Photoperiodic phytochrome-mediated vegetative growth responses of container-grown citrus nursery trees

• Vegetative growth of trifoliate orange-type rootstocks is regulated by photoperiod.
• Citrus photoperiod response is phytochrome, not carbohydrate, mediated.
• Night interrupt treatment can overcome short photoperiod response in citrus.

In Florida, most citrus trees are grown on rootstocks with trifoliate orange parentage. Nurserymen have long noted that these rootstocks exhibit much slower growth during the winter than their non-trifoliate counterparts (e.g., ‘Volkamer’ lemon, ‘Sour Orange’). Since 2007 citrus nursery trees in Florida must be grown in greenhouses to protect them from the Asian citrus psyllid, the vector of huanglongbing (citrus greening). Greenhouse production of citrus nursery trees has greatly increased production costs and, thus, the desire to determine why trifoliate-type rootstocks grow poorly during the winter. We hypothesized that trifoliate-type rootstocks, because of their deciduous habit, respond to photoperiod and exhibit slow growth under short days (photoperiods <12 h). Our objective was to determine the effect of photoperiod on the growth of container-grown trees of the two most common trifoliate-type rootstocks, ‘Carrizo’ citrange and ‘Swingle’ citrumelo with and without ‘Hamlin’ sweet orange scions. Three weeks after budding, all trees were placed in growth chambers under three different photoperiods: short days (SD – 10 h photoperiod), long days (LD – 14 h) and short days + night interrupt (SD-NI – 10 h photoperiod + 1 h night interrupt) for 14 weeks, and maintained at 28/21 °C day/night temperature. All trees, regardless of being budded or not, had reduced growth under SD conditions, whereas the trees under SD-NI grew similar to those under LD. Average tree growth during the 14-weeks was 19, 52 and 55 cm, across all rootstock-scion combinations for SD, LD and SD-NI treatments, respectively. The difference in growth between budded and non-budded trees in the SD treatment was not significant, but was highly significant in LD and SD-NI. Across rootstock-scion combinations, the average numbers of new nodes produced per tree were 13, 30 and 32 nodes in the SD, LD and SD-NI treatments, respectively, indicating that the increased growth under LD and SD-NI conditions was not simply a result of internode elongation. Net CO2 assimilation was higher under the SD and SD-NI treatments compared with LD, but there were no significant differences in whole-plant total nonstructural carbohydrate concentrations as a result. The ability of a 1-h night interrupt to overcome the SD response indicates that the photoperiod effect observed is a phytochrome-mediated response.