LED light in primocane raspberries grown in polytunnel at high latitudes

Growing of primocane raspberries in mid-Norway is challenging, because of short days and low light levels in autumn. However, temperatures in coastal areas are normally high enough in summer and early autumn for growth and fruit development, and higher temperatures can be expected in future according to climatic scenarios. Therefore, improved light conditions were examined when photosynthetic active radiation (PAR) was below critical level for development of high fruit quality (400 mu mol m(2) s(-1)), growing 'Polka' in 2011-2013. An experimental block design with different light treatments: (i) ambient light; (ii) LED lamps (300 W); or (iii) SON (400 W) (the latter not employed in 2013) placed 40 cm above the top of the canopy. Fruit yield, degrees Brix, fruit firmness and arthropods on leaves were recorded. Results over three years indicate that LED had influence on degrees Brix and possibly on fruit yield, but not on fruit firmness. Highest light level (900 W under LED) was beyond saturation point for photosynthesis in periods of low temperatures (mainly beyond October 1). The arthropod fauna was sampled 2-3 times per season, revealing a build-up of spider mites from near zero in August 2011 to 265 mites leaf(-1) two years later. Light treatment did not influence spider mite or aphid numbers. It can be suggested that LEDs had a positive influence on fruit yield and degrees Brix, but not on fruit firmness, and that spider mites and aphids were not promoted by the extra light. However, to secure the whole crop to be harvested some heat should be added beyond October 1 to increase efficiency of LED lighting and secure ripening of total yield potential.