Effects of photoperiod and paclobutrazol on growth dynamics of petioles instrawberry (Fragaria x ananassa)

Petiole elongation of strawberry plants (Fragaria x ananassa) is modified b y photoperiod, and gibberellins may mediate this response. To test the rela tionship further, we measured petiole growth dynamics under short and long days, in comparison with responses to paclobutrazol, which reduces gibberel lin biosynthesis. Growth rate in short days was lower after 2 days, and at its maximum was 60% of long-day controls. Petiole length at maturity was 75 %, and was accounted for primarily by decreased cell length. Subsequent lea ves showed similar patterns, but in the third leaf, cell number was also re duced. Petiole growth rate after treatment with paclobutrazol (5 mg plant(- 1)) was maximal at 2 days, 44% of the control maximum at day 7. Mature leng th was 30% of controls, again due to reduced cell length. Second leaves had even more severely reduced growth rates. Increasing paclobutrazol concentr ation over the range 0.025-1.0 mg progressively reduced growth, but a gibbe rellin-independent component accounted for 20% of elongation. Effects of co mbined paclobutrazol and photoperiod treatments were additive. The normal s hort-day response therefore occurred in paclobutrazol-treated plants with a lready slowed growth and reduced capacity for gibberellin synthesis. We con clude that assessment of the function of gibberellins requires direct measu rement of gibberellins in petioles during growth.