Reflective ground cover - spread at various times - improves microclimate i.e. light reflection, fruit quality, fruit colouration, fruit sugar and taste of apple cvs. 'Elstar' and 'Jonagold' under black and white hailnet.

摘要

Fruit colouration of the bi-coloured apple cvs Elstar Elshof and Jonagold Wilmuta on M9 under hailnet is often hampered due to light deprivation in the autumn, caused by shorter photoperiod, declining solar angle and decreasing light intensity. Therefore, reflective ground cover (ExtendayReg.) was spread early (beginning of July) or late (beginning of August) either side of the tree rows under white (88-92% light transmission) or black (80-84% light transmission) hailnet at the University of Bonn, Campus Klein-Altendorf (50.4 degrees N), Germany, to improve fruit quality, particularly fruit colouration and taste, particularly sugar content. The five-year-old apple trees on M9 rootstock, spaced at 3.5 m x 1 m, were planted in N-S orientation to optimise light utilisation; adjacent uncovered trees without reflective ground cover without hailnet served as control. The ground covers improved light reflection perpendicular from the ground in the middle of the alleyways by 14% at 90 degrees angle, but by 5-fold at a 45 degrees angle into the periphery of the tree canopy, both measured 1 m above ground. The diurnal course of light reflection 1 m from the ground differed distinctly from that of incident radiation: The latter depended on the East or West side of the tree row. By contrast, light reflection values depended solely on the use of reflective ground cover irrespective of hailnet and row orientation. Relative humidity and air temperature appeared unaffected by the use of reflective mulch; sunburn did not appear in any of the treatments. 1,200 colour measurements showed that the reflective ground covers improved red colouration of the downward-facing, lower and tree-facing, inner side of apple fruit of both varieties from 91-99 degrees hue colour angle (green) in the uncovered control to 51-59 degrees hue (red) irrespective of the date of spreading the mulch i.e. early or late. By contrast, no differences were observed in the sun-exposed, outer side of the fruit. The lowest fruit firmness and lowest sugar content were observed in apple fruit under the black hail-net. The firmest fruit with the largest sugar concentration (as a measure of taste) were observed in all trials with reflective mulch, particularly in the lower third of the tree canopy, starting from the uncovered control, the white hailnet to the black hailnet; sugar concentration improved with the time of mulching. Overall, the shorter time of mulching of 4 versus 8 weeks prior to anticipated harvest from the beginning of July (for cv. Elstar) and mid-August (for cv. Jonagold) proved sufficient for improving fruit quality, colouration and taste under both white and black hailnet at 50 degrees N. Based on an expected 10-year lifespan with reduced UV-exposure under hailnet in North-western Europe, this currently most expensive reflective ground cover increased gross returns by 560-EUR 840/ha/year due to more well-coloured class 1 apples and a smaller percentage of poorly coloured apples (<25% blush) compared with the material cost of EUR 700 or 350/ha/year, if used once or twice a year, but without any labour cost involved. Financial returns varied between EUR 210 and 490/ha/year, based on double use per season of the light reflecting material, whereas single use only reached the break-even point. Hailnets are expected to increase due to climate change associated with better light conditions for reflection in the autumn and taller tree training leading to more shade in the lower tree canopy; new alternative reflective, low cost ground covers may then contribute to better utilisation of light otherwise lost in the alleyways and better economy for use in a fruit orchard.