Cultivar Differences in Floral Structures and Effect of Low Air Temperature on Pollen Germination in Fragaria x ananassa Duch.

Abstract

This study was conducted to investigate the causes of fruit malformation in the Korean strawberry Maehyang and to develop cultural techniques to reduce the incidence of the symptoms. The first chapter of this thesis analyzes cultivar differences in morphological characteristics of flower organs. In the second chapter, cultivar differences in pollen germination as affected by low temperature during the dark period are discussed. In Chapter 1, three Korean cultivars Maehyang, Seolhyang, and Keumhyang, and a Japanese cultivar Akihime were grown in a plastic greenhouse, and their flower organs were observed. The shape of the receptacle in Maehyang was longer and narrower in stage 3 than in the other cultivars. Maehyang had the more rows of ovules, which might indicate a difference in pistil development between distal and proximal regions of the receptacle. Among the four tested cultivars, there was no significant difference in pistil length in primary or tertiary flower clusters. However, in the secondary flower cluster, where the incidence of fruit malformation was reported to be greatest, Maehyang had a shorter pistil than Seolhyang, Keumhyang and Akihime, with Keumhyang having the longest pistil. The Stigma surface of Maehyang was smoother than those of the other cultivars and had a larger number of beaks on the stigma. Anther shapes at stage 3 were dependent on cultivar and flower cluster. Among the tested cultivars, Maehyang had the narrower and longer anthers throughout the primary to tertiary flower clusters. Maehyang had a smallest number of pollen grains in the secondary flower cluster, while Keumhyang had the largest number. These morphological differences of flower organs may be related to the higher incidence of fruit malformation in Maehyang. In Chapter 2, the two Korean cultivars Maehyang and Seolhyang were cultivated in a greenhouse. When flower buds became visible in early October, plants of each cultivar were transferred to a plant factory with artificial lighting. Four different air temperatures (5, 10, 15, and 20oC) were tested during the dark-period (14 h d-1), and photoperiod (10 h d-1) air temperature was maintained at 25oC. For the determination of pollen viability and germination, three fully-opened flowers with dehiscent anthers were obtained from each flower cluster of plants grown at the plant factory. An in vitro pollen viability test was performed using the acetocarmine staining technique and pollen grains germinated in germination medium. Maehyang showed lower pollen viability and pollen germination compared with Seolhyang in all temperature treatments. Both cultivars had relatively low pollen viability at 25oC/5oC and 25oC/10oC. The percentage of viable pollen was significantly lower in Maehyang than in Seolhyang at 25oC/15oC in both primary and secondary flower clusters and also lower at 25oC/5oC and 25oC/10oC in the tertiary flower cluster. The percentage of pollen germination in primary flower cluster was lower at high night temperature (25oC/15oC and 25oC/20oC) than at the other two temperature treatments. In the secondary flower cluster, Maehyang showed a lower pollen germination than Seolhyang at 10oC and 20oC, with the lowest percentage of pollen germination occurring at 25oC/20oC in Maehyang but at 25oC/5oC in Seolhyang. In the Tertiary flower cluster, the pollen germination percentage was significantly higher in Seolhyang than in Maehyang at all tested night temperatures except 10oC, and the lowest percentage of pollen germination was observed at 25oC/10oC in both cultivars. The research results of the present study could be used for developing new cultivation techniques to produce high-quality strawberry fruits in winter cultivation, so called forced cultivation.