Among the recent results of this research are two varieties of plums with superior 

 qualities for home gardens and orchards in the Northern Great Plains region. These varieties 

 — named Gracious and Chinook — were introduced by USDA in 1957 after extensive testing 

 at the Northern Great Plains Field Station at Mandan, N. Dak., for trial in that area. Gracious, 

 a seedling of Emerald, strongly resembles another variety, Redwing, in some ways and is 

 probably a cross of Emerald and Redwing but is hardier than the latter. Gracious bears 

 rather large fruit (about 12 to a pound) which is mottled red with yellow flesh. Chinook is 

 a seedling of Ojibwa. Although smaller than Gracious, Chinook is a large plum (about 20 to 

 the pound) which rated very well in cooking tests. The fruit is bright red, round, firm, and 

 semi-freestone. Both of the introductions ripen in late August. 



The Northern Great Plains Field Station is also making progress in breeding improved 

 varieties of hardy plums. By hybridizing high quality Japanese plums with winter hardy 

 native and European varieties, a number of promising selections have been developed and are 

 currently being evaluated. 



Plum breeding work is also going forward at the Irrigation Experiment Station at Prosser, 

 Wash. In this project, several varieties from Vineland, Ontario, Canada, are being crossed 

 with the large-fruited Edwards variety, an Italian Prune type with quality fruit, and with 

 President. 



CITRUS RESEARCH 



Cold damage being the constant threat that it is to citrus growing, USDA plant researchers 

 have long been concerned with the problem. Early investigators of the United States Depart- 

 ment of Agriculture uncovered much useful information regarding the relative cold hardiness 

 of different kinds and varieties of citrus and the many factors affecting the cold hardiness of 

 individual plants. 



An important recent development in citrus breeding is the use of monoembryonic seed 

 parents to speed up the production of hybrids. Unlike polyembryonic seeds, monoembryonic 

 seeds contain only one embryo and are capable of producing only one seedling. The syste- 

 matic use of monoembryonic seed parents to speed up the production of hybrids developed 

 from the discovery that a few citrus varieties, such as Clementine tangerine and Temple orange, 

 produced only hybrid plants when used as seed parents in crossing with other varieties. Prior 

 to this discovery, attempted crosses resulted in a high percentage of nucellar seedlings to 

 true hybrids. Nucellar seedlings inherit only characters from the mother plant. Consequently 

 their inadvertent production in hybridizing attempts has been a delaying factor in the develop- 

 ment of new varieties combining cold hardiness and other desired traits. By using mono- 

 embryonic seed parents for hybridization, plant breeders can avoid using time and land for 

 growing nucellar seedlings which are virtual replicas of the mother plant but often cannot 

 be recognized as such until they are fruited. 



Another major line of research going forward at this time is on the effect of various 

 rootstocks on the cold hardiness of different varieties of citrus scions. Evidence from recent 

 studies supports the generally held belief that rootstocks that are cold resistant themselves tend 

 to induce cold resistance in the scion variety. However, there are indications that physiologi- 

 cal factors in the rootstock-scion combination also have some influence. For example, although 

 the Clementine tangerine is rather cold hardy when fruited on its own roots, it rates lower as a 

 rootstock than some other less cold-hardy varieties in ability to induce hardiness in the scion. 

 On the other hand, sweet oranges have a high cold-hardiness rating when used as rootstocks 

 but when used as scions they are not as cold hardy as many varieties which ranked much lower 

 when rated on the basis of performance when grown from their own roots. On the evidence of 

 studies to date investigators, are generally of the opinion that the degree of cold hardiness 

 induced by rootstocks in scion citrus varieties is related to dormancy — that those rootstocks 

 that induced early and prolonged dormancy in the scion increased its cold hardiness. 



