Minimal temperature of pollen germination controls species distribution along a temperature gradient

Floral thermal biology in relation to pollen thermal performance in an early spring flowering plant

The floral microenvironment impacts gametophyte viability and plant-pollinator interactions. Plants employ mechanisms to modify floral temperature, including thermogenesis, absorption of solar radiation, and evaporative cooling. Whether floral thermoregulation impacts reproductive fitness, and how floral morphological variation mediates thermoregulatory capacity are poorly understood. We measured temperature of the floral microenvironment in the field and tested for thermogenesis in the lab in early spring flowering Hexastylis arifolia (Aristolochiaceae). We evaluated whether thermoregulatory capacity was associated with floral morphological variation. Finally, we experimentally determined the thermal optimum and tolerance of pollen to assess whether thermoregulation may ameliorate thermal stress to pollen. Pollen germination was optimal near 21 °C, with a 50% tolerance breadth of ~18 °C. In laboratory conditions, flowers exhibited thermogenesis of 1.5-4.8 °C for short intervals within a conserved timeframe (08:00-09:00 h). In the field, temperature inside the floral tube often deviated from ambient - floral interiors were up to 4 °C above ambient when it was cold, but some fell nearly 10 °C below ambient during peak heat. Flowers with smaller openings were cooler and more thermally stable than those with larger openings during peak heat. Thermoregulation maintained a floral microenvironment within the thermal tolerance breadth of pollen. Results suggest that H. arifolia flowers have a stronger capacity to cool than to warm, and that narrower floral openings create a distinct floral microenvironment, enhancing floral cooling effects. While deviation of floral temperature from ambient conditions maintains a suitable environment for pollen and suggests an adaptive role of thermoregulation, we discuss adaptive and nonadaptive mechanisms underlying floral warming and cooling.

Pollen thermotolerance of a widespread plant, Lotus corniculatus, in response to climate warming: possible local adaptation of populations from different elevations

One of the most vulnerable phases in the plant life cycle is sexual reproduction, which depends on effective pollen transfer, but also on the thermotolerance of pollen grains. Pollen thermotolerance is temperature-dependent and may be reduced by increasing temperature associated with global warming. A growing body of research has focused on the effect of increased temperature on pollen thermotolerance in crops to understand the possible impact of temperature extremes on yield. Yet, little is known about the effects of temperature on pollen thermotolerance of wild plant species. To fill this gap, we selected Lotus corniculatus s.l. (Fabaceae), a species common to many European habitats and conducted laboratory experiments to test its pollen thermotolerance in response to artificial increase in temperature. To test for possible local adaptation of pollen thermal tolerance, we compared data from six lowland (389-451 m a.s.l.) and six highland (841-1,030 m a.s.l.) populations. We observed pollen germination in vitro at 15 °C, 25 °C, 30 °C, and 40 °C. While lowland plants maintained a stable germination percentage across a broad temperature range (15-30 °C) and exhibited reduced germination only at extremely high temperatures (40 °C), highland plants experienced reduced germination even at 30 °C-temperatures commonly exceeded in lowlands during warm summers. This suggests that lowland populations of L. corniculatus may be locally adapted to higher temperature for pollen germination. On the other hand, pollen tube length decreased with increasing temperature in a similar way in lowland and highland plants. The overall average pollen germination percentage significantly differed between lowland and highland populations, with highland populations displaying higher germination percentage. On the other hand, the average pollen tube length was slightly smaller in highland populations. In conclusion, we found that pollen thermotolerance of L. corniculatus is reduced at high temperature and that the germination of pollen from plant populations growing at higher elevations is more sensitive to increased temperature, which suggests possible local adaptation of pollen thermotolerance.

Pollen viability, longevity, and function in angiosperms: key drivers and prospects for improvement

Pollen grains are central to sexual plant reproduction and their viability and longevity/storage are critical for plant physiology, ecology, plant breeding, and many plant product industries. Our goal is to present progress in assessing pollen viability/longevity along with recent advances in our understanding of the intrinsic and environmental factors that determine pollen performance: the capacity of the pollen grain to be stored, germinate, produce a pollen tube, and fertilize the ovule. We review current methods to measure pollen viability, with an eye toward advancing basic research and biotechnological applications. Importantly, we review recent advances in our understanding of how basic aspects of pollen/stigma development, pollen molecular composition, and intra- and intercellular signaling systems interact with the environment to determine pollen performance. Our goal is to point to key questions for future research, especially given that climate change will directly impact pollen viability/longevity. We find that the viability and longevity of pollen are highly sensitive to environmental conditions that affect complex interactions between maternal and paternal tissues and internal pollen physiological events. As pollen viability and longevity are critical factors for food security and adaptation to climate change, we highlight the need to develop further basic research for better understanding the complex molecular mechanisms that modulate pollen viability and applied research on developing new methods to maintain or improve pollen viability and longevity.

