Summers and Stapleton have shown that tomatoes grown over reflective mulches averaged approximately 7 percent virus-infected plants, while plants grown over bare soil averaged in excess of 50 percent infection with the same viruses. This approach is currently the only viable means of managing virus disease in these production systems. Growers are cautioned to use only metalized reflective mulches when insect anddisease management is the primary objective. Other colors lack the high degree of UV reflectance necessary to repel incoming insects. Also, the mulch must be applied prior to seedling emergence. Plants may be inoculated with aphidborne viruses in the cotyledon stage, and any delay in applying the mulch could lead to an extensive infection. Tomatoes may suffer some leaf burn and early stunting but rapidly outgrow these problems and soon surpass plants grown over bare soil. Eggplant tends to be more sensitive and may suffer prolonged injury. We have observed no problems with any cucurbits grown over metalized mulches. Plants, particularly cucurbits, grown over metalized mulches may be more susceptible to frost injury in the late fall. More widespread adoption of the use of UV-reflective plastic mulches in California has not occurred largely because of added costs associated with their use and disposal and a general lack of familiarity with production techniques required for their successful use.Southern California’s Coachella and Imperial Valleys are major production regions for a wide variety of winter vegetables. Fields in these valleys are typically fallowed during the summer due to high temperatures that preclude the production of most vegetables.
Recently, growing rack researchers at the University of California, Riverside, in conjunction with Riverside County Cooperative Extension, the Indio USDA Natural Resources Conservation Service, and the USDA Agricultural Research Service in Beltsville, Maryland, developed a cover crop mulch production system for this region using cowpea , a warm-season legume. This system relies on the use of large, bush-type cultivars. The cowpea is seeded into preshaped soil beds in June and August. When the cover crop is chopped and converted into a mulch, the above ground biomass can be as much as 2,542 pounds per acre , providing a surface matte that has been shown to control weeds, reduce parasitic nematodes, and provide for vegetable yields comparable to the current bare soil production system. This cover crop mulch production technique has also resulted in increased soil carbon, which is an important management goal of producers in the region.Current agricultural systems rely heavily on the use of herbicides and tillage for weed management, but both have negative impacts on the environment and farm productivity in long-term use . An integrated approach to weed management which incorporates ecological principles and involves using multiple tactics that vary in timing and type of control is needed to reduce the probability of rapid weed adaptation to managemen t practices . Moreover, weed management decisions should aim to prevent soil seedbank inputs rather than just minimize current yield loss for agricultural profitability .
While there have been many studies focused on weed seed biology and seedbank management , research focused on reducing weed seed production by manipulating flowering and seed set is lacking. Flowering plants have developed a wide range of reproductive systems. Most flowering plants are hermaphrodites, having both functional sex organs in the same flower, which is believed to be the ancestral form . Plants have evolved various mechanisms to deviate from this ancestral form such as production of unisexual staminate or pistillate flowers to avoid inbreeding . However, the adoption of any sexual strategy is often accompanied by genetic and demographic trade-offs . For example, dioecy maximizes outcrossing and thereby reduces the likelihood of inbreeding depression . It further allows for more efficient resource allocation between male and female roles by reducing competition between these two functions through sexual specialization . However, successful fertilization in dioecious species relies on proximity and synchronization of male and female flowers. These limitations present an opportunity for the development of novel management strategies for dioecious weeds.Insect sterile technique , an environmentally-friendly and biologically-based method for controlling insect pest, involves sterilizing male insects by irradiation and subsequently releasing the sterile males to mate with wild females , resulting in infertile eggs and reduced insect pest population sizes. Drawing from the success of the IST in controlling insect pests, a similar approach could be effective for weed populations. There are several methods to make pollen grains functionally deficient and thereby reduce seed set .
