Low Night Temperatures Affect Growth of Container-Grown Liner Trees of Trifoliate Orange under Different Photoperiods

This research was conducted at the University of Florida Citrus Research and Education Center, Lake Alfred, FL during 2012 to study the effect of low night temperatures on the growth of container-grown trifoliate orange trees under short day-night interrupt photoperiod treatment. Sixty-eight trees of 'Swingle' citrumelo (Citrus xparadisi x Poncirus trifoliata) were obtained from a commercial citrus nursery. Half of the trees were grown under short day (SD; 10 h) photoperiod and half under short day night interrupt (SDNI; 10 h + 1 h night interrupt) in growth chambers set to maintain 28/21 degrees C day/night temperatures. After 6 weeks, both the growth chambers were changed to SDNI photoperiod, but were set at different night temperatures: one at 28/15 degrees C and the other at 28/10 degrees C. The trees from both SD and SDNI treatments were split between these two new temperature regimes. The trees grown under SD and SDNI treatments began to show significant growth differences after 6 weeks of treatments at 28/21 degrees C. However, after week 6 when the trees were moved to different night temperature regimes, slower growth was observed under 28/10 degrees C compared to 28/15 degrees C. Over 11 weeks of observation, the trees moved from SD 28/21 degrees C to SDNI 28/10 degrees C grew the least (363 cm), and had less total growth compared to the other three treatments. Trees which were moved from SDNI 28/21 degrees C to SDNI 28/15 degrees C had the greatest total growth and had greater growth (727 cm) than trees transferred from SDNI 28/21 degrees C to SDNI 28/10 degrees C (535 cm). These data demonstrate that there is a critical temperature between 15 degrees C and 10 degrees C below which 'Swingle' plants will not respond to photoperiod enhancement.