Plant nutrition and photosynthesis play an important role in all parts of scientific education, e.g. biology teacher education. The knowledge that green plants produce their own food is an ecological key concept, which is essential for understanding nature. However students find the concepts of plant physiology often to be difficult to understand. The metabolism of plants appears abstract and complex. Maybe one reason for difficulties with the ecological key concepts like the photosynthesis in teaching sciences is, that they are rarely subject of didactical research. Less works are handling the topic in conjunction with the biology teacher education (e.g. Ahopelta, I. et al. (2011), Käpylä, M. et al. (2009)). Other studies (e.g. Carlsson, B. (2002), Eisen, Y. & R. Stavy (1988) und Hellden, G (2000)) analyzes the students’ understanding of concepts about plant physiology and plant nutrition, which are part of the Biology curriculum of secondary schools. Many students hold alternative conceptions which are not in line with the generally accepted scientific view. There is agreement among education scientists that the adoption of appropriate scientific concepts is a constructive process requiring the active cognitive engagement of individuals. As a consequence learning environments that support constructivist processes create meaningful opportunities, allowing students being active as well as discovering new explanations (Mandl, H. & G. Reinmann-Rothmeier 2002). A typical learning environment of this encourages activities like experimentation and discussing. They are able to self-regulate their learning, to discuss in their learning group.

But is an unguided way of teaching and learning to such complex topics like nutrition of plants really helpful? Several studies suggest, that students participating in a learning environment constructed according to the principles of constructivistic learning achieve a higher degree of conceptual understanding than students in environments of direct instruction (Christianson & Fisher, 1999; Staub & Stern, 2002; Tynja¨la¨, 1999).Hardy et al (2006) compared constructivistic learning environments varying in instructional support. They found out, that instructional support within constructivistic learning environments fostered conceptual change. It seems to be important to keep a balance of construction and instruction in learning processes to avoid cognitive overload. There is no research on the influence of free-choice experiment orders which means lesser instruction level in the learning environment compared to the other group which get a sequence to do the experiments in the learning environment.

Because of that, the following objectives are in the focus of this study:

Do biology teacher students develop adequate and expandable conceptions about metabolism in plants when learning in a moderate constructivist environment which attaches importance to experiment orders rather than students learning in a moderate constructivist environment which attaches less importance to experiment orders? Are non-performance variables like motivation better promoted in a moderate constructivist learning environment including free-choice experiment orders than in a moderate constructivist learning environment without free-choice experiment orders?

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