The most commonly used is ionizing irradiation with X-rays or gamma rays due to their ease of use, effective penetration, consistent results, and minimal disposal issues . Irradiated pollen can germinate and grow pollen tubes but fails to fertilize egg cells . Moreover, when sterile pollen is artificially applied to stigmas, it can obstruct the fertilization process by fertile pollen, thus disrupting seed production. Pollinating female plants of Amaranthus palmeri with sterile pollen resulted in 40% reduction in the number of newly formed seeds . This also has been demonstrated in various plants like apple, pear, citrus, cacao, and melon . The genus Amaranthus contains about 75 species worldwide . Most Amaranthus species are monoecious whereas a few species are dioecious . Dioecious Amaranthus were originally placed into a single subgenus due to dioecy but in recent phylogenetic studies have been grouped with monoecious species in various clades . Palmer amaranth is indigenous to the southwestern United States and northern Mexico . Initially considered an insignificant weed in fields of cotton, soybean, and maize in 1974, it is now one of the top 10 most troublesome weeds in the US . This is due to a combination of human-assisted seed dispersal and new habitat creation through agricultural expansion . Amaranthus palmeri has emerged as one of the most economically damaging weeds in the United States, causing yield losses of up to 91% in corn , 79% in soybean , and 65% in cotton . Furthermore, A. palmeri has evolved resistance to nine herbicide classes and is able to produce up to one million seeds per plant . This weed is a particularly suitable candidate for exploration of the sterile pollen technique for weed control. As a dioecious species with separate male and female plants, it relies heavily on cross-pollination for successful seed production. Theoretically, this makes it feasible to collect pollen grains from male plants, sterilize them, and subsequently release pollen onto female plants.The separation of sexes is the norm in animals, but it is rare in plants . Most angiospermous plants are hermaphrodites, having both functional sex organs in the same flower, which is believed to be the ancestral form . Plants have evolved various mechanisms to deviate from this ancestral form such as production of unisexual staminate or pistillate flowers to avoid inbreeding . Only about 6% of plant species are dioecious , i.e. having male and female reproductive structures on separate individual plants. Despite its rarity, dioecy has developed from hermaphroditic ancestors independently in numerous taxonomic groups, with at least one dioecious species found in 50% of angiosperm families . Historically, two types of unisexual flowers have been defined based on the developmental mechanisms of the evolution from hermaphroditism. In type I, flowers initiate reproductive structures of both sexes but ultimately become unisexual by arresting the development of the inappropriate sex, such as most dioecious Anacardiaceae . In type II, unisexual flowers have only one set of sex organ primordia that emerges from the floral meristem, such as spinach . It is believed that sex determination of type II occurs before initiation of stamens or carpels ; hence, flowers of either sex exhibit no hermaphrodite stage. Most species can be characterized as type I where the arrest of development of stamens or carpels can be triggered by diverse mechanisms at any point from initiation of stamen and carpel primodia to post-meiosis . For example, hydroponic rack system the development of male and female structures in pistillate flowers resembles hermaphroditic flowers until the end of stage seven in Silene latifolia when stamen development is terminated after anther differentiation . In Celtis iguanaea, gynoecium development arrests before carpal elongation in staminate flowers whereas androecium development in pistillate flowers terminates beforepollen maturation . Thus, it has been speculated that the termination of carpels and stamens rarely happens by a typical procedure or at a certain stage . In angiosperms, sex chromosomes have been identified in about 150 species, with half the species having visible heteromorphic chromosomes . In species with no sex chromosomes, there are often sex-determining genes that either suppress maleness or femaleness .
For instance, in Silene latifolia , there are two important regions in sex determination: one inhibits gynoecium growth and the other supports androecium development . Additionally, programmed cell death has been proposed as a main force in driving the development of unisexuality in some angiosperms ; PCD terminates the gynoecial development in male flowers of Zea mays and anther development in Opuntia robusta . The genus Amaranthus contains about 75 species worldwide and includes crops as well as invasive weeds . Most Amaranthus species have a monoecious breeding system whereas a few species are dioecious . Dioecious Amaranthus were originally placed into a single subgenus due to dioecy but in recent phylogenetic studies have been grouped with monoecious species in various clades . Because of the widespread evolution of herbicide resistance, several weedy Amaranthus species have received increasing attention recently. Palmer amaranth , one of the most devastating weeds in the US, is a dioecious summer annual weed . It ranked as the worst weed in US corn fields in a survey conducted by the Weed Science Society of America . Furthermore,A. palmeri has evolved resistance to nine herbicide classes and is able to produce up to one million seeds per plant . To control this noxious weed, we must adopt a multi-tactic approach that incorporates ecological principles into weed management practices. Understanding the reproductive biology of this species can aid in the development of agronomic strategies for management and mitigation of herbicide resistance . Only a few species have been studied for the potential loss of sexual function in pistillate or staminate organs during floral morphogenesis and we are aware of no study of A. palmeri. Characterizing floral development in A. palmeri is extremely difficult due to the minute flower buds size . The objectives of this study were twofold: to compare floral organogenesis in pistillate and staminate flowers of A. palmeri using scanning electron microscopy and to define stages of floral development in A. palmeri. Considering the higher prevalence of type I , our hypothesis was that A. palmeri flowers produce structures of both sexes during the early stages of development and then the vestigial organs from the opposite sex are aborted as development continues.Flower buds from both male and female plants were dissected for light microscopy . The inflorescence axes from male and female plants were cut into approximately 1cm segments and these segments were further sliced longitudinally. The cut surface was visualized under scanning electron microscopy to locate axillary floral structures. Multiple buds at different angles from male and female plants were prepared to increase throughput and the likelihood of successful imaging. Bracts and tepals of flowers were removed as much as possible without damaging the reproductive structures. Following dissection, tissues were transferred into 70% Formalin-acetic acidalcohol ; 10:1:2:7 Ethanol : Glacial acetic acid: Formalin : Distilled H2O to fix for 24 hours. Samples were transferred through a dehydration series of 70%, 80%, 90%, 95%, 100%, and another 100% ethanol for at least 10 minutes each step. The tissue in 100% ethanol was critical point dried to preserve the surface structure which could be damaged due to surface tension when changing from the liquid state to gas state. Dried samples were mounted on aluminium stubs with adhesive tabs. Samples were coated with a thin gold-palladium layer by sputter coating for 60 seconds at 12mA. Lastly, samples were visualized with a ThermoFisher Quattro ESEM scanning electron microscopy operating at 5 kV and digital images captured as TIFF files.Flowers of both sexes are about 1 mm diameter, white or whitish green and are produced in dense and compact clusters on cylindrical inflorescences with smaller axillary spikes at the base . Each pistillate or staminate flower is enclosed by 1 to 3 stiff awlshaped bracts .