The experiments were done at As, in Norway (60 degsN), where flower initiation normally occurs in early August, at least a month later than in southern England (51 degsN). Single-stemmed plants of various cultivars, including 'Ben Tron' and 'Ben Hope', were propagated from semi-softwood cuttings and then moved to day-lit, controlled-environment rooms on 18 May. There they were grown at 9, 15 or 21 degsC until 21 September.
New leaves were formed faster with increasing temperature in summer but stem growth also ceased earlier. Growth began to slow by 20 July at 21 degsC and this stage was reached approximately two weeks later at 15 degsC and about four weeks later at 9 degsC.
Growth then ceased after a further two weeks and flowers began to be formed. This occurred in mid August at 21 degsC followed about one-to-two weeks later at 15 degsC, but flower formation was very delayed at 9 degsC. Warm temperatures in summer evidently directly promoted flower initiation but they also indirectly promoted it by promoting growth so that plants became adult earlier.
Dr Ken Cockshull, Associate Fellow, Warwick Crop Centre, University of Warwick
Effects of Summer Temperature on Growth & Flowering in Six Black Currant Cultivars (Ribes nigrum L.) by Sonsteby, Opstad and Heide (2012). Journal of Horticultural Science & Biotechnology 87 (2): 157-164. The abstract of the authors' manuscript can be seen in full at www.jhortscib.com.