Effects of photoperiod on reproductive development of peanut (Arachis hypogaea L.) in a cool subtropical environment. I. Field studies

The effects of photoperiod on reproductive development and yield of two Virginia, one Spanish and one Valencia peanut (Arachis hypogaea L.) cultivar were investigated in the field at Kingaroy, Australia. The objective was to assess the importance of photoperiod as a limitation to post-flowering reproductive development in a cool subtropical environment. A total of five experimental sowings were made between the 1986/87 and 1988/89 growing seasons. Treatments consisted of either natural photoperiods (ranging from 11 to 14 h, depending on sowing date and growth stage) or long (16 h) photoperiods during a range of phenological stages, both pre- and post-flowering. Long photoperiods were achieved by using low intensity light from incandescent bulbs with equal extension periods in both morning and evening. Different sowing dates and seasons were used to produce a range of natural photoperiods, radiation and temperature regimes, while shadecloth was used to alter incident irradiance during the 1988/89 experiments. Photoperiod responses were significant in only one experiment (S2, 1986/87), and were due to effects of treatment during the immediate post-flowering period. Numbers of flowers, pegs and pods were reduced under long photoperiods in cv. Robut 33-1, and to a lesser extent, in cv. White Spanish. The Virginia cv. Uf 781 14-5 was unaffected. The effects of long photoperiods during this 30 day post-flowering treatment were not expressed until after the treatment period. Effects were relatively small and reproductive (pod) yields at maturity were not significantly reduced. The lack of strong photoperiod responses in the reportedly sensitive Valencia cv. NC17090 and the occurrence of the strongest response in the relatively insensitive cv. Robut 33-1 were unexpected. Analysis of climatic data from these experiments, and others reported in the literature, suggested interactions between photoperiod and temperature, with photoperiod effects being only significant at higher temperatures, i.e. in our studies, when mean daily temperature during the treatment period was 26-0�C. This finding is of considerable significance in assessing the importance of photoperiod sensitivity for cultivar adaptation to long day/subtropical environments. A subsequent paper in this series examines photoperiod x temperature interactions more closely under controlled environment conditions.