Impact of clear polymer film on the growth and physiology of maize

This thesis investigates the suitability of degradable clear polymer film use to support the production of cold-sensitive crops in Tasmania. Film used in this manner is poorly understood and rarely practiced in Australia but is practiced more widely in cold regions of the northern hemisphere. There has been limited agronomic research conducted in the area of film use to assist crop propagation, and contemporary research efforts in this field focus almost exclusively on film chemistry, particularly in the areas of formulation, spectral properties and degradation rate. Authors currently working in this field instead primarily focuses on clear film use for solarisation in summer for weed/pathogen suppression, and the thermal and reflected spectral effects associated with opaque 'mulch' film use for weed suppression. In addition to these main areas, there is some exciting work being made coupling novel-spectral absorption properties with changes in rates of pest insect development, but to date this has been focused on greenhouse cladding materials and has not yet made the transition across to clear propagation film. This thesis first explores the effects of film use upon temperature conditions and gaseous substrate composition within the film-enclosed growing area, as well as solar radiation transmission properties of film under Tasmanian field conditions. Film use reduced the transmission of solar radiation into the headspace by 20 % and increased the concentration of water vapour and CO\\(_2\\) within the film-enclosed headspace. Film use was shown to increase maximum daily temperatures within the film-growing environment. Film use increased minimum daily temperatures by ~4°C between late spring and early autumn, but reduced minimum daily temperatures during other months. Maximum daily temperatures beneath film varied seasonally in response to solar radiation intensity and cloud shading, increasing temperatures by as much as 10 °C above ambient temperatures during winter and 40 °C above ambient temperatures during summer. Models of these environmental changes were developed from ambient climate data, and were incorporated into APSIM to estimate temperatures at other sites from historical climate data. After discussing the climate effects of film use, this thesis explored the effects of film use upon the agronomy and physiology of maize (Zea mays L.), a C4 model crop species and forage source that is sensitive to cold and frost. Film-enclosed chambers were developed to enable establishment and growth of seedlings under different headspace gas compositions. Use of film was shown to improve all aspects of maize seedling performance under cold seasonal conditions. Film use in winter and early spring protected seedlings from exposure to frost and increased soil and air temperatures, leading to earlier, more uniform crop emergence, faster seedling growth, and improved photosynthetic performance. Increases in seedling chlorophyll content, CO\\(_2\\) assimilation and solar radiation utilisation caused by film use had few persistent effects on maize seedling physiology following removal of the film enclosure. Film use was less beneficial under warmer conditions, causing seedlings to regularly experience acute heat stress when exposed to damaging supra-optimal headspace temperatures above 40 °C. Increased headspace CO\\(_2\\) concentration ([CO\\(_2\\)]) had minimal effect on maize emergence, growth rate or leaf carbon assimilation. Information from climate monitoring and physiology experiments was used to inform APSIM modelling to estimate the effect of the film on crop survival and yield in several scenarios. Modelling suggested the production of above-ambient temperature conditions beneath the film favoured earlier planting dates during winter and early spring, with later dates subject to potentially damaging supra-optimal temperatures. Optimal film use was shown to increase simulated maize forage productivity by 10-15 % above existing industry practices in coastal regions. In inland regions, incorporation of film into early-sowing systems greatly reduced maize exposure to frost and subsequent crop failure, and increased long-term crop yields by 7-10 % above existing industry practices. Yields from film-supported production systems reported in this thesis represent conservative estimates only, and potential increases in yield productivity achieved through film use may exceed those reported in this thesis. The results from this indicate that installation of film can create conditions suitable for maize establishment during winter and spring in this cool temperate environment. Extension of the growing season permits flowering and embryogenesis, facilitates the use of longer-season cultivars in cold-affected areas, and enables crop growth to be realigned to better match temperate winter-spring dominant rainfall patterns. Adoption of this technology may improve dryland maize productivity in some cold-limited regions, and reduce seasonal water consumption for existing irrigated maize producers. These benefits may also promote maize cultivation outside of existing coastal production regions in Tasmania.