100 Lc At Sight Means

[Berenguer Miramontes]

Mostof the senses of sight are concerned with seeing. A wonderful spectacle might be described as a sight, as might the general capacity to see anything ("my sight is not as good as it once was"). Sight is also used in a number of fixed phrases, such as "out of sight, out of mind," "sight unseen," and "set one's sights on." Sight comes from Old English gesiht, meaning "the faculty or act of sight, thing seen."

Site is most often concerned with location; it is related to situate, "to locate," and situation, "relative position or combination of circumstances at a particular moment." A building site is the place where a building is, or will be, located. In contemporary English, site is frequently used as a shortened form of website, to refer to the location of a group of web pages. Site comes from Latin situs, meaning "place, position, site."

Current knowledge suggests that the mechanisms by which plants communicate information take numerous forms. Previous studies have focussed their attention on communication via chemicals, contact and light; other methods of interaction between plants have remained speculative. In this study we tested the ability of young chilli plants to sense their neighbours and identify their relatives using alternative mechanism(s) to recognised plant communication pathways. We found that the presence of a neighbouring plant had a significant influence on seed germination even when all known sources of communication signals were blocked. Furthermore, despite the signalling restriction, seedlings allocated energy to their stem and root systems differently depending on the identity of the neighbour. These results provide clear experimental evidence for the existence of communication channels between plants beyond those that have been recognized and studied thus far.

Copyright: 2012 Gagliano et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Funding: This study was supported by a UWA Postdoctoral Research Fellowship to MG. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Plant communication by means of chemicals, contact or light wavelengths is now well recognised, and the study of these types of communication is well under way. We hypothesised that plants also employ other alternative ways of communicating, based on sound or magnetic waves for example. Therefore the aim of this study was to look for evidence of such alternative means of communication, by testing whether any interaction between plants still occurs when all communication based on recognised means has been blocked. In particular we asked (1) whether the presence of a neighbouring plant could influence germination rates of seeds when above- and below-ground contact, chemical and light-mediated signals are blocked; and if so, (2) whether such effects on germination and growth differed depending on the identity of the neighbouring plant (i.e. conspecific vs heterospecific).

As our model system, we used the seeds of Capsicum annuum (Solanaceae), a widespread chilli species originally native to the Americas where it has been domesticated for over 6,000 years [23], which is now cultivated worldwide in its many varieties. The commercially cultivated types of this flowering plant produce large fruits, which are green in colour ripening into red, and have lost their natural mechanisms for seed dispersal [24]. To test whether the presence of a neighbouring plant influenced how chilli seeds germinated and grew, we chose the Florence fennel plant (Foeniculum vulgare, Apiaceae). F. vulgare was a particularly appropriate heterospecific neighbour for this study, because this species is known to exude chemicals from roots or aerial parts that inhibit growth and even kill its neighbours so is generally grown in seclusion [25]. Hence, we expected the presence of fennel to retard or block germination and/or growth rates of chilli when open contact was possible and to have a progressively smaller negative effect on germination as its signals were partially or totally blocked.

All experiments were conducted at the Plant Growth Facilities at the University of Western Australia. Experiments were done in a 5.30 m2 Controlled Environment Room (CER) fitted with high-intensity discharge lamps. We used custom-designed experimental units (Figure 1), which prevented above and below ground contact and blocked chemical and light-mediated signals plants normally exchange. The experimental units consisted of a group of petri dishes, each one containing chilli seeds, which were sandwiched between layers of 2 mm thick felt to retain moisture and ensure darkness. Petri dishes were arranged in a circle around a sealed central cylindrical box (as per Figure 1a). The seal at the base of the central cylindrical box, which either contained an adult plant or was left empty (control), ensured that seeds were chemically isolated from these adult plants (see Text S1 & Figure S1 for details on the Chemical testing of the experimental unit). All seeds and adult plants within a replicate unit were then housed within 2 different sized square boxes (444450 cm and 323245 cm respectively), one inside the other, with the air in between the two boxes removed using a pump to create a vacuum and thus avoid interference between adjacent experimental units at any time (Figure 1b). Each day, all experimental units were randomly re-interspersed throughout the growth room to avoid any potential artefacts due to their position in the room (e.g. light quantity and quality). Similarly, each day individual petri dishes within each unit were randomly re-arranged in the circular configuration around the central box to avoid any potential confounding effects of their position within the experimental unit. The temperature within the boxes was recorded over a period of 22 consecutive days to ensure that any difference in seed germination or growth measured between treatments was not due to differences in the temperature inside the boxes caused by the presence or absence of adult plants (see Figure S2). All treatments were exposed to identical nutrients, temperature and 12 h light:12 h dark cycle conditions.

