Pomegranate has been grown as a fruit crop since ancient times. Native to central Asia, this tree/shrub is highly adaptive to a wide range of climates and soil conditions and is grown in many geographical regions, including the Mediterranean basin, Asia, and the United States, particularly in California (Holland et al., 2009). It has long been valued for its flavorful, juicy aril sacs and more lately for commercial juice production. Recently, numerous studies have verified the health benefits associated with pomegranates. Pomegranate juice and products are purported to show efficacy against a wide range of conditions, including cancer, coronary heart disease, atherosclerosis, hypercholesterolemia, hyperlipidemia, hypertension, HIV, infectious diseases, aging, and brain disorders (Basu and Penugonda, 2008; Holland et al., 2009; Lansky and Newman, 2007; Seeram et al., 2006). This has led to a higher awareness of the public to the benefits of pomegranate and a prominent increase in the consumption of the fruit and juice (Holland et al., 2009).
Flowering in pomegranate is characterized as having both hermaphroditic (bisexual) flowers and functionally male flowers on the same plant, a condition referred to as andromonoecy. The hermaphroditic flowers have well-formed female (stigma, style, ovary) and male (filaments and anthers) parts and have been referred to as “fertile,” “vase-shaped,” and “bisexual” flowers. Because the hermaphroditic flowers are the type that set fruit, they are commonly referred to as “female” flowers, albeit with some inaccuracy. The male flowers produce well-developed male parts, but on closer examination of the pistil contain reduced female parts. Thus, their role is more accurately depicted as functionally male flowers (i.e., flowers are not strictly male), but rather have degenerated female parts. Male flowers typically drop and fail to set fruit (Holland et al., 2009; Shulman et al., 1984). Chaudhari and Desai (1993) classified pomegranate flowers into three types: male, hermaphroditic, and intermediate. Observations of gradients of flower types in some pomegranate genotypes support this concept. Synonyms for male flowers include “infertile” and “bell” flowers. Herein, we refer to the flower types as “bisexual” and “male” for hermaphroditic and functionally male flower types, respectively.
Andromonoecy as well as other fluctuating sexual expression types is proposed to allow a species to optimize the allocation of limited resources to male and female function (Bertin, 1982; Wilson, 1983). Manipulating the relative ratio of flower types to environmental conditions can be very advantageous. As a result of the high costs associated with female expression, repression of this flower type under poor environmental conditions could be a means to conserve limited resources at a time when maturation of a high fruit/seed yield is not possible. Furthermore, having high numbers of male flowers can be a way to spread genes, because pollen spread is more efficient with more male flowers (Herlihy and Eckert, 2002; Tanurdzic and Banks, 2004).
Under agricultural production conditions, male:female flower ratios in pomegranate can impact crop productivity and yield. Male flowers drop and generally fail to set; thus, fruits develop exclusively from bisexual flowers. A positive relation was found between the percentage of bisexual flowers and bearing capacity (Chaudhari and Desai, 1993; El Sese, 1988). The percentage of flowers that are male in pomegranate can be significant and more than 60% to 70% depending on variety and season (Chaudhari and Desai, 1993; Mars, 2000). Furthermore, the ratio of bisexual and functionally male flowers can vary with season (N. Ravid, personal communication), cultural location, and genotype. Fluctuations of flower types within a season can proceed with a predominant appearance of bisexual flowers followed by male flowers or vice versa.
Although this crop has been grown as an agricultural crop since antiquity, scientific literature on many fundamental aspects of pomegranate development and physiology is lacking, including basic aspects of floral biology. A clear understanding of male and female flowering is lacking. As part of an ongoing project on pomegranate reproductive biology, the present work aims to describe the morphology and anatomy of bisexual and functionally male flower types in pomegranate. Morphological and histological evaluations of hermaphroditic and male flowers were conducted using light microscopy (LM) and SEM to better understand developmental differences between the flower types. Sex expression is labile in this species. A comparison of the form and function of flower parts in the two morphs, including pollen viability, stigma development, and pollen–pistil interactions, would provide information useful in the development of crop production protocols to enhance fruit production.
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