Cold-induced photoinhibition, pigment chemistry, growth and nutrition of Eucalyptus nitens and E. globulus seedlings during establishment

Australia is aiming to treble plantation wood production by 2020. Eucalyptus globulus Labill. and E. nitens (Deane and Maidem) Maiden are the predominant plantation species in southern Australia. This thesis describes physiological strategies employed by these species in response to cold-induced photoinhibition during seedling establishment. A series of experiments was conducted on seedlings prehardened in the nursery. Their physiological and growth responses after planting in the field was investigated. A field trial was established at 350 m asl in early spring 1997. Severe cold-induced photoinhibition caused photodamage which restricted growth of non-hardened E. globulus. Artificial shading alleviated cold-induced photoinhibition and photodamage in both E. globulus and E. nitens, and increased growth in E. globulus. Before planting, nutrient-starved E. nitens were photoinhibited and had high anthocyanin levels. Increased photoinhibition was not measured after planting because of sustained xanthophyll activity and/or light attenuation by high anthocyanin levels. In other treatments changes in anthocyanin levels were related to the severity of coldinduced photoinhibition. Relative to E. nitens, growth of E. globulus was more affected by cold-induced photoinhibition and photodamage. This was possibly due to inherently low levels of carotenoids and lack of acclimation to cold temperatures. The effects of shading on E. globulus and the absence of any effect of cold-hardening on E. nitens stresses the importance of incident light and pigment levels in cold-induced photoinhibition. In a second field trial, an early winter planting of E. nitens was established at 700 m asl in June 1998. Shading may have increased biomass production because of alleviation of cold-induced photoinhibition. Growth in non-shaded than shaded seedlings was greater overall due to higher biomass production in spring and summer. Seedlings grew taller when shaded due to apical dominance. Fertilised seedlings produced more biomass in the field than non-fertilised seedlings. Low growth rates of E. nitens during winter at 700 m asl were associated with high NPQ and sustained xanthophyll activity; photooxidation of chlorophylls, xanthophylls and 13-carotene (which decreased light absorption), and increases in lutein and neoxanthin (which indicated an antioxidant role). In general fertilised seedlings had higher pigment levels which maintained higher levels of light utilisation and dissipation. A controlled environment experiment which induced cold-induced photoinhibition, confirmed that galloylglucoses and flavonoids can act as antioxidants during seedling establishment. Sideroxylonals were also implicated in this role. Anthocyanin kinetics during seedling establishment indicated absorption of irradiance between 400 and 590 nm during periods of greatest cold-induced photoinhibition. Chemical fractionation of leaf N and P indicated that E. globulus is more efficient at acquiring N and P than E. nitens. After planting, re-translocation of stored foliar protein N and inorganic P to roots occurred in both species. Greater amounts of retranslocation in fertilised seedlings may contribute to their superior growth in the field. Planting seedlings involves risk. Planting nutrient-starved seedlings may decrease the risk of severe cold-induced photoinhibiiton and photodamage. However, this is at the expense of optimal growth performance. Planting altitude and season will determine whether fertilised or nutrient-starved seedlings should be planted.