Phase change in citrus: The effects of main stem node number, branch habit and paclobutrazol application on flowering in citrus seedlings

Abstract

SummaryTwo studies were carried out to identify the effects of several factors including plant size (defined as the number of main stem nodes), growth habit and paclobutrazol application on the length of the juvenile period of citrus seedlings. In the first group of experiments, seedlings were grown from seed of ‘West Indian’ lime (Citrus aurantifolia (Christm.) Swingle), ‘Marsh’ grapefruit (Citrus paradisi Macf.) and ‘Meiwa’ kumquat (Fortunella × crassifolia Swingle (pro sp.)). Seedlings were initially grown under rapid-growth controlled environment conditions of 30/25°C day/night temperatures, 16 h days of 650 µmol s-1 m-2 and high relative humidity for 5–8 months and maintained as single stemmed plants until the main stems had more than 100 nodes. They were then treated with one of four rates of paclobutrazol as a root-drench and transferred to mild environmental conditions in a greenhouse (25°C maximum/15°C minimum) to assess subsequent branch development and flowering response. ‘West Indian’ lime and ‘Meiwa’ kumquat seedlings treated with paclobutrazol flowered within 11 months from germination, but plants without paclobutrazol treatment did not flower until 14 months. ‘Marsh’ grapefruit did not flower during the period of the study (14 months). Paclobutrazol application reduced the number of branches, reduced branch internode length and, in ‘West Indian’ lime, increased the intensity of flowering. In the second group of experiments, ‘West Indian’ lime and ‘Meiwa’ kumquat seedlings were grown to sizes of 32, 52, 82 and 102 main stem nodes under rapid-growth conditions where they were maintained either as single- stemmed or as branched 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 profusely. Some 52-node seedlings of both species flowered, but only those ‘West Indian’ lime 52-node seedlings that had been treated with paclobutrazol did so. No 32-node seedlings of either species flowered. Floral branches were formed predominantly in the apical regions of all seedlings. There were no differences in the final numbers of either vegetative or floral branches between the seedlings which were originally grown either as branched or as single-stemmed seedlings. Recommendations are made for applying these results to achieve rapid generation turnover in citrus breeding programmes.