臺灣五種全寄生植物與寄主植物的養分關係探討

Abstract

全寄生植物缺乏葉綠素，生長所需養分皆來自寄主。臺灣的全寄生被子植物獲取養分的機制以及與寄主間的營養關係尚未被報導過。本研究分析五種全寄生植物 (野菰、穗花蛇菰、菱形奴草、臺灣奴草、平原菟絲子) 與其寄主間的養分關係。已知這些寄生植物在選擇寄主的專一性程度、寄生部位 (根或莖/葉) 和寄生形式 (內寄生或外寄生) 有所不同。 首先製作植物切片以確認這些全寄生植物與寄主植物的連接構造，接著分析寄生植物與寄主的碳、氮、磷、鈣、鎂、鉀、鈉元素含量以及碳 (δ13C)、氮 (δ15N) 穩定性同位素比值，目的為：一、探討全寄生植物與寄主植物被寄生部位的營養元素含量是否不同，並比較單一寄主型與廣寄主型寄生植物的營養元素含量是否不同；二、檢視全寄生植物和其寄主是否有相同的碳、氮穩定性同位素比值，據以評估利用穩定性同位素比值追蹤根寄生性全寄生植物之寄主植物的可行性。 在野菰及平原菟絲子的吸器切片可觀察到寄生植物的薄壁細胞入侵寄主組織，且寄生植物及寄主植物的木質部相連接。穗花蛇菰的塊莖切片中有複合維管束，複合維管束包含寄生植物及寄主組織且中央有傳遞細胞。兩種奴草則是在寄主根的皮層及韌皮部中形成大細胞團，屬內寄生。 相較於寄主植物，全寄生植物有較高的碳、磷、鉀含量、較低的鈣含量，以及顯著較高的鉀/鈣比值，因此推測全寄生植物可能藉由累積鉀含量以降低滲透勢和水勢能，藉此機制可從寄主獲取水分和溶質，且這些寄生植物主要由寄主韌皮部獲得養分。兩種奴草比其他三種寄生植物有顯著較高的碳含量及較低的鉀含量，可能和其寄主專一性程度較高且為內寄生有關。 相較於寄主植物被寄生的部位，大部分的全寄生植物都有顯著較高的δ13C及δ15N值，且寄生植物和寄主植物的δ13C及δ15N值呈現正相關性，因此碳及氮穩定性同位素比值應該可以用於追蹤根寄生性全寄生植物的寄主植物。

Holoparasitic plants lack chlorophylls and gain all the nutrients only from host plants. The absorption mechanism and nutrient status of the holoparasitic plants in Taiwan have not been studied. This research investigated the nutrient relationship between five holoparasites (Aeginetia indica, Balanophora laxiflora, Mitrastemon kanehirai, Mitrastemon kawasasakii, Cuscuta campestris) and their hosts. These studied parasitic plants differ in degree of host-specificity, parasitic location on host (root or stem/leaf) and parasitic forms (endoparasite or ectoparasite). Tissue sections were made and investigated to confirm whether there are connecting tissues between holoparasites and their hosts. Subsequently, carbon (C), nitrogen (N), phosphorus (P), calcium (Ca), magnesium (Mg), potassium (K), sodium (Na) contents and carbon (δ13C), nitrogen (δ15N) stable isotopes of the parasitic plants and their hosts were analyzed. The following objectives were studied: 1) to investigate the nutrient contents of holoparasites, and to compare whether the holoparasites of different degree of host-specificity are different in nutrient contents; 2) to investigate whether the holoparasites and their hosts have identical δ13C and δ15N values, and to evaluate the possibility of using the stable isotope ratio for tracing the hosts of holoparasites. The structures of haustoria of A. indica and C. campestris revealed that these two parasitic plants invaded hosts by the parenchyma cells and had xylem connection with their hosts. In the tuber of B. laxiflora, there were composite bundle consisting both the holoparasite and the host tissues and the transfer cells were found in the center. The two Mitrastemon spp., belonging to endoparasite, formed large masses between cortex and phloem of host roots. The results showed that the studied holoparasites had significantly higher C, P, K, lower Ca contents and higher K/Ca ratio than their hosts. Accordingly, by accumulation of K for lowering the osmatic and water potential, the holoparasites might gain water and solute from their hosts. Besides, the high K/Ca ratio indicated that these holoparasites obtained nutrient mainly from host phloem. The two Mitrastemon spp. had significantly higher C but lower K contents than other three holoparasites which might be related to their high degree of host-specificity and being endoparasitic. The holoparasites had significantly higher δ13C and δ15N values than their hosts. In addition, there were positive correlations in δ13C and δ15N values between holoparasites and their hosts. Thus, it is possible to use the ratio of carbon and nitrogen stable isotopes ratio as an indicator for tracing the hosts of the holoparasites.