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ScienceWeek
ECOLOGY: PLANT STOMATA AND ENVIRONMENTAL CHANGE
The following points are made by A.M. Hetherington and F.I. Woodward (Nature 2003 424:901):
1) Stomata are small pores on the surfaces of leaves and stems, bounded by a pair of guard cells, the pores controlling the exchange of gases -- most importantly water vapour and CO2 --between the interior of the leaf and the atmosphere. In this capacity the stomata make major contributions to the ability of the plant to control its water relations and to gain carbon.
2) Gas exchange is regulated by controlling the aperture of the stomatal pore and the number of stomata that form on the epidermis. Environmental signals such as light intensity, the concentration of atmospheric carbon dioxide, and endogenous plant hormones control stomatal aperture and development. The acquisition of stomata and an impervious leaf cuticle are considered to be key elements in the evolution of advanced terrestrial plants(1), allowing the plant to inhabit a range of different, often fluctuating environments but still control water content.
3) The application of knowledge from cognate disciplines is providing new insights into how stomata evolve and are able to process information from simultaneous, often conflicting and sometimes rapidly changing signals. Although it is too early to say whether these recent advances will result in paradigm shifts in our understanding of how plants both respond to and drive environmental change, it is quite clear that stomata are a key experimental tool to investigate these phenomena.
4) Although the total stomatal pore area may be only 5% of a leaf surface(2), the rate of water vapour loss may reach as high as 70% of a similar structure without a cuticle. Stomata exert major controls on both the water and carbon cycles of the world. Annual precipitation over the land is about 110,000 km^(3), or 110 x 10^(15) kg (3), and evaporation and transpiration total about 70 x 10^(15) kg. The contribution of stomatal transpiration alone to the global water cycle can be determined by using a dynamic vegetation model(4). The greatest rates of transpiration occur in the uniform and warm forested areas between the tropics with 32 x 10^(15) kg/yr of water vapour passing through stomata. This is double the water vapour content of the atmosphere (15 x 10^(15) kg/yr). Terrestrial gross photosynthesis annually fixes about 120 x 10^(15) gC (440 x 10^(15) gCO2) from the atmosphere's 730 x 10^(15) gC (5). The global distribution of this flux parallels the distribution of transpiration, indicating the closely coupled controls of stomata on CO2 and water vapour diffusion.
5) In summary: Stomata, the small pores on the surfaces of leaves and stalks, regulate the flow of gases in and out of leaves and thus plants as a whole. They adapt to local and global changes on all time-scales from minutes to millennia. Recent data from diverse fields are establishing their central importance to plant physiology, evolution, and global ecology. Stomatal morphology, distribution, and behavior respond to a spectrum of signals, from intracellular signaling to global climatic change. Such concerted adaptation results from a web of control systems reminiscent of a scale-free network whose untangling requires integrated approaches beyond those currently used.
References (abridged):
1. Raven, J. Selection pressures on stomatal evolution. New Phytol. 153, 371-386 (2002)
2. Willmer, C. & Fricker, M. Stomata 2nd edn (Chapman & Hall, London, 1996)
3. Jackson, R. B. et al. Water in a changing world. Ecol. Appl. 11, 1027-1045 (2001)
4. Cramer, W. et al. Global response of terrestrial ecosystem structure and function to CO2 and climate change: results from six dynamic global vegetation models. Global Change Biol. 7, 357-373 (2001)
5. Ciais, P. et al. A three-dimensional synthesis study of O18 in atmospheric CO2. I. Surface fluxes. J. Geophys. Res. Atmos. 102, 5857-5872 (1997)
Nature http://www.nature.com/nature
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EVOLUTIONARY BIOLOGY: ON THE ORIGIN OF PLANT LEAVES
The plant kingdom includes organisms that range in size from a tiny moss to a giant tree. All plants are multicellular and eukaryotic, each plant cell possessing a membrane-bound nucleus that contains the chromosomes. Most plants contain photoreactive pigments (e.g., chlorophylls a and b, and carotenoids), which play a central role in converting solar radiation into chemical energy (photosynthesis), the process involving reduction of carbon dioxide.
The evolutionary history of plants is recorded in fossils, with some fossils showing the external forms of plant parts, other fossils showing cellular features, and still other fossils consisting of microfossils of pollen and spores. The fossil record reveals a pattern of accelerating rates of evolution coupled with increasing diversity and complexity of biological communities. This diversity and complexity increased enormously with the invasion of land and the progressive colonization of the continents. At present, fossil evidence for the first land plants dates to the Ordovician time-frame (510 to 439 million years ago).
The term "vascular plants" (tracheophyta) refers to plants that have vascular tissues (xylem and phloem) through which water and nutrients are transported.
One of the most important organs of contemporary plants is the leaf, a thin and usually green expanded tissue borne on a node on the stem of the plant. Leaves typically have a stalk and blade (lamina), and they are the main site of solar radiation input and carbon dioxide absorption and photosynthesis. The fossil record indicates that for the first 40 million years of their invasion of land, land plants had no leaves, merely small spine-like appendages, and an important question in evolutionary biology is, How did leaves evolve?
The following points are made by Paul Kenrick ((Nature 2001 410:309):
1) The author points out that vascular plants have leaves of two main types. The "microphyll" is usually a short spine-like leaf and is characteristic of club mosses; the "megaphyll" is generally larger, with a substantial lamina and a complex pattern of veins like the leaves of ferns and flowering plants. Almost all living plants have megaphylls or their derivatives.
2) The 40-million-year gap between the earliest fossil evidence of vascular plants and the advent of megaphylls is puzzling. Megaphylls are ubiquitous today, and their photosynthetic proficiency makes their evolution seem inevitable. The structural framework necessary to assemble the primitive leaf was in place approximately 408.5 million years ago. The fossil record indicates that megaphylls evolved from the photosynthetic branching systems of early vascular plants that became flattened and later webbed to produce a broad lamina in several groups by 362.5 million years ago (the end of the Devonian).
3) Beerling et al propose a novel explanation for the long delay in the advent of leaves. They use a model based on biophysical principles applied to living plants and involving details of water use and heat and gas exchange. They also draw on anatomical and environmental data from the fossil record. They conclude that leaves with a broad lamina evolved in response to a massive drop in atmospheric carbon dioxide during the Devonian period (408.5 to 362.5 million years ago).
4) The author (Kenrick) asks: "If Beerling et al are right, and leaf evolution was driven by a large fall in atmospheric carbon dioxide, what then drove that fall in carbon dioxide? Remarkably, the answer appears to be plants." The scheme is as follows: The physical and chemical effects of root systems on rocks and soils increase rates of weathering, which is thought to have been responsible for removing enormous quantities of carbon dioxide from the Devonian atmosphere. Thus, roots may have played a key role in the evolution of leaves by way of the *carbon cycle.
5) The author (Kenrick) states: "The results of modeling systems that are so remote from the modern world have to be treated with skepticism. How can one be confident in a result for which so many parameters have to be estimated? This is a genuine concern, but Beerling and colleagues' calculations are based on independent biochemical, paleobotanical, and geochemical evidence, and seem reasonable."
Nature http://www.nature.com/nature
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Notes:
carbon cycle: In its most general outline, the term "carbon cycle", in geochemistry and Earth science, refers to the movement of carbon from an atmospheric inorganic state to a biospheric organic state and then back to an atmospheric inorganic state.
ScienceWeek http://scienceweek.com
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