Prevention of high temperature induced rosette and flowering acceleration of Dwarf Eustoma ‘Little Belle Blue’ by optimum temperature treatment

The objective of this research is to produce dwarf Eustoma grandiflorum (Raf.) Shinn. ‘Little Belle Blue’ potted plants to flower in 130 days after sowing without exhibiting high temperature induced (HTI) rosette. High temperature induced rosette is defined as “the abnormal growth and development of plants exhibiting a compressed stem near the ground after forming 3 – 4 pairs of true leaves resulting from exposure of seedlings to high temperatures, and fail to form elongated internodes and to flower”. The effect of temperature and photoperiod treatments during various growth and development stages was tested on the accelerated flowering without exhibiting the HTI rosette (Expt. 1, 2, 3), and to induce flowering in 130 days preventing the HTI rosette (Expt. 4, 5, 6). The treatments at 32/27 °C (day/night) during 0 – 30 days after sowing (0 – 30D) (32/27 °C/0 – 30D and 27/22 °C/31 – 60D should be avoided because 39% of plants exhibited HTI rosette (Expt. 1). Lower temperatures and shorter periods (24/22 °C/61 – 90D, 24/22 °C/31 – 60D) prevented rosette growth (Expt. 2). The treatment 23/21 °C/34 – 74D is considered acceptable when coupled with 21/19 °C/74 – 115D to promote flowering and to prevent the HTI rosette (Expt. 3). The earliest flowering (157 days) plants without the HTI rosette suggested that temperatures during various 20-day stages are 25/23 °C/0 – 20D, 21/19 °C/21 – 40D, 12/10 °C/61 – 80D, and 25/23 °C/81 – 100D. Early flowering occurred at 12/10 °C/61 – 80D and 25/23 °C/81 – 100D. During these periods, it was considered that flower bud initiation and development might occur, and this effect was especially pronounced in the treatment 25/23 °C/61 – 80D (Expt. 4). Flowering was the earliest (148 days) when 45-day old seedlings after sowing seeds in January were exposed for 30 days (46 – 75D) to a long-day (LD) photoperiod at 21/18 °C (Expt. 5). The total cropping time was reduced to 133 days without inducing HTI rosette and reducing the number of flowers by the following treatments: Sowing seeds as late as February and growing at 23/21 and 25/23 °C/30 – 50D, either under a short-day (SD) or LD/51 – 70D, and growing at 21/19 and 23/21 °C under LD/71 – 90D (Expt. 6). A clear distinction should be made between seedlings exhibiting physiologically normal growth during the vegetative juvenile stages that last about 60 days forming up to 3 to 4 pairs of small leaves and HTI rosette that occurs predominantly when seedlings are exposed to 29/27 °C/34 – 74D. Eustoma grandiflorum is an annual that does not exhibit seed dormancy since germination occurs in 12 – 14 days at 23/21 °C, and ‘Little Belle Blue’ does not require vernalization to induce flowering in 130 days after sowing seeds without exhibiting HTI rosette. Seeds are sown at 21/16 °C/0 – 30D in February and growing at 25/23 °C/31 – 50D, 21/19 °C/51 – 70D/LD, 23/21 °C/71 – 90D/LD, and finally at 23/21 °C/91 – 130D is recommended.