介質含水量與鹽度對‘V3’蝴蝶蘭根部構造和生長的影響

Abstract

蝴蝶蘭以水苔作為主要的栽培介質。栽培過程中根包覆於水苔內，對於水苔的環境變化較地上部敏感。當根生長不佳，會限制水分和養分的吸收，而根外觀不佳，也降低商品的價值。本研究探討蝴蝶蘭氣生根和介質根的構造差異，並比較蝴蝶蘭根部在不同水苔含水量及施用不同肥料濃度下構造和生長的差異。 氣生根因懸垂於空氣中，相較於介質根，內根被的細胞壁厚度增加55%，而外皮層細胞壁厚度亦較厚，且隨根部成熟度增加，有增厚的趨勢，可保護根部免於機械性傷害並增加保水能力。而木質部的數量和內皮層通道細胞數較介質根多，可加速水分的運輸效率。介質根附著於水苔表面，內根被和外皮層細胞壁較氣生根薄，且外皮層的通道細胞數增加，根部的分化傾向於增加水分和養分的吸收。研究中發現介質根的內皮層細胞壁厚度大於氣生根，於後續試驗中發現，蝴蝶蘭根部在越高鹽度的水苔內生長，內皮層細胞壁越厚，因此推測介質根為了適應鹽度較高的環境，內皮層細胞壁較氣生根厚。 蝴蝶蘭在含水量恆定於20%、30%、40%、50%或60%下的水苔中栽培。隨含水量上升根部的乾重下降，以含水量60%的處理根部的生長最差，較含水量20%的處理乾重下降20%。將根部品質分為1至5級，以第5級最佳。隨水苔含水量由20%增加到60%，根部品質從4.5級下降到3.8級。而水苔含水量對地上部的生長量則無影響。隨水苔含水量提高，根半徑、根被厚度、外皮層通道細胞數、皮層厚度、皮層的細胞層數、內皮層通道細胞數、中柱直徑、木質部的數量和面積皆有下降的趨勢。而外皮層和內皮層的細胞壁厚度不受水苔含水量影響，厚度只隨根部愈成熟而增加。根的保水能力以水苔含水量60%的處理最差，而水苔含水量20%的處理根保水能力最佳，顯示生長於高水苔含水量的環境，根的保水能力下降。 蝴蝶蘭施用0、100、200、400或800 mg∙ L- 1 N五種不同濃度之液肥(15N-2.2P-12.5K; Peters Excel 15-5-15)。根部的生長量以肥料濃度100 mg∙L-1 N的處理較佳，且根部品質達3.9級，而隨施用的肥料濃度提高，根的生長量下降。800 mg∙L-1 N處理植株根部的鮮重及乾重最低，和100 mg∙L-1處理相比下降40%。當肥料濃度由0增加至400 mg∙L-1 N，地上部的生長量提高，但800 mg∙L-1 N的處理對植株產生鹽分逆境，地上部的生長量下降。隨施用的肥料濃度由0上升至400 mg∙L-1 N，根半徑和皮層厚度於根尖以上1、3、5、7 cm處增加，而當施用的肥料濃度上升至800 mg∙L-1 N，產生高鹽分逆境毒害植株，造成根生長畸形，於根尖以上1 cm和3 cm處隨施用的肥料濃度提高根半徑和皮層厚度增加，而根尖以上5 cm和7 cm處厚度則下降。澆灌肥料的濃度越高，內皮層細胞壁厚度越厚，推測其功能為防止過多鹽分進入中柱運送至地上部。外皮層通道細胞數和內皮層通道細胞數隨施用的肥料濃度上升而增加，可加速水分運輸。中柱直徑只在根尖以上1 cm處隨肥料濃度提高而增加，顯示肥料濃度對中柱直徑的影響較小。根被厚度、外皮層細胞壁厚度、木質部的數量和面積則不隨施用的肥料濃度增加而改變，且不同肥料濃度處理不會影響根的保水能力。 綜合試驗結果發現，水苔含水量維持在20%和30%蝴蝶蘭的生長勢較良好，而施用的肥料濃度則以100 mg∙L-1 N (EC = 0.66 dS∙m-1)的處理植株生長較好。蝴蝶蘭屬附生蘭，可藉由根部構造的改變以適應水分逆境，研究結果發現蝴蝶蘭在缺水乾旱和高鹽度導致的生理性乾旱下，植株根部構造分化的趨勢不相同。在缺水乾旱下，蝴蝶蘭的根被厚度增加以保留較多的水分，增加外皮層和內皮層的通道細胞數量、木質部的數量和面積以加速水分的吸收和運輸；而生理性乾旱下，蝴蝶蘭根部的外皮層和內皮層通道細胞數量增加，以加強水分的吸收和運輸效率。