Flowers that self-shade reduce heat stress and pollen limitation

PREMISE

Plants are facing increased risk of heat stress with global climate change. Reproductive tissues are particularly heat-sensitive, which can result in lower plant fitness. Floral shading and closure are possible mechanisms to limit heat stress although most previous work on petal orientation has considered adaptations to raise temperatures. We hypothesized that floral shading could reduce temperature and increase reproductive success.

METHODS

We measured floral temperatures of four species that exhibited intraspecific variation in flower closure (Opuntia ficus-indica, Oenothera elata, Convolvulus arvensis, and Romneya coulteri). We also wired newly opened R. coulteri flowers so that they were either permanently open or permanently closed; controls were not wired.

RESULTS

Individual flowers of all four species that shaded their pistils were exposed to temperatures 3-8°C lower than those that remained open and unshaded. In our wiring experiment, unencumbered R. coulteri controls were 40% more likely to produce seeds than flowers that were either permanently open or closed. Without added pollen, control flowers produced 2× more seeds than flowers wired open and 8× more than those wired closed. However, pollen addition eliminated the effects of wiring and increased capsule mass and seed production. This effect of pollen addition suggests that pollen limitation was responsible for observed differences in the wiring treatments. Pollinators may prefer control flowers over those that were wired open or closed; petal shading may make flowers cooler and more attractive to pollinators.

CONCLUSIONS

Petal shading may be a behavior that allows flowers to reduce heat stress and increases their chances of pollination and seed set.

Microbial Turnover and Dispersal Events Occur in Synchrony with Plant Phenology in the Perennial Evergreen Tree Crop Citrus sinensis

Emerging research indicates that plant-associated microbes can alter plant developmental timing. However, it is unclear if host phenology affects microbial community assembly. Microbiome studies in annual or deciduous perennial plants face challenges in separating effects of tissue age from phenological driven effects on the microbiome. In contrast, evergreen perennial trees, like Citrus sinensis, retain leaves for years, allowing for uniform sampling of similarly aged leaves from the same developmental cohort. This aids in separating phenological effects on the microbiome from impacts due to annual leaf maturation/senescence. Here, we used this system to test the hypothesis that host phenology acts as a driver of microbiome composition. Citrus sinensis leaves and roots were sampled during seven phenological stages. Using amplicon-based sequencing, followed by diversity, phylogenetic, differential abundance, and network analyses, we examined changes in bacterial and fungal communities. Host phenological stage is the main determinant of microbiome composition, particularly within the foliar bacteriome. Microbial enrichment/depletion patterns suggest that microbial turnover and dispersal were driving these shifts. Moreover, a subset of community shifts were phylogenetically conserved across bacterial clades, suggesting that inherited traits contribute to microbe-microbe and/or plant-microbe interactions during specific phenophases. Plant phenology influences microbial community composition. These findings enhance understanding of microbiome assembly and identify microbes that potentially influence plant development and reproduction. IMPORTANCE Research at the forefront of plant microbiome studies indicates that plant-associated microbes can alter the timing of plant development (phenology). However, it is unclear if host phenological stage affects microbial community assembly. Microbiome studies in annual or deciduous perennial plants can face difficulty in separating effects of tissue age from phenological driven effects on the microbiome. Evergreen perennial plants, like sweet orange, maintain mature leaves for multiple years, allowing for uniform sampling of similarly aged tissue across host reproductive stages. Using this system, multiyear sampling, and high-throughput sequencing, we identified plant phenology as a major driver of microbiome composition, particularly within the leaf-associated bacterial communities. Distinct changes in microbial patterns suggest that microbial turnover and dispersal are mechanisms driving these community shifts. Additionally, closely related bacteria have similar abundance patterns across plant stages, indicating that inherited microbial traits may influence how bacteria respond to host developmental changes. Overall, this study illustrates that plant phenology does indeed govern microbiome seasonal shifts and identifies microbial candidates that may affect plant reproduction and development.

