cold hardiness. 



SMALL FRUITS 



With small fruits, as with tree fruits, greater cold hardiness is among the characteristics 

 sought by plant scientists in their quest for improved varieties. Aiding in the search is the 

 relatively shorter time required to get most small-fruit plants into production. For example, a 

 minimum of about 10 to 15 years is required for an apple tree grown from seed to bear fruit 

 and be initially evaluated, as compared to 4 years for blackberries and only 2 years for straw- 

 berries. 



Among bush fruits, raspberries are generally more hardy than blackberries and blue- 

 berries vary widely as a group in their ability to withstand cold. Although currants and goose- 

 berries are relatively cold hardy, new plantings of these fruits are not advised at this time 

 because of serious pest problems involved. 



Another common fruit grown in cold areas is the strawberry. However, the strawberry 

 does not properly belong in the group of hardy fruits. An evergreen, the strawberry is sus- 

 ceptible to extreme cold but frequently survives severe freezes because it grows close to the 



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