Effects of altered gravity on growth and morphology in Wolffia globosa implications for bioregenerative life support systems and space-based agriculture

Understanding the response of plants to varied gravitational conditions is vital for developing effective food production in space bioregenerative life support systems. This study examines the impact of altered gravity conditions on the growth and morphological responses of Wolffia globosa (commonly known as "water lentils" or "duckweed"), assessing its potential as a space crop. Although an experiment testing the effect of simulated microgravity on Wolffia globosa has been previously conducted, for the first time, we investigated the effect of multiple gravity levels on the growth and morphological traits of Wolffia globosa plants. The plant responses to simulated microgravity, simulated partial gravity (Moon), and hypergravity environments were evaluated using random positioning machines and the large-diameter centrifuge. As hypothesized, we observed a slight reaction to different gravitational levels in the growth and morphological traits of Wolffia globosa. The relative growth rates (RGR) of plants subjected to simulated microgravity and partial gravity were reduced when compared to those in other gravity levels. The morphological analysis revealed differences in plant dimensions and frond length-to-width ratios under diverse gravity conditions. Our findings showed that Wolffia globosa is responsive to gravitational changes, with its growth and morphological adaptations being slightly influenced by varying gravitational environments. As for other crop species, growth was reduced by the microgravity conditions; however, RGR remained substantial at 0.33 a day. In conclusion, this study underscores the potential of Wolffia globosa as a space crop and its adaptability to diverse gravitational conditions, contributing to the development of sustainable food production and bioregenerative life support systems for future space exploration missions.

Plant and microbial science and technology as cornerstones to Bioregenerative Life Support Systems in space

Long-term human space exploration missions require environmental control and closed Life Support Systems (LSS) capable of producing and recycling resources, thus fulfilling all the essential metabolic needs for human survival in harsh space environments, both during travel and on orbital/planetary stations. This will become increasingly necessary as missions reach farther away from Earth, thereby limiting the technical and economic feasibility of resupplying resources from Earth. Further incorporation of biological elements into state-of-the-art (mostly abiotic) LSS, leading to bioregenerative LSS (BLSS), is needed for additional resource recovery, food production, and waste treatment solutions, and to enable more self-sustainable missions to the Moon and Mars. There is a whole suite of functions crucial to sustain human presence in Low Earth Orbit (LEO) and successful settlement on Moon or Mars such as environmental control, air regeneration, waste management, water supply, food production, cabin/habitat pressurization, radiation protection, energy supply, and means for transportation, communication, and recreation. In this paper, we focus on air, water and food production, and waste management, and address some aspects of radiation protection and recreation. We briefly discuss existing knowledge, highlight open gaps, and propose possible future experiments in the short-, medium-, and long-term to achieve the targets of crewed space exploration also leading to possible benefits on Earth.

Perspectives for plant biology in space and analogue environments

Advancements in plant space biology are required for the realization of human space exploration missions, where the re-supply of resources from Earth is not feasible. Until a few decades ago, space life science was focused on the impact of the space environment on the human body. More recently, the interest in plant space biology has increased because plants are key organisms in Bioregenerative Life Support Systems (BLSS) for the regeneration of resources and fresh food production. Moreover, plants play an important role in psychological support for astronauts. The definition of cultivation requirements for the design, realization, and successful operation of BLSS must consider the effects of space factors on plants. Altered gravitational fields and radiation exposure are the main space factors inducing changes in gene expression, cell proliferation and differentiation, signalling and physiological processes with possible consequences on tissue organization and organogenesis, thus on the whole plant functioning. Interestingly, the changes at the cellular and molecular levels do not always result in organismic or developmental changes. This apparent paradox is a current research challenge. In this paper, the main findings of gravity- and radiation-related research on higher plants are summarized, highlighting the knowledge gaps that are still necessary to fill. Existing experimental facilities to simulate the effect of space factors, as well as requirements for future facilities for possible experiments to achieve fundamental biology goals are considered. Finally, the need for making synergies among disciplines and for establishing global standard operating procedures for analyses and data collection in space experiments is highlighted.

Applying productivity and phytonutrient profile criteria in modelling species selection of microgreens as Space crops for astronaut consumption

Introduction

Long-duration missions in outer Space will require technologies to regenerate environmental resources such as air and water and to produce food while recycling consumables and waste. Plants are considered the most promising biological regenerators to accomplish these functions, due to their complementary relationship with humans. Plant cultivation for Space starts with small plant growth units to produce fresh food to supplement stowed food for astronauts' onboard spacecrafts and orbital platforms. The choice of crops must be based on limiting factors such as time, energy, and volume. Consequently, small, fast-growing crops are needed to grow in microgravity and to provide astronauts with fresh food rich in functional compounds. Microgreens are functional food crops recently valued for their color and flavor enhancing properties, their rich phytonutrient content and short production cycle. Candidate species of microgreens to be harvested and eaten fresh by crew members, belong to the families Brassicaceae, Asteraceae, Chenopodiaceae, Lamiaceae, Apiaceae, Amarillydaceae, Amaranthaceae, and Cucurbitaceae.

Methods

In this study we developed and applied an algorithm to objectively compare numerous genotypes of microgreens intending to select those with the best productivity and phytonutrient profile for cultivation in Space. The selection process consisted of two subsequent phases. The first selection was based on literature data including 39 genotypes and 25 parameters related to growth, phytonutrients (e.g., tocopherol, phylloquinone, ascorbic acid, polyphenols, lutein, carotenoids, violaxanthin), and mineral elements. Parameters were implemented in a mathematical model with prioritization criteria to generate a ranking list of microgreens. The second phase was based on germination and cultivation tests specifically designed for this study and performed on the six top species resulting from the first ranking list. For the second selection, experimental data on phytonutrients were expressed as metabolite production per day per square meter.

Results and discussion

In the final ranking list radish and savoy cabbage resulted with the highest scores based on their productivity and phytonutrient profile. Overall, the algorithm with prioritization criteria allowed us to objectively compare candidate species and obtain a ranking list based on the combination of numerous parameters measured in the different species. This method can be also adapted to new species, parameters, or re-prioritizing the parameters for specific selection purposes.

