Dependence of yearly fluctuation of sugar beet (Beta vulgaris L.) yield on depth of fibrous root distribution in different soil types

This study evaluated the major factors that cause yearly fluctuation in the difference in sugar beet yield between the Eastern area and the Western area, characterized by various soil types, of Abashiri, Hokkaido. We focused on the dependence of yearly fluctuation of sugar beet yield on rooting depth in different soil types and under different weather conditions. The results obtained were as follows: 1) The difference in sugar and root yield between the higher-productivity Eastern area and the lower-productivity Western area was significant and was positively related to mean temperature in August. The harvest index increased with increasing temperature in August regardless of soil type. In the haplic andosol occupying the Eastern area, the total yield decreased slowly and the root yield tended to increase in hotter summer years. In contrast, the total yield decreased rapidly and the root yield tended to decrease in the cambisol belonging to the Western area. In the umbric andosol belonging to the Western area, the fluctuation tendency of total and root yield was intermediate. 2) During hot, sunny daytime conditions, sugar beet plants grown in haplic andosol having the deepest root system maintained the greatest stomatal aperture regulating CO2 uptake and water loss, whereas plants grown in cambisol having the shallowest root system showed the most stomatal closing tendency, even under sunshine. The extent of stomatal aperture of plants grown in umbric andosol having moderate rooting depth showed an intermediate tendency. However, during the daytime at near-optimum temperature, stomata of plants grown in umbric andosol and cambisol were both open. 3) The major factor controlling yearly changes in the difference in sugar beet yield between the Eastern area and the Western area can be attributed to the difference in photosynthetic ability, which is based on the rooting depth and the water supply in response to summer temperature regimes.