Studies on Competition between Upland Crops and Weeds : IV. Change of light environment in crop canopies and hypothesis about the period for weed-free maintenance

[摘要] The changes of light environment in crop canopies were investigated with four upland crops which were typical in the Kanto plain. And, the growth of competitive weeds was made clear clear, then we hypothesized about the period for weed-free maintenance after seeding required to escape crop yield reduction due to weeds. 1. The start of shading the surface of the ground by crop canopies after seeding was earliest in corn, but reduction of relative light intensity under crop was slow, and the degree of shading was small. The start of shading by soybean was a little late as compared with that of corn, but reduction of relative light intensity was rapid, and the degree of shading was large. The start of shading was the latest in peanut, but reduction of relative light intensity was the most rapid and the degree of shading was large as well as soybean. The characteristics of shading in upland rice were medium among these crops. 2. High negative correlation was found between logarithms of relative light intensity in the center of crop-rows on the surface of the ground and the number of days after seeding, then the regression lines were obtained. From these regression lines, the times when relative light intensity reduced under 20% were estimated, that is, about 76, 78, 63 and 71 days after seeding in upland rice, peanut, soybean and corn, respectively. And similarly, the times when reduced under 10%, were about 89, 86 and 73 days after seeding in upland rice, peanut and soybean, respectively. Reduction under 10% was not found in corn. 3. The linear regression was obtained between logarithms of relative light intensity in the center of crop-rows on the surface of the ground and LAI. From these regression lines, LAIs at the time when relative light intensity reduced under 20% or 10%, were estimated, that is, 4.9 or 6.6, 2.7 or 3.7, 2.3 or 3.3 and 5.2, in upland rice, peanut, soybean and corn, respectively. 4. At the time when relative light in the center of crop-rows on the surface of the ground reduced under 20% or 10%, the space of 40 or 30 cm-high from the surface of the ground in upland rice, 20 or 15 cm-high in peanut, 60 or 50 cm-high in soybean and 100 cm-high in corn kept the same value of relative light intensity, respectively. 5. In the early summer season when relative light intensitywas reduced by crop canopies, plant length or main stem length of weeds was 1-2, 7-13, 16-28, 39-52 cm in large crab-grass (Digitaria adscendens Henr.), 1-2, 6-12, 12-26, 20-43 cm in Chufa (Cyperus microiria Steud.), and 1>, 4-14, 9-25, 17-33 cm in common purslane (Portulaca oleracea L.)at 10, 20, 30, 40 days after seeding, respectively. 6. From above results. we build up hypothesis about the period for weed-free maintenance after seeding required to escape crop yield reduction due to weeds, as follows. Growth of large crab-grass is greatly diminished at the condition of relative light intensity under 10%. The relative light intensity in crop canopies reduced under 10% at the time of about 89 days after seeding, and the space of 30 cm-high was the same value in upland rice. On the other hand, plant length of large crab-grass grew about 30 cm at the time of 30 days after weed seeding. Therefore, 59 days which went back from 89 days to 30 days were estimated as the period for weed-free maintenance, in the community of upland rice and large crab-grass. It was expected that the growth of large crab-grass which emerged on and after 59 days after crop seeding was greatly diminished by crop canopies. The relative light intensity did not reduce under 10% in corn, but from its characteristics, less than 30 days were estimated, as the period for weed-free maintenance. The period for weed-free maintenance were estimated shorter in common purslane or Chufa. Their growth are greatly diminished at the condition of relative light intensity under 20%.

[学科分类] 农业科学（综合）