Multiple-Tropism: Gravity, Nutrient and Water Interaction of Stimuli for Root Orientation in Microgravity (MULTI-TROP) separately evaluates the role of three stimuli – gravity, water and nutrients – on plant growth. Tropism refers to an organism directional response to an external stimulus, such as plant roots growing downward into soil in response to gravity on Earth. Previous research shows that plant roots grow randomly without specific direction in microgravity, presenting a challenge when developing facilities to cultivate plants in space.

In order to grow healthy and functioning plants in space, it is necessary that the three plant organs (root, stem and leaf) are morphologically well developed and physiologically interactive. On Earth, roots grow into soil to anchor the plant and absorb water and nutrients. Growth direction is determined by the interaction of three stimuli: gravity (gravitropism), water (hydrotropism), and nutrients (chemotropism). The first is generally considered stronger than the others in the process of shaping the root apparatus. In microgravity, it has been proven that root apexes grow randomly without a specific direction. This phenomenon makes the development of facilities for plant cultivation in microgravity challenging.

The aim of the MULTI-TROP investigation is to further investigate this critical issue to define if, and to which extent, hydrotropism and/or chemotropism can direct root growth in microgravity. MULTI-TROP has three main goals: a) education, to enhance secondary school pupils’ interest in space biology; b) scientific, to disentangle the role of gravity from the other stimuli for root growth; and, c) applied, to address technical issues in the design of growth chambers. Results are expected to give insights for basic scienti c questions on root development processes and technical proposals to develop new hardware for plant cultivation in microgravity.

The experiment is performed utilizing the BIOKON equipped with the YING-B2 Experiment Unit. Within each of the four growth chambers two substrates are included: one with water only, and the other with a water nutrient solution. Two seeds are sown in each chamber.

Activities are organized in three phases: pre- ight phase, in- ight phase and post- ight phase. The pre- ight phase includes the seed selection, according to in ight environmental conditions, and set up of the experiment container. The in- ight phase consists of the seed germination and sample storage in chemical xative. The post- ight phase includes a ground reference experiment, processing of plant samples, data analysis and modelling.

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