This study was carried out in the nursery of the Institute of Forestry and Environmental Sciences, University of Chittagong, Bangladesh. The nursery is situated at the intersection of 22°30’ N latitude and 91°50’ E longitude, which enjoys typically tropical climate, characterized by hot humid summer and dry winter. Mean monthly temperature varies from 21.8°C to 29.2°C in summer and 15°C to 26°C in winter. Relative humidity is the lowest (64%) in February and highest (95%) in June through September. Annual rainfall in the area is about 3000 mm that mostly takes place between June and September.
Stock Plant Management and Shoot Production Cuttings were collected from the hedge orchard established with vegetative propagules from a 20-year old P. neriifolius tree in the nursery. Branches of the stock plants in the hedge orchard were trimmed at the beginning of the study in March, for shoot production. The sprouted juvenile shoots developed on the trimmed branches were collected for cutting preparation in June. Shoots were soaked in water immediately after getting them separated from the stock plants and brought to the laboratory for further processing.
Preparation of Cuttings and Setting in the Propagator for Rooting Two to four nodal cuttings with two leaves trimmed to one third were used for rooting trials. Cutting length (5.8 - 6.6 cm) and diameter (3.5 - 3.7 mm) were kept indifferent to avoid possible non-treatment variation among the treatments. The cuttings were then briefly treated with fungicide, Diathane M45 (Rohm & Co. Ltd., France; 2g L-1 of water), to avoid potential fungal infection and kept in shade for 10 minutes. Effects of exogenous rooting hormone IBA on the rooting ability of the cuttings were explored by treating the cuttings with 0%, 0.2% (2000 ppm), 0.4% (4000 ppm) and 0.8% (8000 ppm) (w/v) IBA solutions. Cutting bases were dipped into the IBA solution for one minute and planted into rooting media (perforated plastic trays filled with coarse sand mixed with fine gravel at a ratio of 4:1) and finally placed into a nonmist propagator for rooting. The trays with cuttings were arranged in the propagator following randomized complete block design.
In the design, 30 cuttings were assigned to each of the four treatments (0%, 0.2%, 0.4% and 0.8% IBA solution). Each block was replicated thrice making a total of 360 cuttings in the experiment. Cuttings of each treatment (90 cuttings) were then planted in 9 trays, 3 trays (10 cuttings each) for each replication. The cuttings were watered once only just after setting them in the propagator. A light water spray was done every day early in the morning (before 7 am) and late afternoon (after 6 pm) until transferring the rooted cuttings from the propagator.
Propagator Environment Relative humidity of around 85-95% was maintained in the propagator. Propagator was kept open for a short period of time once in the morning (before 7:0 am) and once in the afternoon (after 6:0 pm) every day for facilitating gas exchange. A bamboo shed lined with jute mat was placed over the propagator to avoid excessive solar heat. Thus, the photosynthetic photon flux measured with quantum sensors (SKP 215, Skye Instruments Ltd., UK) and data logger (Datahog2, SDL5360, Skye Instruments Ltd., UK) inside the propagator was reduced to 12% of the full sun. During the rooting experiments, mean maximum and minimum temperature ranged between 31°C and 23°C.
Transferring the Rooted Cuttings to Polybags The cuttings rooted after 14 weeks of setting them in rooting media in propagator. The rooted cuttings were weaned (hardened) before transferring them to polybags by keeping the propagator open at night for three subsequent days and at day and night for another three days. After weaning, all the rooted cuttings were transferred into polybags filled with forest soil mixed with decomposed cow dung at a ratio of 3:1. Before planting into the polybags, rooted cuttings were assessed for length and diameter, root and shoot number and the length of longest root and shoot developed in each cutting. After transferring the rooted cuttings into the polybags, they were kept under shade for one week and then placed under sun for growing. One year after transferring the rooted cuttings into the polybags, survival percentage and total height of each rooted cutting were measured for their growth assessment.
Data Recording and Statistical Analysis Mean growth variations attributed to treatments were evaluated using the analysis of variance (ANOVA) and Duncan’s Multiple Range Test (DMRT). Rooting percentages were adjusted accordingly using arcsine transformation formula before placing the data into analysis of variance. Rooting percentages, number and length of roots and shoots developed for control and the treatments were compared at p≤0.05 (ANOVA and DMRT).