Extracellular ionic fluxes suggest the basis for cellular life at the 1/f ridge of extended criticality

The criticality hypothesis states that a system may be poised in a critical state at the boundary between different types of dynamics. Previous studies have suggested that criticality has been evolutionarily selected, and examples have been found in cortical cell cultures and in the human nervous system. However, no one has yet reported a single- or multi-cell ensemble that was investigated ex vivo and found to be in the critical state. Here, the precise 1/f noise was found for pollen tube cells of optimum growth and for the physiological ("healthy") state of blood cells. We show that the multi-scale processes that arise from the so-called critical phenomena can be a fundamental property of a living cell. Our results reveal that cell life is conducted at the border between order and disorder, and that the dynamics themselves drive a system towards a critical state. Moreover, a temperature-driven re-entrant state transition, manifest in the form of a Lorentz resonance, was found in the fluctuation amplitude of the extracellular ionic fluxes for the ensemble of elongating pollen tubes of Nicotiana tabacum L. or Hyacintus orientalis L. Since this system is fine-tuned for rapid expansion to reach the ovule at a critical temperature which results in fertilisation, the core nature of criticality (long-range coherence) offers an explanation for its potential in cell growth. We suggest that the autonomous organisation of expansive growth is accomplished by self-organised criticality, which is an orchestrated instability that occurs in an evolving cell.

The thermal ecology of flowers

BACKGROUND

Obtaining an optimal flower temperature can be crucial for plant reproduction because temperature mediates flower growth and development, pollen and ovule viability, and influences pollinator visitation. The thermal ecology of flowers is an exciting, yet understudied field of plant biology.

SCOPE

This review focuses on several attributes that modify exogenous heat absorption and retention in flowers. We discuss how flower shape, orientation, heliotropic movements, pubescence, coloration, opening-closing movements and endogenous heating contribute to the thermal balance of flowers. Whenever the data are available, we provide quantitative estimates of how these floral attributes contribute to heating of the flower, and ultimately plant fitness.

OUTLOOK

Future research should establish form-function relationships between floral phenotypes and temperature, determine the fitness effects of the floral microclimate, and identify broad ecological correlates with heat capture mechanisms.

An Unexplored Side of Regeneration Niche: Seed Quantity and Quality Are Determined by the Effect of Temperature on Pollen Performance

In 1977, Peter Grubb introduced the regeneration niche concept, which assumes that a plant species cannot persist if the environmental conditions are only suitable for adult plant growth and survival, but not for seed production, dispersal, germination, and seedling establishment. During the last decade, this concept has received considerable research attention as it helps to better understand community assembly, population dynamics, and plant responses to environmental changes. Yet, in its present form, it focuses too much on the post-fertilization stages of plant sexual reproduction, neglecting the fact that the environment can operate as a constraint at many points in the chain of processes necessary for successful regeneration. In this review, we draw the attention of the plant ecology research community to the pre-fertilization stages of plant sexual reproduction, an almost ignored but important aspect of the regeneration niche, and their potential consequences for successful seed production. Particularly, we focus on how temperature affects pollen performance and determines plant reproduction success by playing an important role in the temporal and spatial variations in seed quality and quantity. We also review the pollen adaptations to temperature stresses at different levels of plant organization and discuss the plasticity of the performance of pollen under changing temperature conditions. The reviewed literature demonstrates that pre-fertilization stages of seed production, particularly the extreme sensitivity of male gametophyte performance to temperature, are the key determinants of a species' regeneration niche. Thus, we suggest that previous views stating that the regeneration niche begins with the production of seeds should be modified to include the preceding stages. Lastly, we identify several gaps in pollen-related studies revealing a framework of opportunities for future research, particularly how these findings could be used in the field of plant biology and ecology.

Cold Tolerance of the Male Gametophyte during Germination and Tube Growth Depends on the Flowering Time

In temperate climates, most plants flower during the warmer season of the year to avoid negative effects of low temperatures on reproduction. Nevertheless, few species bloom in midwinter and early spring despite severe and frequent frosts at that time. This raises the question of adaption of sensible progamic processes such as pollen germination and pollen tube growth to low temperatures. The performance of the male gametophyte of 12 herbaceous lowland species flowering in different seasons was examined in vitro at different test temperatures using an easy to handle testing system. Additionally, the capacity to recover after the exposure to cold was checked. We found a clear relationship between cold tolerance of the activated male gametophyte and the flowering time. In most summer-flowering species, pollen germination stopped between 1 and 5 °C, whereas pollen of winter and early spring flowering species germinated even at temperatures below zero. Furthermore, germinating pollen was exceptionally frost tolerant in cold adapted plants, but suffered irreversible damage already from mild sub-zero temperatures in summer-flowering species. In conclusion, male gametophytes show a high adaptation potential to cold which might exceed that of female tissues. For an overall assessment of temperature limits for sexual reproduction it is therefore important to consider female functions as well.