Maize root behavior as three-inputs-three-outputs logical gates due to positive gravitropism and nutritropism

Root growth and their interactions can provide valuable information for the development of asynchronous logic systems. Here, maize root behavior due to positive gravitropism and nutritropism is evaluated as three-inputs-three-outputs logical gates. Using plant roots as the element for unconventional computing, the Boolean functions of each root tropism were constructed through arithmetic-logical operations. One gravity gate (rGG) and two nutrient gates (rNG1 and rNG2) were fabricated using additive manufacturing. The rGG platform was oriented with roots directly pulled down by gravity which computes (x, y, z) = (xz + yz, x + y¯z+yz¯, xy + yz), whereas specific output channels in rNG1 and rNG2 were fertigated with high phosphorus concentration resulting in (x, y, z) = (x + y + z, xy + xz, xyz) for rNG1 and (x, y, z) = (xyz, xy¯z+xyz¯, x + y + z) for rNG2. For rGG, rNG1, and rNG2, the symbols x, y, and z pertain to "root presence" in the related channel, whereas top bar on the symbols indicates "root absence". Anatomical traits of roots were evaluated to assess possible differences in vascular tissues due to gravitropic and nutritropic responses. Overall, maize primary roots showed prominent positive gravitropism and nutritropism, and the roots that were most attracted by gravitational or nutritional stimuli showed an increase in the diameter of phloem and xylem. The logic exhibited by roots was dependent on the gravitropic and nutritropic stimuli to which they were exposed in the different logic gates. The responsiveness of maize roots to environmental stimuli such as gravity and nutrients provided valuable information to be used in computational bioelectronics.

High temperatures during microsporogenesis fatally shorten pollen lifespan

Many crop species are cultivated to produce seeds and/or fruits and therefore need reproductive success to occur. Previous studies proved that high temperature on mature pollen at anther dehiscence reduce viability and germinability therefore decreasing crop productivity. We hypothesized that high temperature might affect pollen functionality even if the heat treatment is exerted only during the microsporogenesis. Experimental data on Solanum lycopersicum 'Micro-Tom' confirmed our hypothesis. Microsporogenesis successfully occurred at both high (30 °C) and optimal (22 °C) temperature. After the anthesis, viability and germinability of the pollen developed at optimal temperature gradually decreased and the reduction was slightly higher when pollen was incubated at 30 °C. Conversely, temperature effect was eagerly enhanced in pollen developed at high temperature. In this case, a drastic reduction of viability and a drop-off to zero of germinability occurred not only when pollen was incubated at 30 °C but also at 22 °C. Further ontogenetic analyses disclosed that high temperature significantly speeded-up the microsporogenesis and the early microgametogenesis (from vacuolated stage to bi-cellular pollen); therefore, gametophytes result already senescent at flower anthesis. Our work contributes to unravel the effects of heat stress on pollen revealing that high temperature conditions during microsporogenesis prime a fatal shortening of the male gametophyte lifespan.

A novel device to study altered gravity and light interactions in seedling tropisms

Long-duration space missions will need to rely on the use of plants in bio-regenerative life support systems (BLSSs) because these systems can produce fresh food and oxygen, reduce carbon dioxide levels, recycle metabolic waste, and purify water. In this scenario, the need for new experiments on the effects of altered gravity conditions on plant biological processes is increasing, and significant efforts should be devoted to new ideas aimed at increasing the scientific output and lowering the experimental costs. Here, we report the design of an easy-to-produce and inexpensive device conceived to analyze the effect of interaction between gravity and light on root tropisms. Each unit consisted of a polystyrene multi-slot rack with light-emitting diodes (LEDs), capable of holding Petri dishes and assembled with a particular filter-paper folding. The device was successfully used for the ROOTROPS (for root tropisms) experiment performed in the Large Diameter Centrifuge (LDC) and Random Positioning Machine (RPM) at ESA's European Space Research and Technology centre (ESTEC). During the experiments, four light treatments and six gravity conditions were factorially combined to study their effects on root orientation of Brassica oleracea seedlings. Light treatments (red, blue, and white) and a dark condition were tested under four hypergravity levels (20 g, 15 g, 10 g, 5 g), a 1 g control, and a simulated microgravity (RPM) condition. Results of validation tests showed that after 24 h, the assembled system remained unaltered, no slipping or displacement of seedlings occurred at any hypergravity treatment or on the RPM, and seedlings exhibited robust growth. Overall, the device was effective and reliable in achieving scientific goals, suggesting that it can be used for ground-based research on phototropism-gravitropism interactions. Moreover, the concepts developed can be further expanded for use in future spaceflight experiments with plants.

The World Smallest Plants (Wolffia Sp.) as Potential Species for Bioregenerative Life Support Systems in Space

To colonise other planets, self-sufficiency of space missions is mandatory. To date, the most promising technology to support long-duration missions is the bioregenerative life support system (BLSS), in which plants as autotrophs play a crucial role in recycling wastes and producing food and oxygen. We reviewed the scientific literature on duckweed (Lemnaceae) and reported available information on plant biological traits, nutritional features, biomass production, and space applications, especially of the genus Wolffia. Results confirmed that the smallest existing higher plants are the best candidate for space BLSS. We discussed needs for further research before criticalities to be addressed to finalise the adoption of Wolffia species for space missions.

Spectral effects of blue and red light on growth, anatomy, and physiology of lettuce

Characterizing spectral effects of blue and red light ratios on plants could help expand our understanding of factors that regulate growth and development, which is becoming increasingly important as narrowband light-emitting diodes become common for sole-source lighting. Herein we report growth, physiological, and anatomical responses of two lettuce cultivars grown indoors under various blue and red ratios including monochromatic treatments. When used in combination with red, increasing the proportion of blue light generally reduced growth but increased chloroplast abundance and single-leaf photosynthetic efficiency. However, when used as single wavebands, both blue and red light increased leaf area and epidermal cell area, but reduced root dry mass, SPAD index, stomatal density, and leaf thickness compared to dichromatic light. In addition, chloroplast abundance and single-leaf physiological responses were higher in plants grown under monochromatic blue compared to red light, but the opposite trend was measured for shoot biomass. Our results show that spectral effects on morpho-anatomical leaf responses can largely influence plant growth and single-leaf physiological responses. However, a significant blue light reduction in radiation capture ultimately limits growth and productivity of lettuce plants when dichromatic blue and red light is used.

