Abstract
The bleeding rate from the stem cut end is considered to reflect the root physiological activity since the bleeding is caused by the active water absorption by root. We analyzed factors affecting the bleeding rate using soybean seedlings in order to elucidate the relationship between the root respiration and bleeding rate in soybean plants. The bleeding rate was (1) exponentially decreased when the soil moisture percentage (w/w) dropped from 40% to 30%, (2) higher around 10 a.m. than in the early morning and evening, (3) rapidly declined after stem cutting for 6 hours and was constant thereafter, (4) affected by soil temperature and showed a nearly constant rate at 25 to 30°C, and (5) varied with the plant growth period corresponding to the variation of the root respiration rate showing the maximum level around 45 days after emergence, which is the flower bud initiation stage. When the above variable factors were set constant, the bleeding rate closely correlated with the root respiration rate, which can be expressed as the product of whole root weight and the respiration rate per root weight. Even though the bleeding rate was correlated with the whole root weight more closely than with the respiration rate per root weight, we could demonstrate that a higher respiration rate per root weight resulted in higher bleeding rate when the whole root weight was the same.
