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An investigation into the influence of variation in controlled environment plant research facilities on growth responses

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Cummings, Ian (2008) An investigation into the influence of variation in controlled environment plant research facilities on growth responses. Research Master thesis, University of Tasmania.

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Abstract

In this study, controlled environment plant growth facilities were examined through
both physical measurement and plant growth response studies in order to characterise
the degree of variation between environments and to identify those variations that may
influence experimental results. Plant growth facilities consist of greenhouses, where
temperature and light is influenced by seasonal variations, and growth chambers,
where temperature and light quantity is considered to be accurately controlled, but
where all light is artificial.
Natural light spectral properties were found to be quite consistent temporally and
seasonally, but quantity was highly variable and influenced by greenhouse design and
covering material. In winter, light quantity was found to influence plant morphology,
particularly in greenhouse areas with heavy structural components. Plants showed
increased shoot elongation relative to higher light areas under such conditions. Growth
chamber experiments that varied irradiance but not temperature confirmed shoot
length was closely associated with light quantity, with longest shoot lengths under
lowest irradiance and shortest shoot lengths under highest irradiance. Covering
material also had an influence. In a study of the spectral properties and growth
responses under glass and polycarbonate clad greenhouses with the same design,
orientation and temperature profiles, light quantity was always lower under
polycarbonate relative to glass. In spring, with longer day-length and higher irradiance
relative to winter, this had little influence on plant morphology or development. In
winter, however, plants under polycarbonate showed significant shoot elongation
relative to plants grown under glass. The minor differences in spectral properties
between glass and polycarbonate (polycarbonate had lower UV and blue, and higher
far-red proportions relative to glass and natural light) did not appear to be a significant
influence on results, as flowering node was not significantly different between
treatments. The UV reduction under polycarbonate and laminated glass relative to
natural light and horticultural glass also did not appear to be a significant influence on
plant morphology, as supplementing UV back to natural levels did not produce
significant differences between treatments.
Light quantity reductions in winter can be somewhat compensated for by
supplementary lighting. A range of high pressure sodium lamps were tested, and most
would be suitable for this purpose, including some non-plant specific brands. Irradiances of 50- 100 µmol m-2s-I over an 18h photoperiod produced dramatic growth
improvements in pea, with significantly increased leaf size, dry weight and yield.
Although high pressure sodium lamps have a high red to far-red ratio (R:FR), which
could be expected to delay flowering, there was no delay in flowering node relative to
18h extension lighting with a low R:FR. Diffusing covers over the lamps improved
light distribution, and there was no significant benefit from using a moving light
system relative to a fixed system.
Photoperiod control systems were examined, and the importance of total light
exclusion for day-length studies was confirmed. Inductive light levels for pea were
less than 0.1 µmol m-2 s-1 . While traditional photoperiod extension is with incandescent
lamps because of their low R:FR, white, blue, red and far-red light were all inductive
to flowering for pea. The low R:FR of incandescent and far-red light induced typical
shade avoidance responses of increased shoot length and reduced leaf size, which the
other wavelengths did not.
Seasonally, both light quantity and temperature varied widely in the glasshouse
environments. Various shade methods are commonly employed in summer to reduce
radiant load, and a range of these were examined. All of the methods were found to be
spectrally neutral compared to unshaded conditions, and did not influence plant
morphology. Plants grown in summer had significantly reduced shoot length, leaf size,
flowering time and yield compared to plants grown in other seasons. Both growth
chamber and natural light experiments indicated these were primarily responses to
elevated temperature, particularly the reductions in yield.
For more accurate control over environmental parameters, plant growth chambers
are commonly used in plant research. However, all of the light sources used were
found to have very different spectral properties to natural light, even when mixed to
broaden the spectrum. Thermal load was found to be significant with high intensity
discharge lamps even with a separately ventilated light loft, although the use of double
glass barriers and water filters reduced the impact. The addition of incandescent lamps
to the light mix in an attempt to mimic more natural R:FR ratios was found to be
ineffective and significantly increased thermal load. Plants showed clear signs of
temperature influence, with reduced shoot length, leaf size and yield, and did not
flower at a lower node as expected from reduced R:FR. However, far-red light
emitting diodes added to the light mix produced natural R:FR ratios without thermal load influences, and plants responded as expected with increased shoot length and
reduced flowering node.
Spectral distribution and growth responses under fluorescent and mixed metal
halide/high pressure sodium lamps were quite similar at equal temperature and
irradiance. However, plants grown under metal halide flowered at a significantly
earlier node than the other sources, while under high pressure sodium lamps, shoot
length was significantly longer. Metal halide has high blue, and high pressure sodium
has low blue irradiance. Supplementation of high pressure sodium with blue light
induced reduced shoot length and flowering node. However, R:FR also varied
between light sources and natural light. The role of blue light was further investigated
using photo-selective shade screens, which were found to alter blue proportion but not
R:FR relative to natural light. Under red shade cloth (low blue, high red proportions)
shoot length was significantly increased and under blue shade cloth (high blue, low
red proportions) shoot length was significantly reduced relative to spectrally neutral
shade cloth. Blue light receptor cry] mutant plants did not respond to shade cloth
treatment, as shoot elongation was not significantly different in cryl mutant plants
grown under neutral, red or blue shade. This indicates a clear role of blue light
quantity in pea shoot length responses, and specifically, the cryl photoreceptor in
these changes.
This study has identified that light and temperature are the most important factors
that vary between controlled environments, and are a potential influence on results.
Taken together, the results from this study will allow future plant researchers, and
facility managers, to identify the equipment variations that may influence plant
responses.

Item Type: Thesis (Research Master)
Copyright Holders: The Author
Copyright Information:

Copyright 2008 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Additional Information:

Available for library use only and copying in accordance with the Copyright Act 1968, as amended. Thesis (MSc)--University of Tasmania, 2008. Includes bibliographical references

Date Deposited: 09 Dec 2014 00:12
Last Modified: 11 Mar 2016 05:53
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