The effects of temperature and day length on species of Eucalyptus

Seedlings of Eucalyptus bicostata, E. blakelyi, E. niphophila, E. pauciflora, E. polyanthemos, E. rubida, and E . Stelldata were grown under various defined temperature and day-length regimes and response was assessed in terms of height, dry weights of stems, roots, leaves, and branches, number of leaves and branches, and leaf shape. Seedlings of E. bicostata, E. blakelyi, E. niphophila, E. pauciflora, and E. Rubida were capable of continuous growth under short days (8 hr) provided temperature conditions were appropriate. These species can be allocated, therefore, to Class C of Nitsch (1957). Growth was greater under long days of 18 hr than under short days of 8 hr. Irrespective of the temperature regime operating, the order of dry weight increase over 60 days was: E. bicostata > E. blahelyi > E. polyanthemos > E. niphophila. Growth of E. bicostata, E. blalcelyi, and E. polyanthemos increased as mean temperature levels increased thus: 14/8°C, 19/11°C, 20/17°C, 25/20°C. Further increase to 27/24°C had an adverse effeot upon growth of E. polyanthemos, but not on that of E. bicostata and E. blakelyi. Increasing night temperature from 6°C to 24°C when the day temperature was either 11, 20, 24, or 28°C generally increased height, dry weights of stems, roots, and leaves, and total leaf area of E. blakelyi. Similar effects were produced when day temperatures were increased from 11°C to 2S°C, night temperatures being either 6, 16, 19, or 24°C. Consequently, under short days of winter, the 28/24°C regime was most nearly optimal for growth. Adverse effects on growth appeared to be produced when night temperature exceeded a low day temperature (as under the 11/24°C regime). Average day and night temperature interacted in a statistically significant way upon growth. In E. paucifiora, increasing night temperature from 8°C to 24°C when the average day temperature was either 14, 20, 25, or 27°C, and increasing day temperature from 14°C to 27°C when night temperature was 8, 17, 20, or 24°C increased growth. Thus, the regime 27/24°C approached the optimum for growth, as in E. blakelyi. In E. blakelyi and E. paucifiora, branching was most restricted in plants which were tallest. There were indications that the magnitude of diurnal temperature fluctuations might be important in determining the degree of lateral branching (apical dominance). Leaf size, shape, and number were related to day length and temperature conditions. Leaf shape (as measured by length/breadth ratios) was a function of day and night temperatures, not being significantly related to either alone. Day length appeared to exert a more pronounced effeot upon leaf shape (long days tending to shift shape in the direction juvenile → adult) than temperature. The ratios top/root and leaflstem, especially the latter, were significantly lower in E. blakelyi than in E. pauciflora.