公園綠地平戶杜鵑維護管理技術之改進

Abstract

杜鵑花(Rhododendron spp.)種類眾多且花期長，於景觀上常運用於盆栽、綠籬花屏及群植花叢等，且為許多國家和城市之國花與市花。杜鵑花為臺灣大學之校花，校園栽植以平戶杜鵑(Hirado azalea)為主，其重要的品種有豔紫杜鵑(R. pulchurm Sweet)、粉白杜鵑(R. mucronatum G. Don cv. Akemono)及白琉球杜鵑(R. mucronatum G. Don)等。每年3月天氣回暖時，臺灣大學之椰林大道兩旁的杜鵑花盛開，因而有「杜鵑花城」美譽。近年來氣候較往年相對異常，影響校內杜鵑花的生長及開花，而溫度、光線、水分及肥培管理，皆為重要的影響因子。本研究目的探討平戶杜鵑維護管理之水分、肥料管理及溫度對生長與開花之影響，以期改善植栽生育品質並預測花期。 在水分管理方面，藉由介質水分含量測定儀(water content, electrical conductivity and temperature sensor, WET)定出杜鵑花栽培之適宜介質含水量(volumetric water content, VWC)。校園群簇花叢土壤田間容水量VWC約為60%，當VWC小於18%時，植株出現萎凋的現象，推測VWC小於田間容水量的三分之一前需灌溉。此外，以五寸盆平戶杜鵑為材料，於臺大園藝分場溫室進行介質含水量試驗，以泥炭土：真珠石=3:1(v/v)為介質，藉由WET測定VWC，其田間容水量約為70%，並調查植株外觀形態及葉片水勢，得知平戶杜鵑達暫時凋萎點時，其介質VWC約為20%、葉片水勢為-1.38 MPa，起灌點約23% VWC，此時葉片水勢為-1.03 MPa、第一片完全展開葉與枝條角度為75˚。而豔紫杜鵑在17% VWC下，淨光合作用、蒸散作用速率及葉片水勢均顯著低於在23%、30%及45% VWC處理下之植株。於17% VWC復水處理栽培六週後，其第一片新完全展開葉之葉片厚度最薄(0.39 mm)，且葉綠素計讀値(chlorophyll meter reading, CMR)最高，已影響豔紫杜鵑之生理。故推薦平戶杜鵑栽培介質含水量不宜低於23%VWC，否則長期可能造成其生長不佳與品質下降。 肥料管理方面，氮是影響植物生長的主要元素，豔紫杜鵑之葉綠素計讀值與葉片氮濃度有高度相關，可利用葉綠素計讀值推估葉片的氮濃度，然而葉綠素計讀値(chlorophyll meter reading, CMR) 3/1比色法較無法對豔紫杜鵑進行明確地氮素診斷。杜鵑花喜偏酸性土壤，調查顯示校園部分土壤之酸鹼度偏高，鋅與鐵元素的有效性降低。而缺鋅或缺鐵會導致平戶杜鵑新葉出現黃化之現象，可由黃化位置和葉面積判定。缺鋅和缺鐵的新葉皆會黃化，而缺鋅主要出現在脈間，有小葉之現象；缺鐵者，葉面積與正常葉片一樣。若平戶杜鵑出現缺鐵症狀，建議連續澆灌螯合鐵(Fe(III)-Ethylenediaminetetraacetic acid, Fe-EDTA) 或硫酸亞鐵(Ferrous Sulfate, FeSO4) 4 mg∙L-1兩週，即可改善缺鐵黃化恢復綠色。若考量成本，可葉施2 mg∙L-1 或4 mg∙L-1 之Fe-EDTA或FeSO4四週，能改善新葉缺鐵黃化之現象，但無法完全改善已出現缺乏症狀之葉片。 預測花期方面，藉由2013年至2015年調查臺大校園平戶杜鵑之數據和氣象資料，估算花苞寬為5.8 mm至開花率50%之度積溫。比較推估與實際之盛花期，利用變異係數(Coefficients of variations; CV)之最小值作為基本溫度標準計算者，3年的累積誤差天數為6天；利用標準偏差(Standard deviation; STDEV)作計算者累積誤差天數為7天。故以CV值之基本溫度12 oC估算，當度積溫為318.7 oC時，可達盛花。 綜合上述研究，水分管理部分，藉由WET測定介質VWC，土壤水分達田間容水量的三分之一、並配合觀察第一片完全展開葉與枝條角度為75°時建議進行灌溉。肥料管理方面，可由葉片黃化位置和葉面積判定平戶杜鵑為缺鋅或是缺鐵，而施用Fe-EDTA或FeSO4可改善缺鐵造成的新葉黃化的現象。豔紫杜鵑之葉綠素計讀值與葉片氮濃度有高度相關，可利用葉綠素計讀值推估葉片的氮濃度。花期預測方面，以CV值之基本溫度12 oC估算，當花苞寬為5.8 mm到開花率50%的度積溫為318.7 oC時，可達盛花。

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.