Managing Growth and Flowering of Geranium under Limited Growth Environment

Cultivation of ornamental plants in terrariums is common, but the use of flowering plants in such environment is difficult and rarely seen. Common geranium (Pelargonium hortorum L.H.) is the most well-known potted and garden plant in the top of 25 popular world's market rankings. Today, one of the main goals of commercial producers is the production of the uniform plants in terms of morphological traits such as uniformity in the height that needs some management. To achieve this objective plant genetic potential, managing the growth and environmental factors, restrictions on root environment, water and nutrition to a level that does not affect the quality as well as application of chemical plant growth retardants (PGRs) could be considered. Paclobutrazol (PBZ) commercially known as Bonzi and chlromequat chloride (CCC) known as cycocel are commonly used to control the height of some pot plants. Additionally, Benzyladenine (BA) as a synthetic cytokinin can influence the growth characters of plants. This experiment conducted to evaluate the effect of paclobutrazol, cycocel and benzyladenine on the growth and flowering of geranium as a flowering terrarium plant. The effect of PBZ, CCC and BA evaluated as a factorial experiment based on CRD on the growth and flowering of geranium (Pelargonium hortorum L.H. Bailey ‘Horizon’) as terrarium plant. At first, geranium seeds planted in tray cells as plugs filled by sieved black peat to below 2 mm in early autumn, Oct. 2015. Seedlings were grown in a greenhouse conditions with an average day/night temperatures of 25/20 ± 2 °C, nourished with 1:2 ratio diluted Hoagland nutrient solution as irrigation demand, until 2-4 leaves stage then transferred into terrarium containers (with 20 cm of middle diameter and 15 cm of height). Inside container relative air humidity was about 75±5% and growing environment light intensity (PPF) was about 500 µmole m-2S-1. Irrigation and nutrition also applied at seedling stage in terrarium. Terrarium glass container totally filled to a quarter of volume considering a drainage layer of gravel, activated charcoal, a layer of substrate barrier (plastic net), potting soil containing of 20 vol.% sieved field loam soil, 10 vol.% of fine perlite and 70 vol.% of sieved black peat. PBZ at the levels of 0, 25 and 50 ppm and CCC at the levels of 0, 1000 and 2000 ppm applied one month after transplanting (Feb. 2015) and BA treatment for flowering management applied at the levels of 0, 50 and 100 ppm four months after transplanting (May 2016). Growth and flowering characters evaluated thoroughly while root and shoot fresh and dry weight, photosynthetic pigments of chlorophyll, carotenoids, and anthocyanin index assessed at the end of the experiment. Both growth retardants PBZ and CCC led to a significant reduction in plant height in high concentrations. Thus, the effect of PGRs on plant height was significant, while the effect of BA and its interactions by PGRs on this trait was not considerable. Effective treatments on this trait were PBZ in concentration of 50 ppm and then CCC in 2000 ppm. In particular, the use of these concentrations without BA treatment led to the shortest plants. Comparison of plants cultivated in the pot and terrarium conditions showed that the growth conditions had a considerable and significant impact on plant height and growth. Stem diameter, number of leaves and leaf area significantly reduced by PBZ compared to the control, but CCC did not show a significant effect on these traits. Smaller stem diameter occurred through 25 ppm of PBZ together with 50 ppm of BA. Application of PBZ especially at 25 ppm resulted in a significantly reduced number of plant leaves and leaf area compared to the control and application of CCC. Application of CCC at the level of 2000 ppm combined with BA of 50 ppm caused to a significant increase of leaf area compared to the control. Results on the number of lateral branches showed that application of PGRs had no effect on this character, while restriction of growth in terrarium conditions led to decrease in the number of lateral branches. Number of lateral branches raised by application of BA and CCC, while less number of branches observed with PBZ treatments especially at the level of 25 ppm. Chlorophyll and anthocyanin content of leaves decreased by both retardants. Days to flowering shortened by PBZ treatment of 50 ppm and slightly by CCC treatment of 1000 ppm in terrarium conditions. In general, flowering process accelerated via these treatments, while PBZ of 25 ppm and CCC of 2000 ppm delayed the flowering of plants compared to the control. The acceleration effect of 50 ppm PBZ was superior to the effect of 1000 ppm of CCC. The effect of BA on flowering time was insignificant despite of initial prospect. Finally, the photosynthetic pigments, leaf area and stem diameter increased because of BA, while flowering characters not influenced by means of BA. In general, 50 ppm of PBZ and without BA treatment was able to improve production characters of geranium plants in terrarium conditions. The goal of this research was managing the growth and flowering of geranium in the restricted terrarium conditions by PGRs. It was found that treatment of plants by 50 ppm of PBZ could properly control the plant height and whereas positively accelerated flowering without and negative side effects on the plant performance. It seems that a good hormonal balance performed by this concentration of PBZ compared to CCC and BA. Early flowering is a positive quality trait for the most flowering ornamental plants. However, BA application itself and in interaction with CCC could enhance the photosynthetic pigment contents and thus improved the growth characters but it could not influence flowering traits even though delayed the flowering, significantly. Restriction of the root area via planting in terrarium could considerably limit the vegetative growth characters and delayed the flowering compared to the potted plants.