Effect of coloured shade nets and plastics on Eucalyptus hybrid mini-hedge stock plant morphology and subsequent cuttings rooting potential.
Eucalyptus grandis (W. Hill) × E. nitens (Maiden) (G × N) clonal hybrids are bred to produce trees more cold-tolerant than E. grandis alone due to the cold-resistant E. nitens parent. Some of these selected hybrid clones have superior wood and pulp properties but are considered “difficult-to-root”; thus, any technique that improves their rooting percentage is valuable to the industry. As the radiation spectrum can affect growth and development of plants, manipulation of the spectrum received by mini-hedge stock plants through cultivating them under certain shade nets and plastics could alter the rooting potential of the subsequent mini-cuttings. Therefore, the effects of the covering factor at eight levels (shade nets: black 30 %, green 40 %, Apple blue 20 %, Photo red 30 % and silver Aluminet® 40 % and plastics: Clarix E Blue® as well as Patilite®, plus the control (no covering)) on morphology and subsequent rooting potential on the G x N clone factor at two levels (GN018B: difficult-to-root or PP2107: easy-to-root) were evaluated. A further factor, fertilizer, at two levels (inorganic or organic) was also evaluated. Statistical analysis was carried out using GenStat®. Data were analysed using analysis of variance (ANOVA) where data were orthogonally distributed. Where data were not orthogonally distributed the algorithm restricted maximum likelihood (REML) was used to estimate variance parameters in the multivariate linear mixed model. To pinpoint the effect of the covering employed, firstly, the alteration of environmental parameters (temperature, relative humidity (RH) and radiation spectrum) were analysed as well as the seasonal effect on stock plants and rooting of mini-cuttings and root system quality of mini-cuttings were analysed. One greenhouse was used for stock plants and one for mini-cuttings. In the stock plant tunnel the shade nets and plastic were draped over wire trellising over bricked beds where the top and two sides were covered but the ends left open to allow sufficient ventilation. Thermometers and HOBOs® were hung above the plants and a spectroradiometer placed centrally under a specific covering to measure environmental parameters. It was found that Aluminet® and black nets and Patilite® plastic act as neutral covers with regard to radiation transmission, while the blue, red and green shade nets as well as Clarix E Blue® plastic covers altered the transmission spectrum; thus, they can be considered photoselective. Similarly, the red to near infrared ratios (R:NIR) were altered significantly by the coverings, however, this did not significantly affect the shoot internode length or leaf area (LA) of stock plants. Leaf area was determined with a leaf area meter using ten of the first fully expanded leaves collected per treatment and averaging them to run analyses based on LA per leaf. The irradiance levels in the stock plant tunnel were lowest under Aluminet® 40 % (PPFD of 204.6 μmol s-1 m2) during winter, resulting in low rooting percentages, while higher irradiance under blue 20 % shade net and control (no cover) (PPFD 604.4 and 931.5 μmol s-1 m-2, respectively) during spring 2011, achieved average rooting percentages. There was no clear trend of an optimal radiation intensity to expose G × N stock plants to, in order to achieve high rooting percentages; however, a tendency was documented, whereby black 30 % and green 40 % at 200 to 450 μmol s-1 m-2 PPFD gave good rooting percentages.