Studies on the water/nutrient delivery system and the control of root growth for growing vegetable crops in space.

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To grow plants under microgravity conditions in space, it is necessary to develop a water/nutrient delivery system and to establish a method for the control of root growth, which solve problems caused by weightlessness environment. In the present study, we have studied the control mechanism of root hydrotropism and its relation to the gravitropic response, aiming at the use of root hydrotropism on the water/nutrient delivery system in space.On Earth, hydrotropism in roots is interfered by gravitropism, and the interaction between the two tropisms differ depending on plant species. This diversity of the responsiveness to gravity and moisture gradient was also confirmed in the roots of wheat plant. Simulation experiments suggested that normal gravitropic roots show strong hydrotropic response under a microgravity condition in space. We have developed a model system to establish moisture gradients for the study of hydrotropism. It was found that the root cap senses a moisture gradient of less than 1% RH mm^<-1> and a water potential gradient as small as 0.5 MPa mm^<-1>. The hydrotropic response, differential growth in the elongation zone, occurred following perception of water potential gradient by the root cap. The differential growth occurred due to a difference in cell wall extensibility between the dry side and the humid side of the tissue. It was also found that calcium was required for the induction of hydrotropism and that abscisic acid promoted the hydrotropism in roots. Some information that causes a difference in cell wall extensibility in the elongation zone may transmit from the root cap, and calcium and abscisic acid may be involved with the signal transduction or signal transmission mechanism. To screen and obtain plant species more suitable for the water/nutrient delivery system in space environment, we will clarify the genetic mechanism for hydrotropism in roots.