How and when the earliest plants made the first move on to land is always a hot topic for palaeobotanists. We know that early land plants likely evolved from freshwater algae, gaining a bunch of necessary adaptations in the process. Plants needed to support themselves, protect themselves from drying out and from the harmful effects of UV light, and gain water and nutrients from a finite supply on land. A study published last week by Mariusz Salamon and colleagues described fossils that push back the earliest evidence of land plants to around 445 million years ago.
The new fossils come from mudstones in central Poland, in beds that have been dated using other, much more common and cosmopolitan, fossils. The plant remains are tiny, branched fragments, up to about 3mm long. Some specimens appear to have spore-cases at the top of their branches, similar to those seen in younger, better-known early land plants such as Cooksonia. The preservation of the plants means details are hard to discern, but Salamon and colleagues present a single, tantalising stoma, or air pore, on one of the fragments as a key piece of evidence. If this plant had stomata for gas exchange, it was likely to have been living in land, a good 15 million years earlier than previously known plant fragments.
During the Cretaceous and early Tertiary, when the climate was warm and the atmospheric CO2 concentration ([CO2]) was at least double that of the present-day, polar forests populated high latitude landmasses.
We investigated the density and length of stomata and other epidermal cells of two deciduous and three evergreen “living fossil” tree species representative of these ancient forests. These tree species were grown in a simulated Cretaceous high latitude environment at either ambient (400 ppmv) or elevated (800 ppmv) [CO2] during four years.
After 4 years growing at elevated [CO2], the leaf stomatal density and index (percentage of leaf epidermal cells that are stomata) of these plants were similar to those of their counterparts growing at ambient [CO2]. While the CO2 enrichment only modified the stomatal pore length in two of the five studied species, it increased significantly the overall length of the epidermal cells of all the species, reducing their density.
These results revealed that leaf epidermal cells of these “living fossil” species were more sensitive than stomata to an experimental doubling of atmospheric CO2 concentration.
Recently a new method to reconstruct paleoelevation has been developed, based on the adjustment in number of stomata on leaves to the predictable decrease in CO partial pressure with altitude (McElwain, 2004). The first time 2 application of this method focuses on determining the altitude of growth of early Miocene Quercus pseudolyrata leaves from the northern Sierra Nevada. Calibration of the fossil stomatal numbers to altitude is based on the stomatal response of the modern day California black oak (Quercus kelloggi), whose leaves are indistinguishable from Q. pseudolyrata and grows in the same region. The clear correlation between increasing stomatal density and index vs. increasing altitude of growth found in Quercus kelloggii can for a large part be attributed to the predictable decrease in CO partial pressure. Many plants, 2 among which several Quercus species, show this increase in stomatal frequency with decreasing CO and CO is the only environmental parameter that globally and predictably changes with elevation. 2 2 However, several other climatic variables can also change with elevation and potentially influence stomatal density. The most important two are light intensity and temperature. In a natural setting, the relative influence of all environmental parameters on the stomatal change over altitude transect is extremely difficult to pry apart. Therefore, saplings of Quercus kelloggi were grown under controlled conditions in growth chambers, varying the levels of light and temperature. We will present preliminary data on the effect of both light levels and temperature regimes on stomatal frequency and epidermal cell morphology of Quercus kelloggii leaves.
Vascular plants have evolved a suite of different strategies to optimize carbon uptake against water loss via developmental, physiological and cellular level control of stomatal function.
Alterations to the development of stomatal number and size in response to atmospheric CO2 concentration enable some species to regulate stomatal conductance on time-scales of weeks to decades.
Other species demonstrate physiological control of stomatal aperture allowing for rapid control of stomatal function in response to abiotic factors on time scales of seconds to minutes.
Competing hypotheses have been proposed for the evolution of stomatal function in land plants with suggestions on one side that stomatal function is highly conserved across the plant phylogeny and the alternative view that stomatal function has increased incrementally across vascular plants from monilophytes to spermatophytes.
Observations of stomatal response to both instantaneous and long-term exposure to elevated atmospheric CO2 in UCD PÉAC (Programme for Experimental Atmospheres and Climate) do not completely support either competing hypothesis. Rather they suggest that the evolution of stomatal function is more complex and does not display a strong phylogenetic signature.
We show that loss of one aspect of stomatal function in response to an external stimulus can be readily compensated by improved functionality in another.
Experiments also show that there may be a trade-off between physiological control of stomatal conductance via stomatal aperture opening/closing and morphological control of conductance through developmental alteration to stomatal density and pore size.
This “stomatal trade- off hypothesis” is supported by the observation of a significant negative correlation between the magnitude of response of conductance to instantaneous change in CO2 and the magnitude and sign of response of stomatal density to long term elevated CO2 exposure. In other words, species which show rapid instantaneous changes in stomatal conductance do not alter stomatal density inversely under elevated CO2 and vice versa.
This has obvious and important implications for future development and application of the stomatal-CO2 proxy method which uses the inverse relationship between SD and CO2 to reconstruct palaeo-CO2 concentration in the geological past.
This will be discussed in relation to new estimates of palaeo-CO2 spanning the Eocene-Oligocene boundary between 40 and 25 million years ago when the Earth transitioned between an ice-free (greenhouse) and glaciated (icehouse) state.
This year marks the 100th anniversary of the first in a series of papers on the biota of a 407-million-year-old hot spring system that opened a window onto early life on land (1). The site near the village of Rhynie in Aberdeenshire, Scotland, is exceptional because fossilization occurred in microcrystalline silica (chert), preserving organisms to the cellular level and shedding light on community structure and interactions among the plants, arthropods, fungi, algae, and cyanobacteria. Recent research on these remarkable fossils and advances in understanding plant developmental genetics are beginning to reveal how major changes in life cycle had an early influence on the direction of plant evolution.
