|Carter, John - UNIVERSITY OF MINNESOTA|
|Brennan, Rex - SCOTTISH CROP RESEARCH|
Submitted to: Journal of American Society of Horticulture Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 1, 1999
Publication Date: N/A
Interpretive Summary: The loss of flowers on deciduous fruit crops in early spring due to freeze damage is a constant problem for fruit growers. Each year, spring frosts represent a potential loss to yield. In some years, entire crops are lost resulting in severe economic hardship to growers and increased prices for produce for the consumer. Breeding more cold hardy cultivars is the main way of trying to overcome this problem. The cold hardiness of particular selections and cultivars,as determined in laboratory tests, does not always correlate with the performance of plants under field conditions. The cold hardiness of genotypes of blackcurrant is an example of this contradiction. This suggests that a better understanding of how plants freeze and how ice propagates in plant tissues is needed. We have used infrared thermography in the current study to directly visualize the freezing process in several cultivars of blackcurrants. This is a novel use for this technology and it thas provided unique insights into the freezing process that will help to formulate new approaches to freeze protection. We have been able to demonstrate that in black currants the ability of flowers to survive a frost episode is more dependent on the existence of barriers to ice propagation, which prevent the growth of ice from stem tissues into sensitive flowering tissues, than the actual freezing tolerance of the flowers. Research is continuing to better understand the nature of the ice barrier and to determine if this can be used as a selection tool in breeding programs that aim at increasing cold tolerance.
Technical Abstract: The low-temperature tolerance of flowers from three blackcurrant (Ribes nigrum L.) cultivars,'Brodtorp', 'Ben Tirran,' and 'Baldwin,' was determined at two stages of floral development. Plants were maintained either at 4C in a growth cabinet under a 16 h photoperiod or outdoors in Scotland during Spring, 1997. The three cultivars together represent a large part of the available genetic base for this sub-genus of Ribes. Observed genotypic differences in survival of spring frosts are shown to be unconnected to differences in LT50 of the flowers, and observations of freezing damage to flowers on intact plants suggest that the flowers can often survive episodes of frost by supercooling. This hypothesis is partly confirmed by the finding that flowers from all three cultivars have the capacity to supercool to at least -9C. Ice nucleation in blackcurrant stem tissue, however, was found to occur at or above -2C. That flowers on intact plants can apparently survive episodes of frost by supercooling, together with the finding that ice nucleation in stem tissue occurs at temperatures well above the LT50 of flowers, indicated the presence of barriers to propagation of ice from stem tissue to flower strig. Barriers to propagation of ice within individual flower strigs are also indicated by patterns of freezing damage to flowers on intact plants cooled to -5C.