PHASE-CHANGE IN CITRUS - THE EFFECTS OF MAIN STEM NODE NUMBER, BRANCHHABIT AND PACLOBUTRAZOL APPLICATION ON FLOWERING IN CITRUS SEEDLINGS

Two studies were carried out to identify the effects of several factor s including plant size (defined as the number of main stem nodes), gro wth habit and paclobutrazol application on the length of the juvenile period of citrus seedlings. In the first group of experiments, seedlin gs were grown from seed of 'West Indian' lime (Citrus aurantifolia (Ch ristm.) Swingle), 'Marsh' grapefruit (Citrus paradisi Macf.) and 'Meiw a' kumquat (Fortunella x crassifolia Swingle (pro sp.)). Seedlings wer e initially grown under rapid-growth controlled environment conditions of 30/25-degrees-C day/night temperatures, 16 h days of 650 mumol s-1 m-2 and high relative humidity for 5-8 months and maintained as singl e stemmed plants until the main stems had more than 100 nodes. They we re then treated with one of four rates of paclobutrazol as a root-dren ch and transferred to mild environmental conditions in a greenhouse (2 5-degrees-C maximum/15-degrees-C minimum) to assess subsequent branch development and flowering response. 'West Indian' lime and 'Meiwa' kum quat seedlings treated with paclobutrazol flowered within 11 months fr om germination, but plants without paclobutrazol treatment did not flo wer until 14 months. 'Marsh' grapefruit did not flower during the peri od of the study (14 months). Paclobutrazol application reduced the num ber of branches, reduced branch internode length and, in 'West Indian' lime, increased the intensity of flowering. In the second group of ex periments, 'West Indian' lime and 'Meiwa' kumquat seedlings were grown to sizes of 32, 52, 82 and 102 main stem nodes under rapid-growth con ditions where they were maintained either as single-stemmed or as bran ched plants. Paclobutrazol was applied once the seedlings had reached the designated main stem node numbers. Most 82- and 102-node seedlings in both the paclobutrazol-treated and control groups flowered profuse ly. Some 52-node seedlings of both species flowered, but only those 'W est Indian' lime 52-node seedlings that had been treated with paclobut razol did so. No 32-node seedlings of either species flowered. Floral branches were formed predominantly in the apical regions of all seedli ngs. There were no differences in the final numbers of either vegetati ve or floral branches between the seedlings which were originally grow n either as branched or as single-stemmed seedlings. Recommendations a re made for applying these results to achieve rapid generation turnove r in citrus breeding programmes.