Biology and crop production in Space environments: Challenges and opportunities

Long-term manned space-exploration missions and the permanence of human colonies on orbital stations or planetary habitats will require the regeneration of resources onboard or in-situ. Bioregenerative Life Support Systems (BLSSs) are artificial environments where different compartments, involving both living organisms and physical-chemical processes, are integrated to achieve a safe, self-regulating, and chemically balanced Earth-like environment to support human life. Higher plants are key elements of such systems and Space greenhouses represent the producers' compartment. Growing plants in Space requires the knowledge of their growth responses not only to all environmental factors acting on Earth, but also to specific Space constraints such as altered gravity, ionizing radiations and confined volume. Moreover, cultivation techniques need to be adjusted considering such limitations. The type and intensity of environmental factors to be taken into account depend on the mission scenarios. Here, we summarize constraints and opportunities of cultivating higher plants in Space to regenerate resources and produce fresh food onboard. Both biological and agro-technological issues are considered briefly going through experiments both ground-based on Earth and in Space.

ConservePlants: An integrated approach to conservation of threatened plants for the 21st Century

Even though plants represent an essential part of our lives offering exploitational, supporting and cultural services, we know very little about the biology of the rarest and most threatened plant species, and even less about their conservation status. Rapid changes in the environment and climate, today more pronounced than ever, affect their fitness and distribution causing rapid species declines, sometimes even before they had been discovered. Despite the high goals set by conservationists to protect native plants from further degradation and extinction, the initiatives for the conservation of threatened species in Europe are scattered and have not yielded the desired results. The main aim of this Action is to improve plant conservation in Europe through the establishment of a network of scientists and other stakeholders who deal with different aspects of plant conservation, from plant taxonomy, ecology, conservation genetics, conservation physiology and reproductive biology to protected area's managers, not forgetting social scientists, who are crucial when dealing with the general public.

Root Tropisms: New Insights Leading the Growth Direction of the Hidden Half

Tropisms are essential responses of plants, orienting growth according to a wide range of stimuli. Recently, considerable attention has been paid to root tropisms, not only to improve cultivation systems, such as those developed for plant-based life support systems for future space programs, but also to increase the efficiency of root apparatus in water and nutrient uptake in crops on Earth. To date, the Cholodny–Went theory of differential auxin distribution remains the principal tropistic mechanism, but recent findings suggest that it is not generally applicable to all root tropisms, and new molecular pathways are under discussion. Therefore, an in-depth understanding of the mechanisms and functions underlying root tropisms is needed. Contributions to this special issue aimed to embrace reviews and research articles that deepen molecular, physiological, and anatomical processes orchestrating root tropisms from perception of the stimulus to bending. The new insights will help in elucidating plant–environment interactions, providing potential applications to improve plant growth on Earth and in space where microgravity diminishes or nullifies the gravitropism dominance.

Subsequent inclusion/exclusion criteria to select the best species for an experiment performed on the ISS in a refurbished hardware

The interest in re-using flown hardware for new and different space biology experiments is increasing. To match the constraints of the flown hardware with the requirements of the new biological system, innovative methodological approaches are necessary. MULTITROP was a successful plant biology experiment that was performed on the ISS to investigate multiple-tropism interactions during the early stage of seedling growth. We used the hardware designed and flown for the IFOAM experiment in 2009. The main challenge was to implement seeds of a crop species in a growth chamber conceived for yeast culture and to grow the seedlings in microgravity condition but activating seed germination on ground before the launch. Our approach was to adapt the biological system to the hardware constraints and also to the experiment timing and the environmental factors expected during the prelaunch, launch and flight operations. We looked for an objective and repeatable method to effectively select the best suited species. Innovatively, we applied the method of inclusion/exclusion criteria to adapt a new biological system to a reused hardware. The list and the consecutive order of the specific inclusive/exclusive criteria turned out to be a valid support to guide the science team in objectively choosing the most suitable species for the experiment. Among the 50 initial food species, the carrot seeds resulted as the best in satisfying all technical requirements and post-flight data confirmed the expectations.

Fiber-optic refractometer for in vivo sugar concentration measurements of low-nectar-producing flowers

Sugar concentration in floral nectars is an assessment required in several diverse fields of application. The widely used analysis, consisting of nectar extraction with a microcapillary and sugar concentration measurement with a light refractometer, is not reliable when the nectar is secreted in small quantities, unextractable with a microcapillary. Ancillary methods adopted in such cases are destructive, rather complicated and often provide much less precise and accurate results. The microscopic-size, low cost and biocompatibility of optical fibers were exploited to deliver light directly inside the flower with minimal invasiveness and measure instantaneously the refractometric properties of the nectar without extracting it. After comparing the new and old methods using two known nectariferous species, the new approach was validated on Primula palinuri, whose nectar is unextractable with microcapillaries. The fiber-optic probe was able to measure the nectar refractive index in P. palinuri flowers making it possible to highlight a previously undetected significant trend of the sugar concentration throughout the long anthesis of the single flowers. Changes in nectar concentrations are similar in both longistylous and brevistylous flowers. The fiber-optic refractometer is an advancement of light refractometer analysis. Further customization of the laboratory set-up into portable equipment will boost applications.

