FLORAL DEVELOPMENT, ANTHESIS AND POLLINATION
Bud failure prior to anthesis has been identified as a limitation to yield. The cause of the problem has not been identified for walnut, although in other crops there is involvement of high gibberellin levels associated with vine vigour (grapevine), and competition from developing nuts (pistachio). Abnormalities of the mature flower can also reduce yield including sterility due to polyploidy in grapevine and flower death caused by air pollution in blueberry and raspberry. The pistil of most temperate fruit species has a wet papillate stigma with a solid style in the rosaceous genera and a hollow style in kiwifruit. Sex expression in the dioecious mulberry appears to be controlled by ethylene (female) and gibberellin (male) but in Chinese chestnut the reverse occurs. More advance has been made in the understanding of dichogamy mechanisms with faster development of male than female flowers in protandrous cultivars and vice versa for protogynous cultivars of walnut and pecan. Self-incompatibility, controlled by multiple S-alleles, continues to be reported in a wide range of crops and cultivars. The glycoprotein gene product of the S-allele has been identified in cherry and pear and has a MW of between 37,000 and 58,000.
Yield improvement due to the placing of honeybee hives in the orchard has been demonstrated in many insect-pollinated crops. Bee pollination can be further improved by various methods including hive dispensers to dust the bees with pollen, and feeding the bees by placing sugar syrup inside the hive. Methods for pollination improvement not involving bees are based on mechanical spraying or dusting of pollen in the orchard. There is a conflict between pollination efficiency and disease control using fungicides. Inhibition of pollen germination and tube growth is widely reported and reduced ovule longevity occurs in apple in response to the use of Triadimefon. An interesting recent finding is that the optimum temperature for pollen tube growth tends to be higher than that for ovule longevity and this can result in low effective pollination periods in warm weather. There is also a deceleration of pollen tube growth when the tube reaches the ovary, which in almond and hazel has been shown to be a result of ovule immaturity at anthesis. In both these crops compatible pollination is essential for ovule maturation.