Effects of photoperiod and paclobutrazol on growth dynamics of petioles in strawberry (Fragaria × ananassa)

Abstract

Petiole elongation of strawberry plants (Fragaria × ananassa) is modified by photoperiod, and gibberellins may mediate this response. To test the relationship further, we measured petiole growth dynamics under short and long days, in comparison with responses to paclobutrazol, which reduces gibberellin 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 leaves showed similar patterns, but in the third leaf, cell number was also reduced. 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 length was 30% of controls, again due to reduced cell length. Second leaves had even more severely reduced growth rates. Increasing paclobutrazol concentration over the range 0.025–1.0 mg progressively reduced growth, but a gibberellin-independent component accounted for 20% of elongation. Effects of combined paclobutrazol and photoperiod treatments were additive. The normal short-day response therefore occurred in paclobutrazol-treated plants with already slowed growth and reduced capacity for gibberellin synthesis. We conclude that assessment of the function of gibberellins requires direct measurement of gibberellins in petioles during growth.