Fruitfulness in grape-vines: The response of different cultivars to light, temperature and daylength

Plants of five cultivars (Sultana, syn. Thompson Seedless; Muscat Gordo Blanco; Rhine Riesling; Shiraz; and Ohanez) of grape-vine (Vitis vinifera L.) were grown for 13 weeks in controlled environment growth cabinets, and then the number of bunch primordia present in buds was counted after dissection. Sultana (S) and Ohanez (0) had very few bunch primordia under all conditions. Of the other cultivars all were barren after growth at 15° C, but Rhine Riesling (R) and Shira". (Sh) had some primordia at 20° C, and Gordo (G) at 25° C. Primordia number of R, Sh and G increased with increase in temperature up to approximately 35° C. R had most at all temperatures. At a light intensity of 900 foot candles (f. c.) R had some primordia, but other cultivars were barren. At 1800 f. c. both R and G had a moderate number of primordia, but Sh was barren. At 3600 f. c. R, G and Sh all had fruitful buds. R had the greatest number of primordia at each intensity. R and Sh had more primordia when 16 hours of light per day were given continuously than when in two portions separated by 4-hour dark periods. S and O produced very few bunch primordia under any of the conditions tested. Evidence is presented that this result was due to a later differentiation of bunch primordia in buds of these cultivars, such that primordia were still unrecognisable at 13 weeks. Of the other cultivars, R had more primordia at all temperatures and light intensities; each cultivar performed well at high temperature and light intensity, but only R performed well under poor conditions.