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

为了在空间微重力条件下生长植物，有必要开发一种水分/养分输送系统和建立一种控制根生长的方法，以解决失重环境带来的问题。在本研究中，我们研究了根的向水性及其与重力反应的关系，旨在将根的向水性应用于空间水分/养分输送系统。在地球上，根的向水性受到向力性的干扰，两种向水性之间的相互作用因植物种类而异。这种对重力和水分梯度响应的多样性在小麦根中也得到了证实。模拟实验表明，在空间微重力条件下，正常的重力根表现出很强的水力响应。我们已经开发了一个建立水分梯度的模型系统，用于研究亲水作用。结果表明，根冠的水分梯度小于1%RH mm~(-1)，水势梯度小于0.5 Mpa mm~(-1)；在根冠感觉到水势梯度后，在伸长区发生了水溶反应，即不同的生长。这种差异生长是由于组织的干燥侧和潮湿侧的细胞壁延伸性不同所致。研究还发现，根的亲水性诱导需要钙，脱落酸促进根系的亲水性。导致伸长区细胞壁伸长性差异的一些信息可能是从根冠传递的，钙和脱落酸可能参与了信号转导或信号传递机制。为了筛选和获得更适合空间环境中水分/养分输送系统的植物物种，我们将阐明根部亲水性的遗传机制。

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.