Salt accumulation and ion imbalance caused by residual fertilizer in greenhouse cultivation have been serious problems in Japan since the 1970s. Because an excess of sulfate ions was considered to be one of the causes of these problems, low-sulfate fertilizers were developed to solve this problem. However, when low-sulfate fertilizers were used in root restricted cultivations, chlorophyll indexes were decreased comparing with in the non-restricted cultivations, because total available sulfate resources to tomato plants were limited by rooting zone restriction. The chlorophyll indexes were higher on the treatment covering over soil surface with mulch of silver vinyl film than those of without mulch treatment. Especially, in the drip fertigation, salt accumulation including sulfate was caused on the surface part of soil surrounding area of dripping spot without mulch. Because the condition of rhizosphere of soil surface area was improved as for water and ion concentration by using mulch, the roots distributed there were able to absorb inorganic elements, especially to sulfate ion.

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An important factor in determining the architecture of fibrous root systems is the growth direction of individual nodal roots. However, little is known about the mechanism of root plagiogravitropism. Therefore, we examined the relationship between the change of growth angle of nodal roots with root elongation and the morphology of the root cap in field-grown maize and sorghum. Root length, diameter of root tips and growth angles of each nodal root were recorded. Moreover, longitudinal sections of nodal roots were prepared to measure the length and width of the root cap. The nodal roots of both maize and sorghum turned downwards with root elongation after emerging from the stem. Additionally, although the diameter of the root tip and the width of the root cap decreased with root elongation, the length of the root cap increased. In order to examine the changes of morphology of the root cap, ratios of the width and length of the root cap to the diameter of the root tip were measured. Although the ratio of the width of the root cap did not change, the ratio of the length increased with root elongation and showed a significant correlation with the growth angle of the nodal root. These results suggest that nodal roots change their growth direction downwards, possibly due to a relative increase of root cap size.

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Discovery of aquaporin (water channel) genes have facilitated investigations into the molecular mechanisms of cellular water permeability and transport. We isolated genes of plasma-membrane type aquaporins in barley roots and demonstrated the water transport activity, tissue localization, and diurnal change in expression of a particular aquaporin, HvPIP2;1, in barley. Over-expression of HvPIP2;1 increased the shoot/root ratio and raised salt sensitivity in transgenic rice plants. Furthermore, we noticed the increase in internal CO₂ conductance of transgenic plants. This finding indicated that HvPIP2;1 can facilitate permeation of a CO₂ as well as water molecule.

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