When the Rhynie Chert fossils were first reported, the plants, in particular, caused quite a stir because they seemed to capture an early stage in the adaptation to life on land. Most lack key organs familiar in living species, such as leaves or roots. Moreover, they are small (<20 cm), with simple bifurcating axes that terminate in spore-bearing sacs. Later research has revealed that their life cycles also differed in important ways from those of living species.
Land plants inherited their biochemistry and cell biology from ancestral green algae, but their fundamental organs and tissues evolved on land. Their closest algal relatives have a haplontic life cycle, which typically features a simple multicellular haploid sexual phase (a gametophyte) and a unicellular diploid zygote (2). This implies that the ancestor of land plants was also haplontic, probably with a filamentous, weakly differentiated multicellular phase (3).
The transition to land entailed changes in life cycle in which the diploid zygote also became multicellular (4). This change was accompanied by substantial somatic development, resulting in the evolution of tissues and organs basic to plants, such as stems, leaves, roots, a vascular system, stomata (the minute pores that facilitate gaseous exchange), and sex and dispersal organs. The multicellular diploid phase evolved into a highly successful spore-producing dispersal unit (the sporophyte), laying the foundations of modern plant diversity. Changes in life cycle thus underpinned the early diversification of plants on land, but how such changes evolved remains a puzzle.
Life-cycle phases are known in varying degrees of detail for four of the six fossil plant species from the Rhynie Chert. Like ferns, the gametophytes and sporophytes lived as independent plants, but unlike any living land plants, tissues such as rooting structures, a vascular system, and stomata were expressed in both parts of the life cycle (6). Despite these similarities, sporophytes and gametophytes were not identical. Although both were axial and leafless, the gametophytes were smaller. Overall, habit and size are still poorly understood for several gametophytes, but one factor known to influence their size was the degree of development of their branching systems (1). Also, in several species, gametophyte axes terminated in an expanded cup-shaped structure that bore the sexual organs. These differences notwithstanding, the gametophyte bore much greater similarity to the sporophyte than it does in living species, and it developed tissues of greater diversity.
Stomatal data of Platanus neptuni leaves were used as CO2 proxy for the Oligocene.
CO2 calculation was based on a gas exchange model.
The data indicate quite a constant level of about 400 ppm throughout the Oligocene.
Platanus neptuni (Ettingshausen) Bůžek, Holý and Kvaček is a deciduous and preferentially azonal taxon of temperate to warm-temperate vegetation in Europe from the Late Eocene to the Late Miocene. The high fossilization potential of its leaves and easily identifiable stomata and epidermal cells make P. neptuni an excellent source of stomatal data that can be utilized as a CO2proxy. Moreover, it was found in former studies that CO2 data based on stomatal frequency data of P. neptuni overlapped to a high degree with CO2 results which are provided by other, contemporaneous taxa. In this study, the stomatal CO2 signal of P. neptuni is expanded to include the early Oligocene and is analyzed in more detail with three aims: 1) to evaluate the CO2 signal of P. neptuni stomatal data, 2) to check SI and SD data of P. neptuni for consistency, and 3) to contribute additional terrestrial CO2 data to the Oligocene record. During the Oligocene, full scale Antarctic glaciation occurred, punctuated by various distinct glaciation events. There is evidence that Oligocene glaciation was coupled to atmospheric CO2 level. Presently, the main proxy sources for Oligocene CO2 levels are alkenones and boron-isotope data, both obtained from marine sediments.
Since P. neptuni is an extinct taxon, CO2 was reconstructed by using an ecophysiological modeling approach to plant gas exchange which utilizes various other data in addition to stomatal density. Material was considered from sites which are dated to the following time intervals: early Oligocene — 33.9 to 32 Ma, late Oligocene — 27 to 26.2 Ma and 25.3 to 23 Ma, and latest Oligocene — around 24 Ma. Comparison of raw SI and SD data of P. neptuni revealed partially conflicting results, with the SD data indicating a decrease in CO2from the early to the late Oligocene whereas SI data indicate an increase. In contrast, CO2 results calculated with the gas exchange model indicate relatively stable CO2 for the considered time intervals, with levels of about 400 ppm. The reconstructed CO2 data points are similar to other proxy data and are consistent with the general climate development during the Oligocene.
Stomatal density of plants may vary depending on environmental factors, such as CO2concentration. Under the current atmospheric conditions, it is expected that leaves have different stomatal density than they had hundreds or thousands of years ago, due to the rise in CO2 in the atmosphere.
Coprolites of the extinct Myotragus balearicus from Cova Estreta (Pollença, Mallorca), with a radiocarbon age of 3775–3640 cal. bc, have been used to study the diet of this bovid. A significant amount of epidermal fragments of Buxus was found in the faecal material. Three coprolites were used to estimate the stomatal density and stomatal index of Buxus epidermal fragments from this period.
Samples of the endangered Buxus balearica, the sole species of Buxus currently present on Mallorca, and samples of the Buxus sempervirens, present in the nearest mainland, were also collected in different locations. Leaves were examined using microscopy to determine and compare the stomatal density and stomatal index between current plant material and coprolite material.
The results indicated a higher value for stomatal index (12.71) and stomatal density (297.61 stomata/mm2) in leaves from the coprolites versus leaves of the living B. balearica and B. sempervirens species (7.99 and 227.77 stomata/mm2 respectively).
These results could provide a palaeobotanic evidence of a change in atmospheric CO2 concentration since mid-Holocene in the Mediterranean basin.