Root Tropisms: Investigations on Earth and in Space to Unravel Plant Growth Direction

Root tropisms are important responses of plants, allowing them to adapt their growth direction. Research on plant tropisms is indispensable for future space programs that envisage plant-based life support systems for long-term missions and planet colonization. Root tropisms encompass responses toward or away from different environmental stimuli, with an underexplored level of mechanistic divergence. Research into signaling events that coordinate tropistic responses is complicated by the consistent coincidence of various environmental stimuli, often interacting via shared signaling mechanisms. On Earth the major determinant of root growth direction is the gravitational vector, acting through gravitropism and overruling most other tropistic responses to environmental stimuli. Critical advancements in the understanding of root tropisms have been achieved nullifying the gravitropic dominance with experiments performed in the microgravity environment. In this review, we summarize current knowledge on root tropisms to different environmental stimuli. We highlight that the term tropism must be used with care, because it can be easily confused with a change in root growth direction due to asymmetrical damage to the root, as can occur in apparent chemotropism, electrotropism, and magnetotropism. Clearly, the use of Arabidopsis thaliana as a model for tropism research contributed much to our understanding of the underlying regulatory processes and signaling events. However, pronounced differences in tropisms exist among species, and we argue that these should be further investigated to get a more comprehensive view of the signaling pathways and sensors. Finally, we point out that the Cholodny-Went theory of asymmetric auxin distribution remains to be the central and unifying tropistic mechanism after 100 years. Nevertheless, it becomes increasingly clear that the theory is not applicable to all root tropistic responses, and we propose further research to unravel commonalities and differences in the molecular and physiological processes orchestrating root tropisms.

Chemotropic vs Hydrotropic Stimuli for Root Growth Orientation in Microgravity

Understanding how plants respond to spaceflight and extraterrestrial environments is crucial to develop life-support systems intended for long-term human explorations. Gravity is a main factor influencing root development and orientation, typically masking other tropisms. Considering that reduced levels of gravity affect many plant responses in space, the interaction of other tropic stimuli in microgravity represents the frontier to be investigated aiming at life-support systems optimization. In this paper we report on MULTITROP (Multiple-Tropism: interaction of gravity, nutrient and water stimuli for root orientation in microgravity), an experiment performed on the International Space Station during the Expedition 52/53. Scientific aim of the experiment was to disentangle hydrotropism from chemotropism for root orientation in absence of the gravity stimulus. Among several species relevant to space farming, Daucus carota was selected for the experiment because of its suitability with the experimental hardware and setup. At launch site, carrot seeds were placed between two disks of inert substrate (one imbibed with water and the other with a disodium phosphate solution) and integrated into a hardware developed, refurbished and flight-certificated by Kayser Italia. Post-flight, a Ground Reference Experiment was performed. Root development and orientation of seedlings grown in microgravity and at 1g condition were measured through 3D-image analysis procedures after imaging with X-ray microtomography. Radicle protruded preferentially from the ventral side of the seed due to the asymmetric position of the embryo. Such a phenomenon did not prevent the achievement of MULTITROP scientific goal but should be considered for further experiments on radicle growth orientation in microgravity. The experiment conducted in space verified that the primary root of carrot shows a positive chemotropism towards disodium phosphate solution in the absence of the gravity stimulus. On Earth, the positive chemotropism was masked by the dominant effect of gravity and roots developed downward regardless of the presence/absence of nutrients in the substrate. Taking advantage of altered gravity conditions and using other chemical compounds, further studies should be performed to deepen our understanding of root chemotropic response and its interaction with other tropisms.

Xylogenesis reveals the genesis and ecological signal of IADFs in Pinus pinea L. and Arbutus unedo L

Background and Aims

Mediterranean trees have patterns of cambial activity with one or more pauses per year, leading to intra-annual density fluctuations (IADFs) in tree rings. We analysed xylogenesis (January 2015-January 2016) in Pinus pinea L. and Arbutus unedo L., co-occurring at a site on Mt. Vesuvius (southern Italy), to identify the cambial productivity and timing of IADF formation.

Methods

Dendrochronological methods and quantitative wood anatomy were applied and enabled IADF identification and classification.

Key Results

We showed that cambium in P. pinea was productive throughout the calendar year. From January to March 2015, post-cambial (enlarging) earlywood-like tracheids were observed, which were similar to transition tracheids. The beginning of the tree ring was therefore not marked by a sharp boundary between latewood of the previous year and the new xylem produced. True earlywood tracheids were formed in April. L-IADFs were formed in autumn, with earlywood-like cells in latewood. In A. unedo, a double pause in cell production was observed, in summer and winter, leading to L-IADFs in autumn as well. Moreover, the formation of more than one IADF was observed in A. unedo.

Conclusions

Despite having completely different wood formation models and different life strategies, the production of earlywood, latewood and IADF cells was strongly controlled by climatic factors in the two species. Such cambial production patterns need to be taken into account in dendroecological studies to interpret climatic signals in wood from Mediterranean trees.

Changes in Leaf Anatomical Traits Enhanced Photosynthetic Activity of Soybean Grown in Hydroponics with Plant Growth-Promoting Microorganisms

The use of hydroponic systems for cultivation in controlled climatic conditions and the selection of suitable genotypes for the specific environment help improving crop growth and yield. We hypothesized that plant performance in hydroponics could be further maximized by exploiting the action of plant growth-promoting organisms (PGPMs). However, the effects of PGPMs on plant physiology have been scarcely investigated in hydroponics. Within a series of experiments aimed to identify the best protocol for hydroponic cultivation of soybean [Glycine max (L.) Merr.], we evaluated the effects of a PGPMs mix, containing bacteria, yeasts, mycorrhiza and trichoderma beneficial species on leaf anatomy, photosynthetic activity and plant growth of soybean cv. 'Pr91m10' in closed nutrient film technique (NFT). Plants were grown in a growth chamber under semi-aseptic conditions and inoculated at seed, seedling and plant stages, and compared to non-inoculated (control) plants. Light and epi-fluorescence microscopy analyses showed that leaves of inoculated plants had higher density of smaller stomata (297 vs. 247 n/mm2), thicker palisade parenchyma (95.0 vs. 85.8 μm), and larger intercellular spaces in the mesophyll (57.5% vs. 52.2%), compared to non-inoculated plants. The modifications in leaf functional anatomical traits affected gas exchanges; in fact starting from the reproductive phase, the rate of leaf net photosynthesis (NP) was higher in inoculated compared to control plants (8.69 vs. 6.13 μmol CO2 m-2 s-1 at the beginning of flowering). These data are consistent with the better maximal PSII photochemical efficiency observed in inoculated plants (0.807 vs. 0.784 in control); conversely no difference in leaf chlorophyll content was found. The PGPM-induced changes in leaf structure and photosynthesis lead to an improvement of plant growth (+29.9% in plant leaf area) and seed yield (+36.9%) compared to control. Our results confirm that PGPMs may confer benefits in photosynthetic traits of soybean plants even in hydroponics (i.e., NFT), with positive effects on growth and seed production, prefiguring potential application of beneficial microorganisms in plant cultivation in hydroponics.

