| Summary: | 碩士 === 國立臺灣大學 === 園藝學研究所 === 103 === Azalea (Rhododendron spp.) is a showy-flowering woody plant family that consists of various horticultural cultivars. They are not only widely used as potted plants, fence and landscape bushes, but also are the symbols of some countries and cities.
Four species of Hirado azalea, including the R. pulchurm Sweet, R. mucronatum G. Don cv. Akemono and R. mucronatum G. Don, that grown on campus of National Taiwan University as the official university flower. The blooming season of azaleas in March decorate the campus with splendid, colorful showy flowers. Recently, the climate change as well as temperature, light, water availability and soil fertility variations have altered the growth habits and flowering of azaleas on campus. This study investigated the effects of soil water availability and fertility on the growth conditions, and the effect of temperature on the flowering period of azaleas, to gain the optimum growth conditions hoping that may enhance the blooming quality and predict the flowering period of azalea on the university campus.
First, water content electrical conductivity and temperature sensor (WET) was used to determine the volumetric water content (VWC) of the growth medium for R. kiusianum. The result showed that the VWC of the campus soil field capacity was approximately 60%. Plants exhibited wilted leaves as VWC declined below 18%. Thus, plants were suggested to be irrigated when VWC declines lower than one third of the field capacity. Additionally, a VWC experiment was conducted in the greenhouse affiliated to the National Taiwan University Horticultural Farm using 5-inch potted azaleas as the plant materials. The field capacity of the growing medium peat:perlite=1:1 was found to be 70%. When the potted plants reached the temporal wilting point at VWC = 20%, the leaf water potential = -1.38 MPa. The irrigation point was approximately at VWC=23%, with the corresponding leaf water potential of -1.03 MPa and 75 degree angle between the first expanded leaf and the shoot. On the other hand, when the VWC of R. pulchurm reached 17%, the net photosynthetic rate and the transpiration rate were significantly lower comparing to the plants under the conditions of VWC=23%, 30% and 45%. In addition, after the six-week cultivation in which the plants were fully irrigated only when VWC declined to 17%, the first new fully-expanded leaf was the thinnest (0.39 mm), accompanied by the highest chlorophyll meter reading (CMR) value, which had affected the physiological conditions of R. pulchurum. Thus, the VWC of R. kiusianum shall not be lower than 23%, otherwise negative effects may appear on the further growth of R. kiusianum
At the aspect of soil fertility condition, azaleas prefers acidic soil. Our results, however, showed that parts of the campus soil had high pH, which inhibited the availability of zinc and iron that caused the leaf chlorosis of azaleas. Chlorosis position and leaf area change may be used to determine which element is insufficient, i.e. insufficient zinc lead the inter-veinal chlorosis, accompanied with the leaf area decreasing; insufficient iron did not affect leaf area but would lead the whole new leaf chlorosis. To solve the chlorosis caused by insufficient iron, the plants were suggested to be continuously irrigated with 4 mg/L of Fe-EDTA or FeSO4 for two weeks. Foliage spray 2-4 mg/L of Fe solutions were recommended when the budget is limited, but such a movement might not completely enhance the situation for the leaves that already had chlorosis. In addition, nitrogen is the one of the major elements that affects the plant growth. The leaf nitrogen concentration may be determined by CMR. However, CMR3/1 colorimetry was not able to effectively determine the nitrogen concentration for R. kiusianum.
Based on a 3-year meteorological data from 2013 to 2015, the estimation of the heat unit accumulation from 5.8 mm-sized flower bud to 50% full blooming can be used to predict the flowering period for R. kiusianum. When the minimum value (12 oC) obtained from the Coefficients of variations (CV) was used as the base temperature of the heat unit accumulation, the predicted and the actual blooming period had 6-day differences within 3 years. In contrast, when the base temperature of heat unit accumulation was obtained from Standard Deviation (STDEV), the predicted and the actual blooming period had 7-day differences within 3 years. Therefore, the heat unit accumulation estimated with CV was more precise, which showed that the azaleas on campus reaches full blooming when the heat unit was 318.7 day∙oC.
In conclusion, the azaleas on the National Taiwan University campus are recommended to be irrigated once the soil water content reaches one third of field capacity, or when the angle between the first fully-expanded new leaf and the shoot reaches 75 degree. The determination of leaf chlorosis position allows one to know whether zinc or iron is insufficient. Applying Fe-EDTA or FeSO4 may enhance the leaf chlorosis caused by insufficient iron. Plus, The CMR of R. kiusianum is highly correlated to leaf nitrogen concentration. Finally, the R. kiusianum on campus were found full blooming when the heat unit accumulation was 318.7 day∙oC.
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