An investigation into the influence of variation in controlled environment plant research facilities on growth responses
Creators: name: Cummings, Ian
Publisher: name: University Of Tasmania
Year: 2023

description: 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.