Timing of False Ring Formation in Pinus halepensis and Arbutus unedo in Southern Italy: Outlook from an Analysis of Xylogenesis and Tree-Ring Chronologies

Mediterranean tree rings are characterized by intra-annual density fluctuations (IADFs) due to partly climate-driven cambial activity. IADFs are used as structural signals to gain information on relations between environmental conditions and eco-physiological processes during xylogenesis, with intra-annual resolution. To reach an unbiased synchronization of the IADF position within tree rings and seasonal fluctuations in environmental conditions, it is necessary to know the timing of cambial activity and wood formation, which are species- and site-specific processes. We applied the microcoring technique to analyze xylogenesis in Pinus halepensis and Arbutus unedo. To the best of our knowledge, this is the first attempt to study xylogenesis in a hardwood species forming frequent IADFs. Both species co-occur at a site in southern Italy characterized by a Mediterranean climate. To facilitate tree-ring dating and identification of IADFs, we performed traditional dendroecological analysis. We analyzed xylogenesis during summer, which is considered a constraint for xylogenesis and a trigger for IADF formation. We followed the different phases of cell development in the current wood increment with the aim of evaluating whether and which type of IADFs were formed. We additionally analyzed the same phases again in September and in winter to verify the possible formation of IADFs in fall and whether cell production and differentiation was completed by the end of the calendar year. Both species formed the same type of IADFs (earlywood-like cells within latewood), due to temporary growth restoration triggered by rain events during the period of summer drought. At the end of the calendar year, no cells in the phases of enlargement and secondary cell wall deposition occurred. A. unedo was more sensitive than P. halepensis because IADFs were formed earlier in the season and were more frequent in the tree-ring series. The dendro-anatomical approach, combining analysis of tree-ring series and of xylogenesis, helped to detect the period of IADF formation in the two species. Results are discussed in functional terms, highlighting the environmental conditions triggering IADFs, and also in methodological terms, evaluating the applicability of xylogenesis analysis in Mediterranean woods, especially when the formation of IADFs is not uniform around the stem.

Reproducing under a warming climate: long winter flowering and extended flower longevity in the only Mediterranean and maritime Primula

Under the pressure of global warming, general expectations of species migration and evolution of adaptive traits should always be confirmed with species-specific studies. Within this framework, some species can be used as study systems to predict possible consequences of global warming also on other relatives. Unlike its mountain congeneric, Primula palinuri Petagn. has endured all the climatic fluctuations since the Pleistocene, while surviving on Mediterranean coastal cliffs. The aim of this work was to investigate the possible evolution of reproductive biological and ecological traits in P. palinuri adaptation to a warmer environment. Data showed that flowering starts in mid-winter; single flowers remain open for over a month, changing from pendulous to erect. The number of insects visiting flowers of P. palinuri increases during the flowering season, and pollination reduces flower longevity. Overall, the best pollen performances, in terms of viability and germinability, occur at winter temperatures, while pollinator activity prolongs flowering until spring. Moreover, extended longevity of single flowers optimises reproductive success. Both phenotypic plasticity and selective processes might have occurred in P. palinuri. However, we found that reproductive traits of the only Mediterranean Primula remain more associated with cold mountain habitats than warm coastal cliffs. Given the rapid trend of climate warming, migration and new adaptive processes in P. palinuri are unlikely. Response to past climate warming of P. palinuri provides useful indications for future scenarios in other Primula species.

From flower to honey bouquet: possible markers for the botanical origin of Robinia honey

Flowers are complex structures devoted to pollinator attraction, through visual as well as chemical signals. As bees collect nectar on flowers to produce honey, some aspects of floral chemistry are transferred to honey, making chemical markers an important technique to identify the botanical and geographical origins of honey. We applied a new approach that considers the simultaneous analysis of different floral parts (petals, stamens + pistils, calyxes + nectarines, and nectar) and the corresponding unifloral honey. We collected fresh flowers of Robinia pseudoacacia L. (black locust), selected five samples of Robinia honey from different geographical origins, applied SPME-GC/MS for volatile analyses, and defined the chemical contribution added by different floral parts to the honey final bouquet. Our results show that honey blends products from nectar as well as other flower parts. Comparing honey and flower profiles, we detected compounds coming directly from flower parts but not present in the nectar, such as hotrienol and β-pinene. These may turn out to be of special interest when selecting floral markers for the botanical origin of honey.

Drought impact on water use efficiency and intra-annual density fluctuations in Erica arborea on Elba (Italy)

Erica arborea (L) is a widespread Mediterranean species, able to cope with water stress and colonize semiarid environments. The eco-physiological plasticity of this species was evaluated by studying plants growing at two sites with different soil moistures on the island of Elba (Italy), through dendrochronological, wood-anatomical analyses and stable isotopes measurements. Intra-annual density fluctuations (IADFs) were abundant in tree rings, and were identified as the key parameter to understand site-specific plant responses to water stress. Our findings showed that the formation of IADFs is mainly related to the high temperature, precipitation patterns and probably to soil water availability, which differs at the selected study sites. The recorded increase in the (13) C-derived intrinsic water use efficiency at the IADFs level was linked to reduced water loss rather than to increasing C assimilation. The variation in vessel size and the different absolute values of δ(18) O among trees growing at the two study sites underlined possible differences in stomatal control of water loss and possible differences in sources of water uptake. This approach not only helped monitor seasonal environmental differences through tree-ring width, but also added valuable information on E. arborea responses to drought and their ecological implications for Mediterranean vegetation dynamics.

