Covering young 'Reed' avocado trees with shading nets during winter alleviates cold stress and promotes vegetative growth

摘要

Avocado is a commercially important sub-tropical fruit crop. Despite its popularity, most cultivars are susceptible to cold stress, that may result in yield losses and limits avocado geographic expansion. As shading nets are used to protect crops from environmental hazards, the objective of this study was to evaluate the effect of covering young Reed avocado trees with shading nets during winter. Silver 50% and 70% nets were placed over young avocado trees during three consecutive winters while uncovered trees served as control. Measurements show that photosynthetically active radiation (PAR) in the control plots was significantly higher compared to both nets. Minimum air temperature under the shading nets was similar to that of the control, but maximum air temperature was generally lower. Mid-winter ratio of variable to maximum fluorescence (Fv/Fm) rates were lowest in the control trees and highest in trees covered with Silver 70% net. Moreover, leaf chlorophyll content was generally higher in the covered trees, especially under Silver 70%. Furthermore, flowering intensity in trees covered with Silver 70% was significantly lower than in the control trees. Finally, tree trunk diameter was lowest in the control trees and higher in trees covered with both nets, especially Silver 70%. In summary, the effect of the shading nets may be attributed to mitigation of chilling stress via a reduction in irradiation levels during winter. In conclusion, shading nets may be considered as a possible solution to mitigate cold stress in commercial orchards, reducing growers losses and enabling geographic distribution and expansion of avocado orchards to colder areas.