Conifer species adapt to low-rainfall climates by following one of two divergent pathways

Significance A major determinant of plant species distribution on Earth is a specific tolerance to soil drying, yet there are currently no functional or anatomical characteristics that can predict species’ requirement for rainfall. This study examines the systems responsible for controlling water de...

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Published in:Proceedings of the National Academy of Sciences - PNAS 2014-10, Vol.111 (40), p.14489-14493
Main Authors: Brodribb, Timothy J., McAdam, Scott A. M., Jordan, Gregory J., Martins, Samuel C.V.
Format: Article
Language:English
Subjects:
Abscisic Acid - metabolism
Adaptation, Physiological - physiology
Araucariaceae
Biological Sciences
Climate
Conifers
Cupressaceae
Cupressaceae - classification
Cupressaceae - physiology
Dehydration
Drought
Droughts
Ecosystem
Evolution
Flowers & plants
Leaves
Models, Biological
Nonnative species
Pinaceae
Pinaceae - classification
Pinaceae - physiology
Plant Leaves - metabolism
Plant Leaves - physiology
Plant Stomata - physiology
Plants
Rain
Species
Species Specificity
Tracheophyta - classification
Tracheophyta - physiology
Transpiration
Water - metabolism
Water loss
Xylem
Xylem - physiology
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Summary:Significance A major determinant of plant species distribution on Earth is a specific tolerance to soil drying, yet there are currently no functional or anatomical characteristics that can predict species’ requirement for rainfall. This study examines the systems responsible for controlling water delivery and water loss in the leaves of conifers and finds functional evidence of how conifers have evolved in drying climates over the course of the last 150 million years. Two “strategies” for conserving water during water stress emerged. One group relied on the plant hormone abscisic acid to maintain stomata closed during sustained drought, and another, more derived group allowed leaves to dehydrate and resisted damage by producing a water transport system capable of functioning under the extreme tension that develops in water-stressed plants. Water stress is one of the primary selective forces in plant evolution. There are characters often cited as adaptations to water stress, but links between the function of these traits and adaptation to drying climates are tenuous. Here we combine distributional, climatic, and physiological evidence from 42 species of conifers to show that the evolution of drought resistance follows two distinct pathways, both involving the coordinated evolution of tissues regulating water supply (xylem) and water loss (stomatal pores) in leaves. Only species with very efficient stomatal closure, and hence low minimum rates of water loss, inhabit dry habitats, but species diverged in their apparent mechanism for maintaining closed stomata during drought. An ancestral mechanism found in Pinaceae and Araucariaceae species relies on high levels of the hormone abscisic acid (ABA) to close stomata during water stress. A second mechanism, found in the majority of Cupressaceae species, uses leaf desiccation rather than high ABA levels to close stomata during sustained water stress. Species in the latter group were characterized by xylem tissues with extreme resistance to embolism but low levels of foliar ABA after 30 d without water. The combination of low levels of ABA under stress with cavitation-resistant xylem enables these species to prolong stomatal opening during drought, potentially extending their photosynthetic activity between rainfall events. Our data demonstrate a surprising simplicity in the way conifers evolved to cope with water shortage, indicating a critical interaction between xylem and stomatal tissues during the process of e
ISSN:0027-8424
1091-6490
DOI:10.1073/pnas.1407930111