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Effect of different photoperiod and led lighting regimes on the growth and physiology of containerized citrus nursery trees
Nursery citrus trees in California must be grown in insect exclusion facilities to be protected against Huanglongbing (HLB), a deadly disease spread by the Asian citrus psyllid. Faster year-round propagation is critical for citrus nurseries to offset the investment in new exclusion facilities, but nurseries currently face serious problems of poor bud push and slow scion growth in fall-budded container grown trees. Therefore, the purpose of this study was to explore the effect of supplemental LED lighting technique on container citrus tree growth, physiology and total non-structural carbohydrate partitioning to improve the propagation efficiency during winter months. Seventy-two trees were placed in growth chambers under four photoperiod treatments: T1, 10 h LED with low supplemental light extension of day length (EoD); T2, 10 h LED with low supplemental night interruption (NI); T3, 10 h LED with supplemental far-red light; and T4, 10 h LED (Control). Light spectrum of LEDs was adjusted to 90 Red and 10 Blue ratio. The trees were maintained in the growth chambers at 21/13 °C day/night temperatures and 80% RH for 12 weeks. This experiment was run twice. Results showed that there were significantly higher leaf count and average shoot growth in NI and EoD than in 10 h LED in both budded and unbudded trees. Far-red supplemental light treatment was able to significantly increase shoot length in budded trees. The efficacy of low supplemental light intensities (10 μmol. m-2 . s-1 ) below the light compensation point and the partitioning of total non-structural carbohydrates indicated phytochrome- mediated control of growth in citrus nursery trees.