Photoperiod affects floral ontogeny in strawberry (Fragaria × ananassa Duch.) plug plants

Abstract Strawberry (Fragaria × ananassa Duch.) plug plants of the short day (SD) cultivars ‘Earliglow’, ‘Seneca’, ‘Jewel’, ‘Chandler’ and ‘Cavendish’, and the long day (LD) cultivars ‘Seascape’, ‘Fern’ and ‘Selva’ were exposed to 0, 1, 2 or 4 weeks of short-days (8 h) at 15 °C followed by forcing in a long-day (16 h) greenhouse. Leaf, runner and flower growth were monitored weekly. Floral ontogeny and plant phenology of three week old plug plants was sensitive to pre-forcing short-day conditioning and the degree of sensitivity varied with cultivar and parameter examined. In all SD cultivars except ‘Chandler’ conditioning hastened flowering. The length of conditioning needed to enhance precocity and intensity of enhancement varied with cultivar. Conditioning did not alter the time of flowering for LD cultivars. SD conditioning enhanced leaf production under LD forcing in some cultivars. Each cultivar exhibited its own characteristic response to photoperiod for runner production regardless of the traditional photoperiodic category to which it belonged. The general classification for enhanced runner production of SD cultivars under LD and LD cultivars under SD is questionable since runner production responses of cultivars did not follow this categorization: runner production was enhanced by SD conditioning in the SD cultivar ‘Chandler’ and inhibited by SD in the LD cultivar ‘Fern’. Runner and inflorescence production are not necessarily antagonistic processes in short-day cultivars, as sometimes suggested. Reduced runner formation was observed with a concomitant increase in inflorescence production in some cultivars. The reduction in runner production in other cultivars was a direct inhibitory response to short days rather than an inhibition caused by flowering. Runner production was reduced but flowering was not enhanced with short day conditioning. In addition, SD enhanced flowering in ‘Chandler’ did not inhibit runner production. Cultivars did not fall into specific photoperiod response categories (SD or LD) with respect to flowering either. All SD and one of three LD cultivars were vegetative at the start of the experiment. Inflorescence production occurred during long-day forcing in non-conditioned controls indicating that floral initiation occurred during the long days in the greenhouse. Nevertheless, altered flowering under LD forcing of SD conditioned plugs compared to controls in several cultivars indicated that floral ontogeny under long days was altered with pre-forcing, short-day conditioning. All cultivars except ‘Seascape’ and ‘Cavendish’ exhibited enhanced initiation and differentiation following SD conditioning. Each stage of floral development (induction/initiation, development and differentiation) was differentially affected by photoperiod and as many of these stages as possible should be evaluated to provide a clear description of any response. The only general description of flowering response to photoperiod in strawberry is that SD exposure enhances differentiation in both SD and LD cultivars. General categorizations for initiation and development should not be made as responses to photoperiod for these stages are cultivar specific.