Effects of Photoperiod On Liner and Budded Tree Growth in Containerized Citrus Nurseries

As per the new legislation in Florida, citrus nursery trees now must be grown in greenhouses to protect them from the Asian Citrus Psyllid, the vector of greening disease. This shift to indoor container-grown systems has given rise to problems of bud failure and poor/slow scion growth especially during winter months. It was hypothesized that common trifoliate orange rootstocks, because of their deciduous habit, are responsive to photoperiod and show slow growth under short days, but this response is lost when budded with non-trifoliate scion varieties. This study was conducted with the objective of determining the effect of photoperiod on the growth of container grown trees of two trifoliate rootstocks, Carrizo citrange and Swingle citrumelo. Half the trees of each rootstock were budded with Hamlin sweet orange while the other half remained unbudded. Three weeks after budding, the trees were moved into three separate growth chambers set to: short days (SD - 10 hr photoperiod), long days (LD - 14 hr) and short days + night interrupt (SD-NI - 10 hr photoperiod + 1 hr night interrupt). All chambers were set to 28°C/21°C day/night temperature. Both the budded as well as non-budded trees showed reduced growth under SD conditions. However, the plants under SD-NI grew similar to those under LD. The average growth was 19 cm in SD, 52 cm in LD, and 55 cm in SD-NI, across all combinations. The difference in growth between budded and non-budded trees within SD was not significant while it was highly significant in LD and SD-NI. Over 12 weeks, the number of new leaves on non-budded trees increased significantly in LD and SD-NI, but not in SD. The average increase in leaf number was 13 in SD across all combinations, while it was 30 and 32 in LD and SD-NI, respectively. This shows that the trifoliate leaves respond to day length. Although instantaneous photosynthetic rate was higher in SD and SD-NI than in LD, the estimates of total daily net assimilation were similar under all the photoperiods. It was concluded that the growth of trifoliate rootstocks and non-trifoliate sweet orange budded on trifoliate rootstock is reduced under SD. The ability of a 1-hour night interrupt to overcome the SD response indicates that this is a phytochrome mediated photoperiod response and not a photosynthetic response.