Sorghum is grown between latitudes of 42 °N and 42°S. Many of the sorghums grown at low latitudes less than 14 °N and S are highly photoperiod sensitive. Sorghum is a short-day plant and flowering is advanced under short-days. Techniques that can rapidly advance flowering will help breeding programs aimed at improving sorghum for the post-rainy (rabi) season, by advancing two generations a year instead of the one that is currently possible. This would double the rates of genetic gain per year. This study was designed to enhance generation advance in photoperiod and post-rainy sorghums under short days. The objectives of this project was to develop 1): techniques to produce rapidly the seeds for generation advance in photoperiod sensitive sorghum and 2): more lines advanced to next generation in a cost effective and adaptable manner in limited time and space.
Experiments were carried out during 2000-2002 under glasshouse conditions at the International Crops Research Institute for the Semi-Arid Tropics, Patancheru, India. Shortday treatments (8h day length) were imposed by covering the plants from emergence to 35 days after emergence (DAE) with a manually operated blackout facility of 3m long by 1.7m wide by 1.5m high. To determine the optimum time for short-day treatment, experiments were conducted at various periods during the year. Fifteen genotypes ranging in their photoperiod sensitivity and representing possible photo-period responses of cultivated sorghum were used in this study. Though flowering was induced under short day treatments, growing sorghum plants up to maturity in small pots was not effective. Plants showed both depressed growth and nutritional disorders despite receiving adequate nutrients. Therefore, seedling were grown in small Styrofoam cups (6.5 cm diameter of 150 cc) under short-day treatments up to 35 DAE and then seedlings were transplanted into big pots (33 cm diameter of 13,200 cc) or in field and grown until maturity.
Flowering was advanced by short-day treatment in most genotypes. However, the plantings where the mean temperatures during the panicle development were greater than 28°C was not effective in seed set. In these experiments the daily mean temperatures exceeded the reported optimum temperature for sorghum and resulted in delayed flowering under short day treatment. Further in some genotypes the spike-lets were sterile when the daily mean temperatures during panicle development exceeded the optimum temperature. Therefore, any period (except April and May in this study) where the daily mean temperatures during the year do not exceed 28°C is recommended for short day treatment for inducing flowering in sorghum. To accommodate more plants in the short day treatment, it was decided to grow the seedling in small Styrofoam cups up to 35 DAE under short days and then transplant them into bigger pots or into field. Transplanting by uprooting usually sets back the plant growth and delays flowering in sorghum. However, in these experiments the intact transplanting of plants from the Styrofoam cups showed no such depression.
• Any period (except April and May in this study) where the daily mean temperatures during the year do not exceed 28 °C is suitable for short-day treatment to advance time to flowering in sorghum. 2nd April 2001 2
• Short-days (8h day-length) cycles from emergence to 35 DAE are sufficient to induce flowering in most photoperiod sensitive genotypes.
• Short-day treatments can be given to plants grown in small pots (in Styrofoam cups) and transplanted at 35 DAE to either big pots or field.