Soilless cultivation of soybean for Bioregenerative Life-Support Systems: a literature review and the experience of the MELiSSA Project - Food characterisation Phase I

Higher plants play a key role in Bioregenerative Life-Support Systems (BLSS) for long-term missions in space, by regenerating air through photosynthetic CO2 absorption and O2 emission, recovering water through transpiration and recycling waste products through mineral nutrition. In addition, plants could provide fresh food to integrate into the crew diet and help to preserve astronauts' wellbeing. The ESA programme Micro-Ecological Life-Support System Alternative (MELiSSA) aims to conceive an artificial bioregenerative ecosystem for resources regeneration, based on both microorganisms and higher plants. Soybean [Glycine max (L.) Merr.] is one of the four candidate species studied for soilless (hydroponic) cultivation in MELiSSA, because of the high nutritional value of the seeds. Within the MELiSSA programme - Food characterisation Phase I, the aim of the research carried out on soybean at the University of Naples was to select the most suitable European cultivars for cultivation in BLSS. In this context, a concise review on the state-of-the-art of soybean cultivation in space-oriented experiments and a summary of research activity for the preliminary theoretical selection and subsequent agronomical evaluation of four cultivars will be presented in this paper.

Anatomical alterations of Phaseolus vulgaris L. mature leaves irradiated with X-rays

The cultivation of higher plants in Space involves not only the development of new agro-technologies for the design of ecologically closed Space greenhouses, but also understanding of the effects of Space factors on biological systems. Among Space factors, ionising radiation is one of the main constraints to the growth of organisms. In this paper, we analyse the effect of low-LET radiation on leaf histology and cytology in Phaseolus vulgaris L. plants subjected to increasing doses of X-rays (0.3, 10, 50, 100 Gy). Leaves irradiated at tissue maturity were compared with not-irradiated controls. Semi-thin sections of leaves were analysed through light and epi-fluorescence microscopy. Digital image analysis was applied to quantify anatomical parameters, with a specific focus on the occurrence of signs of structural damage as well as alterations at subcellular level, such as the accumulation of phenolic compounds and chloroplast size. Results showed that even at high levels of radiation, general anatomical structure was not severely perturbed. Slight changes in mesophyll density and cell enlargement were detected at the highest level of radiation. However, at 100 Gy, higher levels of phenolic compounds accumulated along chloroplast membranes: this accompanied an increase in number of chloroplasts. The reduced content of chlorophylls at high levels of radiation was associated with reduced size of the chloroplasts. All data are discussed in terms of the possible role of cellular modifications in the maintenance of high radioresistance and photosynthetic efficiency.

Microgravity effects on different stages of higher plant life cycle and completion of the seed-to-seed cycle

Human inhabitation of Space requires the efficient realisation of crop cultivation in bioregenerative life-support systems (BLSS). It is well known that plants can grow under Space conditions; however, perturbations of many biological phenomena have been highlighted due to the effect of altered gravity and its possible interactions with other factors. The mechanisms priming plant responses to Space factors, as well as the consequences of such alterations on crop productivity, have not been completely elucidated. These perturbations can occur at different stages of plant life and are potentially responsible for failure of the completion of the seed-to-seed cycle. After brief consideration of the main constraints found in the most recent experiments aiming to produce seeds in Space, we focus on two developmental phases in which the plant life cycle can be interrupted more easily than in others also on Earth. The first regards seedling development and establishment; we discuss reasons for slow development at the seedling stage that often occurs under microgravity conditions and can reduce successful establishment. The second stage comprises gametogenesis and pollination; we focus on male gamete formation, also identifying potential constraints to subsequent fertilisation. We finally highlight how similar alterations at cytological level can not only be common to different processes occurring at different life stages, but can be primed by different stress factors; such alterations can be interpreted within the model of 'stress-induced morphogenic response' (SIMR). We conclude by suggesting that a systematic analysis of all growth and reproductive phases during the plant life cycle is needed to optimise resource use in plant-based BLSS.

Stochastic changes affect Solanum wild species following autopolyploidization

Polyploidy is very common within angiosperms, and several studies are in progress to ascertain the effects of early polyploidization at the molecular, physiological, and phenotypic level. Extensive studies are available only in synthetic allopolyploids. By contrast, less is known about the consequences of autopolyploidization. The current study aimed to assess the occurrence and extent of genetic, epigenetic, and anatomical changes occurring after oryzaline-induced polyploidization of Solanum commersonii Dunal and Solanum bulbocastanum Dunal, two diploid (2n=2×=24) potato species widely used in breeding programmes. Microsatellite analysis showed no polymorphisms between synthetic tetraploids and diploid progenitors. By contrast, analysis of DNA methylation levels indicated that subtle alterations at CG and CHG sites were present in tetraploids of both species. However, no change occurred concurrently in all tetraploids analysed with respect to their diploid parent, revealing a stochastic trend in the changes observed. The morpho-anatomical consequences of polyploidization were studied in leaf main veins and stomata. With only a few exceptions, analyses showed no clear superiority of tetraploids in terms of leaf thickness and area, vessel number, lumen size and vessel wall thickness, stomata pore length and width, guard cell width, and stomatal density compared with their diploid progenitors. These results are consistent with the hypothesis that there are no traits systematically associated with autopolyploidy.

Morphofunctional traits and pollination mechanisms of Coronilla emerus L. flowers (Fabaceae)

It is accepted that the papilionaceous corolla of the Fabaceae evolved under the selective pressure of bee pollinators. Morphology and function of different parts of Coronilla emerus L. flowers were related to their role in the pollination mechanism. The corolla has a vexillum with red nectar lines, a keel hiding stamens and pistil, and two wing petals fasten to the keel with two notched folds. Pollinators land on the complex of keel and wings, trigger the protrusion of pollen and finally of the stigma from the keel tip. Data on pollen viability and stigma receptivity prove that flowers are proterandrous. The results of hand-pollination experiments confirmed that insects are fundamental to set seed. Interaction with pollinators allows not only the transport of pollen but also the rupture of the stigmatic cuticle, necessary to achieve both allogamy and autogamy. Field observations showed that Hymenoptera, Lepidoptera, and Diptera visited the flowers. Only some of the Hymenoptera landed on the flowers from the front and elicited pollination mechanisms. Most of the insects sucked the nectar from the back without any pollen transfer. Finally, morphological and functional characteristics of C. emerus flowers are discussed in terms of floral larceny and reduction in pollination efficiency.

