Interaction of photoperiod and gibberellin on growth and photosynthesis of high‐latitude Poa pratensis

The relative growth rate of pot‐grown plants of Poa pratensis L. cv. Holt, origin 69s°N, was increased by 20–40% by photoperiod extension with low intensity incandescent light from 8 to 24 h at 9–21°C. The main increase occurred over the 14 to 18 h photoperiod range. The true photoperiodic nature of the response was demonstrated by the effectiveness of night interruption in stimulating growth. Fortnightly sprayings with gibberellic acid (GA 3 ) (3 × 10 ‐6 to 3 × 10 ‐5 M ) mimicked all the effects of long days, whereas (2‐chloroethyl)‐trimethylammonium chloride (CCC) counteracted the effects of long days. Both growth substances exhibited pronounced interactions with photoperiod, GA 3 being most effective in short days and CCC in long days. The growth stimulation, whether caused by long days or GA 3 , was exerted mainly through increases in individual and total leaf area. This was associated with a reduction in CO 2 , exchange rate and a parallel fall in specific leaf weight. Proportionally, however, the increase in leaf area was greater than the fall in CO 2 exchange rate, resulting in a 38 to 118% increase in photosynthesis per leaf. No evidence was found of any direct and promotive effect of transition to long days on the CO 2 exchange rate of already expanded leaves.