Project Number: 5306-21000-018-01
Project Type: Specific Cooperative Agreement

Start Date: Sep 23, 2008
End Date: Sep 30, 2009

Objective:
Pistillate flower abscission (PFA) caused by high pollen loads is a phenomenon in both Juglans nigra (Beineke and Masters, 1976) and J. regia (Kavetskaya and Tokar, 1963; Por and Por, 1990; Szentivanyi, 1990; McGranahan et al., 1994). PFA has emerged as a very serious non-pathogenic problem of some walnut cultivars. It has been shown to cause up to 90% abscission of female flowers followed by a similar reduction in yield under normal circumstances in the field (Catlin et al., 1987). The formerly popular cultivar Serr (Payne x PI159568) is thought to be most susceptible but most of the commercial cultivars are susceptible to some extent (Catlin et al., 1987; Catlin and Olsson, 1990). Growers in California and Chile are now using ¿Retain¿ an ethylene inhibitor, as a spray to maintain yield in several cultivars (R. Beede, pers. com.). It is essential in any breeding or selection program that the problem is avoided and for that reason the response of selections, cultivars and potential parental germplasm to high pollen loads needs to be investigated. Walnuts are monoecious and wind-pollinated and bloom between March and May. The genus has a heterodichogamous bloom habit: Individuals may be protandrous or protogynous. This bloom habit encourages outcrossing but does not completely eliminate the possibility of self-fertilization because the temporal separation of male and female blooming is rarely complete. The extent of overlap between staminate and pistillate bloom varies considerably within species and from year to year. Both the presence of pollenizers and the extent of overlap are thought to be responsible for the damaging high pollen loads on the female flowers (Walnut Research Reports, Walnut Marketing Board, Sacramento , CA). PFA is characterized by cessation of pistillate flower growth when the ovary is about 3-4 mm in diameter followed by flower abscission 1-2 weeks later (Catlin et al. 1987). Near the time of abscission, PFA-type flowers show signs of necrosis at the tips of the stigma, in the integument, and throughout the placental invaginations (Catlin and Polito, 1989). Flower abscission due to PFA is clearly distinguishable from abscission that results from lack of pollination. Unfertilized flowers usually expand to approximately 7 mm in diameter and remain attached for 3 weeks or longer.

Approach:
Ten clones have been selected for initial screening. These represent known sensitive cultivars, the parents of the most seriously afflicted cultivar and germplasm from a range of sources. The trees are growing either at the National Clonal Germplasm Repository or in the University of California germplasm collection. They are: PI 159568 Serr Payne Chandler Alsoszentivani 117 Sinensis 5 85-8 67-13 J. nigra Thomas J. hindsii Rawlins Pollen will be collected from the cultivar Gustine which has the earliest pollen available. Source of pollen in the same species has no effect (McGranahan et al., 1994). Pollen will be stored frozen over magnesium chloride in doubled tubes, the standard pollen storage method for the walnut breeding program. Pollen will be thawed at room temperature before opening of the vessel. As soon as female flowers are visible 50 double flowers on each of two trees will be tagged, and at peak fertility (Forde and Griggs, 1975) when stigmas are in a tight V, pollen will be air-brushed on half of the flowers. An extra set of flowers will be treated and preserved to determine approximate pollen load using standard methods (McGranahan et al., 1994). PFA counts will be made at approximately 3 weeks after pollination. PFA will be determined by the percent of flowers that abscise. Naturally occurring PFA will be assessed on the unpollinated flowers. Data on percent PFA will be submitted to GRIN.