Pomegranate has been grown as a fruit crop since ancient times. Pomegranate fruit are composed of an outer leathery pericarp that contains hundreds of juice-containing arils. It is marketed as whole fresh fruit, extracted arils, juice, syrups, teas, and as an ingredient in a wide range of products. Numerous recent studies have verified the many health benefits associated with pomegranates, which include efficacy against conditions including cancer, coronary heart disease, atherosclerosis, hypercholesterolemia, hypertension, HIV, infectious diseases, and aging (Basu and Penugonda, 2008; Holland et al., 2009; Lansky and Newman, 2007; Seeram et al., 2006). This has led to an increase in the production and consumption of fresh fruit, juice, and other pomegranate products.
The morphological and histological characterization of pomegranate flowers has been recently described (Wetzstein et al., 2011a). Both hermaphroditic (bisexual) and functionally male flowers are on the same plant, a condition referred to as andromonoecy. Although pomegranate flowers are commonly portrayed as falling within either bisexual or male floral types, sex expression appears to follow a spectrum where flowers can range from those with strong pistils to those with vestigial development (Holland et al., 2009; Wetzstein et al., 2011a). This indicates that flower quality is inconsistent, and that even within bisexual flower types, the strength and vigor of individual flowers are variable. This leads to the question of whether flower vigor and quality as expressed in differences in flower size and degree of female development influence fruit production in pomegranate.
Flower and/or ovary size have been shown to impact final fruit size in a number of crops including peach [Prunus persica (Scorza et al., 1991)], rabbiteye blueberry [Vaccinium ashei (Johnson et al., 2011)], and olive [Oleo europaea (Rosati et al., 2009)]. Flower “strength” has been observed to affect fruit-setting ability in apple (Malus ×domestica), which may encompass factors such as bud size, spur position, flower quality, and location (Dennis, 1986). With apple flowers that are typically produced in a cluster of five to six flowers, the terminal or king flower consistently sets more fruit than lateral flowers. Unclear are the relative contributions in this case of differences in flower size, time of development, and sink strength.
In a study characterizing attributes related to fruit size in pomegranate, fruit volume, fruit weight, total aril weight, and total number of arils per fruit were highly correlated (Wetzstein et al., 2011b). In contrast, individual aril weight had no significant relationship to fruit size, indicating that a better approach to optimize size in pomegranate is to increase the number of arils per fruit rather than to promote more extensive aril enlargement. Large fruit were found to contain upward of 1000 arils per fruit. Each aril is derived from a single ovule in which an independent fertilization event occurs. Thus, production of large fruit requires the fertilization of hundreds of ovules. Fruit size can be quite variable even among fruit harvested from the same orchard under similar management conditions. 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. Likewise, the number of ovules in flowers has not been previously reported. Is ovule number per flower a factor that limits aril number and thus fruit size? Flowering in pomegranate can be as single flowers or in flower clusters containing a more advanced terminal flower subtended frequently by two to four lateral flowers (Wetzstein et al., 2011a). Unclear are how flower types differ in size and morphology and whether terminal vs. lateral flowers have divergent fruiting capacity.
As part of an ongoing project on pomegranate reproductive biology, the present work aims to evaluate how bisexual flower quality characteristics influence fruit set and development. Factors evaluated include the morphological characterization of different flower types (i.e., single flowers, terminal flowers, and lateral flowers) to ascertain if flower vigor influences fruit production. Specifically, 1) floral characteristics were measured in populations of different flowers types and flower size distribution patterns determined; 2) the numbers of ovules in individual flowers were determined and related to flower size; and 3) hand pollination studies were conducted to determine if flower size subsequently influences fruit set and fruit size. These studies will provide information to determine if flower type and vigor impact fruit set and size in pomegranate. Establishing how flower quality affects fruiting can have important implications in crop production applications. Optimizing cultural conditions may be a means to promote the development of greater numbers of high-vigor bisexual flowers to obtain maximum fruit set and yield.
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