Pomegranate [Punica granatum (Punicaceae)] is characterized by having two types of flowers on the same tree: hermaphroditic bisexual flowers and functionally male flowers. This condition, defined as functional andromonoecy, can result in decreased yields resulting from the inability of male flowers to set fruit. Morphological and histological analyses of bisexual and male flowers were conducted using light and scanning electron microscopy (SEM) to characterize the different flower types observed in pomegranate plants and to better understand their developmental differences. Bisexual flowers had a discoid stigma covered with copious exudate, elongated stigmatic papillae, a single elongate style, and numerous stamens inserted on the inner wall of the calyx tube. Using fluorescence staining, high numbers of pollen tubes were observed growing through a central stylar canal. Ovules were numerous, elliptical, and anatropous. In contrast, male flowers had reduced female parts and exhibited shortened pistils of variable heights. Stigmatic papillae of male flowers had little exudate yet supported pollen germination. However, pollen tubes were rarely observed in styles. Ovules in male flowers were rudimentary and exhibited various stages of degeneration. Pollen from both types of flowers was of similar size, ≈20 μm, and exhibited similar percent germination using in vitro germination assays. Pollen germination was strongly influenced by temperature. Maximal germination (greater than 74%) was obtained at 25 and 35 °C; pollen germination was significantly lower at 15 °C (58%) and 5 °C (10%).
Pomegranate trees (Punica granatum) produce large numbers of both hermaphroditic (bisexual) flowers that produce fruit and functionally male flowers that characteristically abort. Excessive production of male flowers can result in decreased yields resulting from their inability to set fruit. Within hermaphroditic flowers, sex expression appears to follow a spectrum ranging from those exhibiting strong to weak pistil development. Unknown is the scope that flower quality plays in influencing fruit production. A description of floral characteristics and how they vary with flowers of different sizes and positions is lacking in pomegranate and was the focus of this study. Furthermore, the effects of flower size and position on fruit set and fruit size were evaluated. This study documents that flower size characteristics and ovule development can be quite variable and are related to flower type and position. Single and terminal flowers within a cluster were larger than lateral flowers. In addition, lateral flowers exhibited a high frequency of flowers with poor ovule development sufficient to negatively impact fruiting in that flower type. Ovule numbers per flower were significantly influenced by flower size with more ovules in larger flowers. Pollination studies verified significantly higher fruit set and fruit weight, and larger commercial size distributions were obtained with larger vs. smaller flowers. Thus, flower quality is an important issue in pomegranate. Cultural and environmental factors that influence flower size and vigor may have a direct consequence on fruit production and yield.
Pomegranate fruit is valued for its juice-containing arils and is consumed and marketed as whole fresh fruit, extracted arils, juice, syrup (grenadine), wine, teas, seed oil, and other products. Recent consumption has rapidly increased attributable in part to reported health benefits that include efficacy against coronary heart disease, atherosclerosis, cancer, hypertension, and infectious diseases. Within commercial orchards, the size of fruits produced can be quite variable even with trees of the same genotype grown under similar cultivation practices. Although pomegranates have been cultivated since antiquity, fruit attributes, particularly those related to size, are poorly defined. In this study, compositional changes in pomegranate fruits of the Wonderful cultivar, including volume and weight, aril weight and number, pericarp weight, seed weight, and juice/pulp content, were evaluated in fruits of variable sizes. Correlations between fruit characteristics were determined, and factor analysis established fruit and aril indices. Results indicated that because fruit volume, fruit weight, and total aril weight are closely correlated, any of these characteristics can be used as an indicator of fruit size. The number of arils per fruit was highly correlated with fruit size with larger fruit containing greater numbers of arils. This is in contrast to individual average aril weight, which showed no significant relationship to fruit size. Crop production strategies aimed at increasing aril numbers may be a means for obtaining larger fruit in pomegranate.