Abstract
Growth retardation and wilting of corn seedlings frequently occur in spring in Hokkaido, the northernmost island in Japan. In order to elucidate the process of the growth retardation and wilting, the effect of aboveground and underground environmental factors on the balance between transpiration from leaves and water babsorption by roots and the photosynthetic rate were examined by model experiments. The transpiration rate of corn seedlings increased with increase in light intensity and saturation deficit in the air (Fig. 1 and 2). The evaporation rate from a filter paper atmometer increased steeply with increase in wind velocity, though it was less influential on the transpiration rate (Table 1). The water absorption rate and photosynthetic rate of corn seedlings were strongly affected by the water potential of culture solution (Fig. 4 and 5). The photosynthetic rate decreased rapidly to less than 60 % in about 40 minutes after the water potential in the culture soultion was reduced from 0 to -0.6 bar by ammonium sulfate (Fig. 5). The environmental condition at the seedling stage of corn in the main corn area of Hokkaido is characterized by high solar radiation, strong wind, low precipitation (Table 2) and heavy application of fertilizer (about 10 kg/10 a of N). High transpiration rate under high solar radiation may reduce the water content in corn seedlings. The water uptake by roots may be restricted by the soil water deficiency resulting from both low precipitation and high evaporation due to strong winds. In addition, this may be accentuated by the reduction in soil water potential caused by heavy fertilizer application. The transpiration-water absorption imbalance produced by these environmental conditions is regarded as a main cause of the growth retardation and wilting of corn seedlings. The outline of the growth retardation and wilting process is schematically represented in Fig. 6.