Sphagnum moss has been used as the main substrate for Phalaenopsis cultivation. During cultivation, roots are covered with substrate; thus, roots are more sensitive than shoots when substrate condition changes. Weak root growth limits water and nutrient uptake, and appearance of inferior roots reduces its commercial value. In this study, we compared the anatomical differences between aerial roots and substrate roots of Phalaenopsis Sogo Yukidian ‘V3’, and explored root anatomy and growth of Phalaenopsis under various substrate moisture and salinity levels. Aerial roots suspended in the air, compared with substrate roots, the cell wall thickness of endovelamen increased by 55%. The cell wall thickness of exodermis of aerial roots also increased as the root matured. The change of these structures in aerial roots can protect the root from mechanical damage and increase water retention capacity. The number of xylems and the number of passage cells of endodermis in aerial roots increased, which could accelerate the water conduction. When roots grew in the substrate, the cell wall thickness of endovelamen and exodermis reduced but the number of passage cells of exodermis increased. The differentiation of substrate roots tended to increase the absorption of water and nutrients. The cell wall thickness of endodermis of substrate roots was more than that of aerial roots. In the following experiment, the cell wall thickness of endodermis increased as the salinity of the substrate increased. In order to adapt to a high salinity substrate environment, the cell wall thickness of endodermis of substrate roots was more than that of aerial roots. Phalaenopsis were cultivated in the sphagnum moss which its volumetric water content (VWC) maintained at 20%, 30%, 40%, 50%, or 60%. Root dry weight decreased as the VWC increased. The root dry weight was reduced by 20% in 60% VWC treatment compared with 20% VWC treatment. Root quality was graded from 1 to 5 with grade 5 being the best. Root quality decreased from grade 4.5 to 3.8 as the VWC increased from 20% to 60%. VWC had no effect on the shoot growth of Phalaenopsis. As VWC increased, root radius, velamen width, the number of exodermis passage cells, cortex width, the number of cortex cell layers, the number of endodermis passage cells, stele diameter, the number of xylems, and xylem area also decreased. However, the cell wall thickness of exodermis and endodermis was not affected by VWC, and increased with the maturity of the root. The root water retention capacity was the least under 60% VWC, while 20% VWC treatment was the highest, indicating that roots grown in high substrate moisture had a lower water retention capacity. Phalaenopsis plants were fertigated with 0, 100, 200, 400, or 800 mg∙L-1 N fertilizer (15N-2.2P-12.5K; Peters Excel 15-5-15). The root growth and the root quality (3.9) was the finest in 100 mg∙L-1 N treatment, and with increasing fertilizer concentration, root growth decreased. The 800 mg∙L-1 N treatment had the lowest root fresh weight and root dry weight, which were 40% lower than those of 100 mg∙L-1 N treatment. However, the shoot growth increased as the fertilizer concentration increased to 400 mg∙L-1 N, and the shoot growth stunted in 800 mg∙L-1 N treatment. When the fertilizer concentration increased from 0 mg∙L-1 N to 400 mg∙L-1 N, the root radius and cortex width increased at 1, 3, 5, 7 cm above the root tips. However, when the fertilizer concentration increased to 800 mg∙L-1 N, the plants were under salinity stress, causing root deformity. Root radius and cortex width increased with the increase of fertilizer concentration at 1 cm and 3 cm above the root tip, but they decreased at 5 cm and 7 cm above the root tip. When the fertilizer concentration was higher, cell wall thickness of endodermis increased, which could reduce the entry of excess salt into stele. The number of exodermis passage cells and the number of endodermis passage cells increased with the fertilizer concentration, which could accelerate the transport of water. The stele diameter only increased at 1 cm above root tip as the fertilizer concentration increased, indicating that the effect of fertilizer concentration on stele diameter was little. However, velamen width, the cell wall thickness of exodermis, and the number of xylems were not different among treatments. There was no difference in root water retention capacity among fertilizer concentration treatments. In summary, maintaining the VWC at 20% to 30% was better for the growth of Phalaenopsis. The optimal fertilizer concentration was 100 mg∙L-1 N (EC = 0.66 dS∙m-1). Phalaenopsis is an epiphytic orchid that can tolerate drought. Results of this study showed that under regular drought conditions or physiological drought conditions caused by salt, Phalaenopsis had different tendency of differentiation in root structure. Under regular drought condition, valemen width increased to retain water. The number of exodermis passage cells, the number of endodermis passage cells, the number of xylems, and xylem area were increased to accelerate water absorption and transportation. Under physiological drought conditions passage cells in endodermis and exodermis were increased to accelerate water uptake and transport efficiency.