Plants in dense vegetation compete for resources, including light, and optimize their growth based on neighbor detection cues. The best studied of such behaviors is the shade-avoidance syndrome that positions leaves in optimally lit zones of a vegetation. Although proximate vegetation is known to be sensed through a reduced ratio between red and far-red light, we show here through computational modeling and manipulative experiments that leaves of the rosette species Arabidopsis thaliana first need to move upward to generate sufficient light reflection potential for subsequent occurrence and perception of a reduced red to far-red ratio. This early hyponastic leaf growth response is not induced by known neighbor detection cues under both climate chamber and natural sunlight conditions, and we identify a unique way for plants to detect future competitors through touching of leaf tips. This signal occurs before light signals and appears to be the earliest means of above-ground plant-plant signaling in horizontally growing rosette plants.competition | phenotypic plasticity | thigmomorphogenesis | canopy development P lant growth in dense vegetations is dominated by a fierce battle over resources. In competition for light, small size inequalities can have major effects on light capture and thus competitive power (1-3). Therefore, it is essential for plants in dense stands to timely respond to neighbor detection cues and adjust growth to that of their competitors. Reduction of the red (R):far-red (FR) ratio, signaled through phytochromes and caused by horizontal FR reflection from neighboring plants, is seen as the earliest above-ground detection signal of neighbors. The R:FR is decreased even before true shading occurs through overlap of leaves, and induces shade-avoidance responses such as upward leaf movement (hyponasty) and stem elongation that secure light capture during subsequent plant competition (reviewed in refs. 4-6). Accordingly, low R:FR can induce shade-avoidance responses in plants grown without neighbors (7,8), and plants blinded to FR-enrichment in a dense canopy fail to respond to neighbors at an early stage of competition (9). Although some additional above-ground neighbor detection signals are known, e.g., blue light depletion (10, 11) and volatile ethylene accumulation (11), none of these acts as early as a decrease in the reflected R:FR.The paradigm of decreased R:FR as the earliest neighbor-detection signal in competition for light has been a breakthrough in mechanistic plant competition research. Interestingly, this paradigm is based on research on stem-forming forbs and trees (12-14), but has not been studied in plants that lack an appreciable height growth before competition in the vegetative life stage, e.g., rosette species such as Arabidopsis thaliana and many other species. Here we study early neighbor detection in dense stands of Arabidopsis. Hyponasty appears to be the earliest shade-avoidance response and occurs exclusively in leaves that touch neighboring leaf tips, before a physiologi...

A system of two plant chambers and a downstream reaction chamber has been set up to investigate the emission of biogenic volatile organic compounds (BVOCs) and possible effects of pollutants such as ozone. The system can be used to compare BVOC emissions from two sets of differently treated plants, or to study the photochemistry of real plant emissions under polluted conditions without exposing the plants to pollutants. The main analytical tool is a proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF-MS) which allows online monitoring of biogenic emissions and chemical degradation products. The identification of BVOCs and their oxidation products is aided by cryogenic trapping and subsequent in situ gas chromatographic analysis. Environmental impactIn this paper we describe a new laboratory setup to study the response of volatile plant emissions to different types of environmental stress. The centerpiece is the combination of two plant chambers with a reaction chamber that allows for photochemical processing of the plant emissions. Modern analytical tools such as proton-transfer-reaction time-of-ight mass spectrometry and gas chromatography allow for fast, precise and detailed chemical analysis. Results from a pilot study demonstrate the capacity of the system to investigate environmental issues of high importance, such as the discrepancy between the observed and predicted chemical loss of ozone via reactions with plant emissions; the impact of pollution and UV-B radiation on plant emissions.

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A system of two dynamic plant chambers and a downstream reaction chamber has been set up to investigate the emission of biogenic volatile organic compounds (BVOC) and possible effects from pollutants such as ozone. The system can be used to compare BVOC emissions from two sets of differently treated plants, or to study the photochemistry of real plant emissions under polluted conditions without exposing the plants to pollutants. The main analytical tool is a proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF-MS) which allows online monitoring of biogenic emissions and chemical degradation products. The identification of BVOCs and their oxidation products is aided by cryogenic trapping and subsequent in situ gas chromatographic analysis. The data presented in the paper demonstrates the good performance of the setup