Question

1. There was major period of angiosperm diversification and radiation between 110 and 90
million years ago. For many years the dominant view was that this reflected coevolution
with insects, which were also thought to have diversified during this period. This view is
now thought to be overly simplistic. Briefly outline evidence for and against this view,
and give an account of the various other processes that may have been involved.

2. "Water in the xylem is under negative pressure, or tension". Explain what this
statement means, how this physical situation comes about, what the risks are for plants
because of it, and describe some adaptations that help minimize these risks or cope with
the consequences.

Answer 1

The majority of environments are dominated by flowering plants today, but it is uncertain how this dominance originated. This increase in angiosperm diversity happened during the Cretaceous period and led to replacement and often extinction of gymnosperms and ferns. We propose a scenario for the rise to dominance of the angiosperms from the Barremian to the Campanian based on the European megafossil plant record. These megafossil data demonstrate that angiosperms migrated into new environments in three phases:

(i) Barremian freshwater lake-related wetlands

(ii) Aptian–Albian (ca. 125–100 Ma) understory floodplains (excluding levees and back swamps); and (iii) Cenomanian–Campanian (ca. 100–84 Ma) natural levees, back swamps, and coastal swamps.

This scenario allows for the measured evolution of angiosperms in time and space synthesizing changes in the physical environment with concomitant changes in the biological environment.

Darwin’s “abominable mystery” concerning the sudden appearance of rather modern genera of flowering plants can now be understood. What most bothered Darwin was “the sudden appearance of so many extant taxa of flowering plants in the Upper Chalk” (1). At that time, he was relying on a paleobotanical record produced by people who approached the study of angiosperm fossils with the intent of relating them to extant taxa by using only gross leaf form as a basis for their systematic determinations. More recently, numerous Early Cretaceous angiosperm remains have been described as extinct angiosperm leaf and fruit morphotypes based upon critical observations and evaluations of many leaf and fruit characters. Our scenario supports the view by Darwin that “the presence of even one true angiosperm in the Lower Chalk makes [one] inclined to conjecture that plant[s] of this great division must have been largely developed in some isolated area, whence owing to geographical changes, they at last succeeded in escaping, and spread quickly over the world” (1, p 539). Thus, the rise to dominance of angiosperms was a process that lasted >45 million years. Dilcher pointed out that the modern nature of the fossil angiosperms identified in the Cretaceous were products of the use of limited characters combined with the goal of finding extant angiosperm genera. Probably there are no extant angiosperm genera that extend back to the Cretaceous when careful and detailed character analysis is used.

Angiosperms became abundant and diversified worldwide in coastal environments by the Albian as proposed in the “coastal hypothesis of angiosperm dispersal”. The so-called “sudden appearance of angiosperms” is really a series of ecological successions captured by exceptional depositional and preservational environments. At this time, there are also important rapid changes in the biotic environment as angiosperm and insect coevolution gains momentum. The biotic changes and habitat changes may seem to be sudden events, but in geological time these, events can be seen as occurring in distinct phases.

By the mid-Cretaceous time, some early ancestors of major taxa can be recognized. This diversification and local abundance in particular environments explains why Friis and colleagues found very diverse and abundant angiosperm mesofossil flowers, fruits, and seeds in Portuguese stratigraphic horizons now considered as Late Aptian–Early Albian in age (16), whereas older Aptian mesofossil floras (e.g., Torres Vedras, Catefica) were less diversified and mainly consisted of the early diverging clades [e.g., Austrobaileyales (Anacostia) and Chloranthaceae (Pennipollis) plant; The fossil angiosperms reported by Friis et al. are diverse, abundant, and well preserved so that some can be linked to extant major angiosperm lineages. Their occurrence in fluvial deposits demonstrates the occupation of understory floodplains Overall, the ecological diversification of angiosperms parallels the progressive systematic diversification of angiosperms evidenced from mesofossils.

Crepet et al. , Crepet and Nixon, Grimaldi, and Hu et al. have called attention to angiosperm coevolution with insects. This coevolution appears to have been a crucial step in the rise to dominance of angiosperms. For example, the increase in ecological range and richness of conifers from the beginning of the Albian suggests closed vegetation under humid climates that encouraged the rise of understory angiosperms. This situation implies that angiosperms did not simply overtop other plant groups in different environments, but they benefited from any opportunities offered by global vegetation and climate changes. Furthermore, angiosperms were not the only clade that experienced a diversification during the Cretaceous: Core Leptosporangiate ferns, Pinaceae, Gnetaceae, and Podocarpaceae also underwent extensive diversification. In addition, the heterosporous ferns originated and radiated at this same time. It is important to appreciate the extent of vegetation turnover during the Cretaceous. We need to expand our focus when dealing with angiosperm evolution to consider also an angiosperm evolution in time and space as they pioneered changing physical and biotic environments through Cretaceous time.

2)  Water in the xylem is under negative pressure, or tension

The motion of water from the soil, through a vascular plant, and into the air-occurs by a passive, wicking mechanism. This mechanism is described by the cohesion-tension theory: loss of water by evaporation reduces the pressure of the liquid water within the leaf relative to atmospheric pressure; this reduced pressure pulls liquid water out of the soil and up the xylem to maintain hydration. Strikingly, the absolute pressure of the water within the xylem is often negative, such that the liquid is under tension and is thermodynamically metastable with respect to the vapour phase. Qualitatively, this mechanism is the same as that which drives fluid through the synthetic wicks that are key elements in technologies for heat transfer, fuel cells, and portable chemical systems. Quantitatively, the differences in pressure generated in plants to drive flow can be more than a hundredfold larger than those generated in synthetic wicks. Here we present the design and operation of a microfluidic system formed in a synthetic hydrogel. This synthetic 'tree' captures the main attributes of transpiration in plants: transduction of sub saturation in the vapour phase of water into negative pressures in the liquid phase, stabilization and flow of liquid water at large negative pressures (-1.0 MPa or lower), continuous heat transfer with the evaporation of liquid water at negative pressure, and continuous extraction of liquid water from subsaturated sources. This development opens the opportunity for technological uses of water under tension and for new experimental studies of the liquid state of water.

RISK

In vascular plants, xylem sap is transported under negative pressure in lignified conduits such a transport is constrained by two major limitations:

(a) a risk of collapse of the water columns

(b) a risk of collapse of the conduit walls.

The first constraint originates from the metastable state of water under negative pressures. Under such conditions, vapor nucleation can occur, disrupting the water columns in the xylem and therefore the sap flow. This phenomenon is referred to the risk of cavitation.

The second physical limitation, wall collapse, is due to the centripetal forces exerted on conduit walls that may implode if wall mechanical reinforcement is deficient.

adaptations

Water deficit is considered the main limiting factor for the establishment, survival, and growth of plants mainly in water-limited ecosystems. Plants have evolved a wide range of morphologic and functional mechanisms to adapt to arid environments. However, if the tension in the xylem conduits becomes too high, thus xylem cavitation can occur i.e., water column breakage. This results in the hydraulic disconnection of leaves and above-ground parts from roots because xylem conduits are filled with air and water vapor, and this phenomenon is called embolism. Therefore, the resistance of the xylem to cavitation and embolism is of paramount importance for plant functioning.