Occurrence of morphological and anatomical adaptive traits in young and adult plants of the rare Mediterranean cliff species Primula palinuri Petagna

Cliffs worldwide are known to be reservoirs of relict biodiversity. Despite the presence of harsh abiotic conditions, large endemic floras live in such environments. Primula palinuri Petagna is a rare endemic plant species, surviving on cliff sites along a few kilometres of the Tyrrhenian coast in southern Italy. This species is declared at risk of extinction due to human impact on the coastal areas in question. Population surveys have shown that most of the plants are old individuals, while seedlings and plants at early stages of development are rare. We followed the growth of P. palinuri plants from seed germination to the adult phase and analysed the morphoanatomical traits of plants at all stages of development. Our results showed that the pressure of cliff environmental factors has been selected for seasonal habitus and structural adaptive traits in this species. The main morphoanatomical modifications are suberized cell layers and accumulation of phenolic compounds in cell structures. These features are strictly related to regulation of water uptake and storage as well as defence from predation. However, we found them well established only in adult plants and not in juvenile individuals. These findings contribute to explain the rare recruitment of the present relict populations, identifying some of the biological traits which result in species vulnerability.

Variations of vessel diameter and δ13C in false rings of Arbutus unedo L. reflect different environmental conditions

Woody species in Mediterranean ecosystems form intra-annual density fluctuations (IADFs) in tree rings in response to changes in environmental conditions, especially water availability. Dendrochronology, quantitative wood anatomy and high-resolution isotopic analysis (using a laser ablation technique) were used to characterize IADFs in Arbutus unedo shrubs grown on two sites with different water availability on the island of Elba (Italy). Our findings show that IADF characterization can provide information about the relationship between environmental factors and tree growth at the seasonal level. At the more xeric site, IADFs mainly located in the early and middle parts of the annual ring, showed a decrease in vessel size and an increase in δ(13) C as a result of drought deficit. Opposite trends were found at the more mesic site, with IADFs located at the end of the ring and associated with a lower δ(13) C. Moreover, at the first site, IADFs are induced by drought deficit, while at the second site IADFs are linked with the regrowth in the last part of the growing season triggered by favourable wet conditions. This combined approach is a promising way for dating problematic wood samples and interpreting the phenomena that trigger the formation of IADFs in the Mediterranean environment.

Building and degradation of secondary cell walls: are there common patterns of lamellar assembly of cellulose microfibrils and cell wall delamination?

During cell wall formation and degradation, it is possible to detect cellulose microfibrils assembled into thicker and thinner lamellar structures, respectively, following inverse parallel patterns. The aim of this study was to analyse such patterns of microfibril aggregation and cell wall delamination. The thickness of microfibrils and lamellae was measured on digital images of both growing and degrading cell walls viewed by means of transmission electron microscopy. To objectively detect, measure and classify microfibrils and lamellae into thickness classes, a method based on the application of computerized image analysis combined with graphical and statistical methods was developed. The method allowed common classes of microfibrils and lamellae in cell walls to be identified from different origins. During both the formation and degradation of cell walls, a preferential formation of structures with specific thickness was evidenced. The results obtained with the developed method allowed objective analysis of patterns of microfibril aggregation and evidenced a trend of doubling/halving lamellar structures, during cell wall formation/degradation in materials from different origin and which have undergone different treatments.

Application of Support Vector Machines to Melissopalynological Data for Honey Classification

In this paper, the authors address the problem of the discrimination of geographical origin and the selection of marker species of honeys using Support Vector Machines and z-scores. The methodology is based on the elaboration of palynological data with statistical learning methodologies. This innovative solution provides a simple yet powerful tool to detect the origin of honey samples. In case of honeys from Sorrento Peninsula, the discrimination from other Italian honeys is obtained with high accuracy.

Xylem development and cell wall changes of soybean seedlings grown in space

BACKGROUND AND AIMS

Plants growing in altered gravity conditions encounter changes in vascular development and cell wall deposition. The aim of this study was to investigate xylem anatomy and arrangement of cellulose microfibrils in vessel walls of different organs of soybean seedlings grown in Space.

METHODS

Seeds germinated and seedlings grew for 5 d in Space during the Foton-M2 mission. The environmental conditions, other than gravity, of the ground control repeated those experienced in orbit. The seedlings developed in space were compared with those of the control test on the basis of numerous anatomical and ultrastructural parameters such as number of veins, size and shape of vessel lumens, thickness of cell walls and deposition of cellulose microfibrils.

KEY RESULTS

Observations made with light, fluorescence and transmission electron microscopy, together with the quantification of the structural features through digital image analysis, showed that the alterations due to microgravity do not occur at the same level in the various organs of soybean seedlings. The modifications induced by microgravity or by the indirect effect of space-flight conditions, became conspicuous only in developing vessels at the ultrastructural level. The results suggested that the orientation of microfibrils and their assembly in developing vessels are perturbed by microgravity at the beginning of wall deposition, while they are still able to orient and arrange in thicker and ordered structures at later stages of secondary wall deposition.

CONCLUSIONS

The process of proper cell-wall building, although not prevented, is perturbed in Space at the early stage of development. This would explain the almost unaltered anatomy of mature structures, accompanied by a slower growth observed in seedlings grown in Space than on Earth.

Combined histochemistry and autofluorescence for identifying lignin distribution in cell walls

Histological staining methods commonly used for detecting cellulose and lignin in cell walls were combined with epifluorescence microscopy to visualize differences in lignification between and within cellular elements. We tested our approach on sections of one-year-old branches of Fraxinus ornus L., Myrtus communis L., Olea europaea L., Pistacia lentiscus L. and Rhamnus alaternus L., containing both normal and tension wood. Sections were subjected to various staining techniques, viz. safranin O, safranin O/fast green FCF, and alcoholic solutions of safranin O/astra blue, according to the commonly accepted protocols. Stained and unstained sections were compared using both light and epifluorescence microscopy. Safranin O with or without counterstaining hid the strong fluorescence of vessel walls, cell corners and middle lamellae allowing the secondary wall fibers to fluoresce more clearly. Epifluorescence microscopy applied to stained sections showed more cell wall details than autofluorescence of unstained sections or white light microscopy of counterstained sections. This simple approach proved reliable and valuable for detecting differences in lignification in thick sections without the need for costly equipment.

