The effect of a low-density white shade net on physiology, phenology and production of mandarin (Citrus reticulata 'Nadorcott')

A number of South African mandarin producers cover orchards with shade nets to protect their crop against occasional hail. However, shade nets also affect the light environment and microclimate of the orchard covered. This subsequently affects the tree physiology, growth, production and fruit quality. The aim of this study was to determine the effect of a low-density, white shade on the physiology, vegetative growth and yield of 'Nadorcott' mandarin. The shade net did not have a marked effect on orchard air temperatures, but relative humidity was significantly increased while photosynthetic active radiation (PAR) was significantly decreased. These changes in orchard light and humidity conditions had a significant effect on tree physiology. Trees covered by the shade net had increased stomatal conductance and rates of transpiration and photosynthesis. Vegetative vigour was further increased by the shade net for both years. This was possibly as a result of shade avoidance responses that were triggered by the lower light levels under the shade net. The effect of the shade net on yield was inconclusive because 'Nadorcott' mandarin is an alternate bearer. The large differences in yield obtained between the two years as a result of alternate bearing gives insufficient information on the effect that the shade net might have on yield. Fruit size was also significantly increased under the shade net.