田字草異形葉和水陸環境變化的關係

Abstract

本研究探討兩棲性蕨類-田字草（Marsilea quadrifolia）的異形葉（heterophylly）和水陸環境變化的關係。田字草栽培在不同環境時可分別形成四種異形葉：沉水葉（submerged leaf）、浮水葉（floating leaf）、挺水葉（emergent leaf）和陸生葉（terrestrial leaf），後三者因葉片和空氣接觸故合稱氣生葉（aerial leaf）。本文主要探討田字草氣生葉如何藉由外部形態、內部構造和生理的調整，以適應水陸交替的環境。實驗發現浮水葉和挺水葉的生成與速率會受到水域環境中溫度、光度和水深的影響；將水生植株移植到陸生環境時，其新生葉均為陸生型，移植植株生物量愈重，所生成的第一片葉其葉柄愈長，葉柄單位長度重量比也愈重；移植相同重量的水生植株於不同水陸環境，其所生成的第一片葉的葉柄重量相同但葉型與葉柄長度皆不同；因此植株生物量的大小與環境中的水位變化共同決定田字草葉柄長度是否能高出水面；進而影響葉型的轉變。各種氣生葉在外觀形態、解剖構造與生理特徵上雖有相當的差異，但卻表現相同的光合作用效率和速率，因此推測田字草氣生葉會因為所處環境的不同，藉由各種異形葉的生成、葉部特徵的調整和內部生理改變，使得處在不同環境下的葉片仍可達到最好的光合作用效率。陸生植株給予低溫或Ethephon處理時，會生成類似淹水環境下所產生的浮水葉，由此推測低溫及淹水環境會促使植株大量生成乙烯並累積在植物體內，乙烯進而誘導幼葉發育出浮水葉的特徵。

The heterophylly of Marsilea quadrifolia, an amphibious fern often found growing in paddy field, pond, stream and shore of lake, in its external morphology and internal structure and physiology in response to interchanges between aquatic and terrestrial environments was studied. When cultured in greenhouse, M. quadrifolia plants were able to produce four morphologically distinct leaves, including submerged, floating, emergent and terrestrial leaves, depending upon the culture conditions. Results showed that temperature, light intensity and water depth in aquatic environment affected the production and growth rate of submerged, floating and emergent leaves. In addition, when rhizomes of M. quadrifolia plants of different biomass were transplanted from aquatic to terrestrial environment, the first leaf produced by those with more biomass had longer and heavier petiole than those with less biomass. However, when rhizomes of M. quadrifolia of similar biomass were transplanted from aquatic to different environments, the first leaf produced showed different type with different morphology. Accordingly, leaf type produced by M. quadrifolia might be codetermined by plant biomass and the fluctuation in water level. Differences in leaf morphology, anatomy and nitrogen content existed among aerial leaves, but no significant difference was found in their photosynthetic quantum yield and electron transport rate. These results imply that M. quadrifolia was able to adjust its morphology, anatomy and physiology to achieve maximum photosynthetic rate. Under conditions of chilling or being sprayed with Ethephon, terrestrial grown M. quadrifolia plants produced leaves with characteristics similar to floating leaves. I therefore proposed that the production and accumulation of ethylene might be one of the mechanisms responsible for the development of the characteristics of floating leaf.