Effects of simulated microgravity on male gametophyte of Prunus, Pyrus, and Brassica species

In this study we evaluated the effect of simulated microgravity on pollen germination of both herbaceous and woody species in order to investigate the possibility of applying gametophytic selection for plant growth in the space environment. The behaviour of gametophytes exposed to the stress of clinostat rotation could be used to screen the degree of tolerance of the sporophyte to simulated microgravity. The use of male gametophyte selection overcomes the problems generally encountered by sporophytic selection in space especially for woody plants: the large size of plants and their long juvenile phase. In this experiment, pollen collected from just bloomed flowers of Prunus persica, P. avium, P. domestica, Pyrus communis, and Brassica rapa was subjected to tests assessing its viability by techniques such as fluorochromatic reaction. Once pollen viability was ascertained by fluorescence microscopy, pollen was placed on the growth medium in petri dishes both at 1 g and on the clinostat. After incubating for 1 day at room temperature, pollen was observed under a light microscope in order to detect parameters such as the percentage of germination and the growth direction. Then histochemical analyses were performed in order to verify the presence and distribution of nuclei, cytoplasm, and storage substances. Moreover, the presence, size, and morphology of callose plugs were observed. Results showed that the response of gametophytes to simulated microgravity is dependent on the species, some showing altered metabolism, others being unaffected.

Effects of relative humidity and temperature conditions on pollen fluorochromatic reaction of Rosmarinus officinalis L. (Lamiaceae)

Mediterranean ecosystems are characterized by seasonal and annual fluctuations in humidity and temperature which are considered limiting factors for plant growth and might have played a key role in the selection of species that compose the present vegetation. After anther release, pollen is generally exposed to various changes of temperature and humidity conditions, therefore its viability and consequently successful fruit set are strongly affected by these environmental parameters. The aim of this research was to study the effect of different combinations of humidity and temperature on pollen membrane integrity of Rosmarinus officinalis L. in order to investigate possible relations between pollen features and climatic conditions during flowering. This species is an evergreen perennial shrub, occurring among the sclerophyllous vegetation of Mediterranean maquis. In many areas of Southern Italy, it shows a flowering period starting from the beginning of winter and spreading throughout spring months. The analysis of data showed that duration of pollen viability of R. officinalis is enhanced by the co-occurrence of low temperature and high humidity. Moreover, these conditions are able to newly raise the viability of pollen kept at higher temperatures and lower humidity. These observations indicate that reactivation of the pollen membrane depends on low temperature apart from high humidity. Therefore "vernalization" of rosemary pollen enhances its viability, supporting that pollen behavior is adapted to winter condition and allows flowering in winter and early spring.

The effect of simulated microgravity on seed germination and seedling anatomy of Phaseolus vulgaris L

Seed germination and root anatomy were investigated in seedlings of Phaseolus vulgaris L. developed on a slowly rotating bi-dimensional clinostat and in 1g. Germination time, percent germination, curvature and anatomy of developing root apexes were monitored on the clinostat and compared with the control. Interesting differences were found in germination and root features of the seeds developed on the clinostat compared with 1g ones: the main being germination time, root cap formation, the quantity and distribution of amyloplasts in statocytes. The use of a software to quantitatively analyse root cap anatomy allowed us to detect some differences otherwise unlikely to highlight. Our results showed that prolonged rotation on a bi-dimensional clinostat has an effect on some aspects of germination and on the statocytes that continuously perceives gravity from ever-changing directions.

Classification and counting of fluorescent pollen using an image analysis system

Pollen viability is commonly assessed by fluorochromatic reaction (FCR) because of the high correlation between positive fluorescence of the pollen grains and their ability to germinate. One of the advantages of this method is its simplicity. An experiment to test FCR analysis for reproducibility, however, showed that results are affected by subjectivity. There is little consistency between analysts, and assessment by the same analyst may differ for the same pollen sample image examined at different times. These problems were solved by a computerized image analysis system that provides a method for classifying positive and negative fluorescent pollen and automatic counting of the grains in each class. The computerized image analysis system does not change the biochemistry of the FCR test, but avoids some experimental errors owing to the subjectivity of the analyst. Microscope images of the pollen after FCR were digitized and later analyzed by specially designed software, "Plant Meter." This software deletes the dark background of the image to isolate the grains, and subsequently counts positive and negative fluorescent pollen grains. An experiment was carried out to validate software output and it showed reliable results. Moreover, the software is user friendly and very little training is necessary for analysts to achieve reliable results.

First evidence of myrmecochory in fleshy-fruited shrubs of the Mediterranean region

In the mediterranean ecosystems of South Africa and Australia, the role of ants in the dispersal of seeds is well known and antagonistic interactions as well as myrmecochory occur. The high prevalence of seed dispersal by ants, relative so that by vertebrates, is viewed as evidence of adaptation to the nutrient poor soils typical of these environments. Ant dispersal has been considered unimportant in the Mediterranean region and, until now, no systematic study of ant-seed interaction has been undertaken. In the Mediterranean shrubland of Southern Italy the seed content of functioning nests of the ant Messor minor (André) has been quantitatively analyzed. The existence in the nest of seeds from shrub species, as well as those of the Fabaceae and Poaceae, suggest that ant dispersal may be much more widespread in the Mediterranean environments than has been realized. A large proportion of the seeds came from fleshy-fruited shrub species previously reported to be bird dispersed, e.g. Rhamnus alaternus L., Myrtus communis L., Smilax aspera L., Pistacia lentiscus L. and Phillyrea latifolia L. Analysis of nest mounds showed that, although many seeds bad been partially predated by the ants, a very large number had been collected, transported to the nest and subsequently removed to the mound without damage. Examination of the structure of the fruits and seeds of R. alaternus and M. communis, and summer fruiting phenology of R. alaternus, provided further evidence of specific adaptation to ant dispersal. The presence of an elaiosome on the seed is reported for the first time in both specific. Feeding trials showed that ants selectively remove specific diaspores, preferentially collecting the seeds with the elaiosome.