(a) The seal at the base of the central cylindrical box ensured that chilli seeds arranged in a circle around the adult plant were chemically isolated from it. (b) All seeds and adult plants within a replicate unit were housed within 2 different sized square boxes, one inside the other, with the air in between the two boxes removed using a vacuum pump. The whole experimental unit was custom-made in colourless cast acrylic material (ModenGlas), which transmitted 92% of visible light, but was opaque to ultraviolet and infrared wavelengths.

Ninety-six chilli seeds were randomly apportioned among 3 treatments (Chilli, Fennel, and Control), each replicated 4 times. The experimental units consisted of a group of 8 seeds, individually sowed into small pots (337 cm) filled with coco fiber substrate (Organic Nutrifield Coco), which were positioned c.10 cm from each other in a circle around the sealed central cylindrical box as per above. All seeds and plants within a replicate unit were then housed within the boxes and the entire unit was maintained in isolation from adjacent ones as described above. The coco fiber substrate was kept moist by watering and fertilizing every 4th day. Seeds were maintained individually in pots throughout the experiment and allowed to grow in isolation from siblings to avoid the confounding effects of root interactions and unequal acquisition of resources such as water, nutrients and light on germination and growth. On the 7th day after sowing, all seeds were inspected by lightly brushing away the top coco fibers to expose the seed using a fine paintbrush. Germination rates in each treatment were recorded and monitored for the initial 20 d of the experiment after which the number of germinating seeds reached an asymptote. Emergence rates, maximum stem height (as an estimate of above-ground growth) and number of leaves were monitored and recorded over the course of the experiment with the number of branches recorded at the conclusion at 38 d. At the end of the experiment, the roots of all seedlings were carefully washed clean of all coco fibre and photographed against a scale bar. Maximum root length (as an estimate of below-ground growth) was then measured from these calibrated digital images using the image analysis programme, OPTIMAS 6.5.

Atmospheric Effect: an effect caused by the presence of the plant that acts through atmospheric contact, such as volatile chemical signals, and is thus blocked by the central cylindrical box (note that this may also incorporate some light signals based on far-red light, since the barrier blocking chemical signals also blocked far-red light).

and we thus defined binary present/absent factors for each of the four effects across the five treatments. It was then possible to test the significance of each of the four effects directly by comparing models as follows:

Light Effect: two models with and without the Light effect fitted to a data subset consisting of all treatments except the F open treatment. (In this case we needed to account for the effects of Other and Masking as well, so we included both these effects in both the models).

Each measured variable was analyzed separately using GLMMs. For the 2010 experiment, the number of branches at 38 days was modeled with a Poisson GLMM (appropriate for count data) with a fixed effect for treatment and a categorical random effect for plant nested within experimental container. The number of seeds germinating over time and the number of seeds emerging over time were both modeled with a binomial GLMM with fixed effects for treatment, time and an interaction between them, and a continuous time random effect for plant nested within experimental container. The number of leaves on the plant over time was modeled with a Poisson GLMM with fixed effects for treatment, time and an interaction between them, and a continuous time random effect for plant nested within experimental container. For germination, emergence and leaf number, all times were included in a single analysis. The height of the plant over time was modeled with a Gaussian GLMM (appropriate for continuous data with approximately normally distributed residuals) with fixed effects for treatment, time and an interaction between them, and a continuous time random effect for plant nested within experimental container. Only plants that had emerged by day 14 were included in this height analysis. Furthermore, since initial data exploration indicated that heights diverged over time with maximum divergence at day 29, one analysis was done with all times included, a second analysis with the last four measurement times (days 25, 29, 34 and 38) together, and a third analysis with just the day 29 measurement. The third analysis had no time effect included in the model of course. Stepwise model simplification based on AIC values was used to test whether the random effect for experimental container and the fixed effect for treatment should be included in the model. Where treatment was significant, we made specific contrasts by defining a new factor based on grouping two of the treatments at a time, refitting the model, and comparing the refitted model to the original model.

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