Photosynthetic activity of vegetable plants and its horticultural significance

Abstract

After-effect of low-CO2 experience upon the growth and physiology for tomato and cucumber plants was examined by means of isotopic tracers under controlled conditions.The fixation of 14CO2 at normal CO2 concentration, was decreased slightly in the tomato plants which had been kept for 2 hours in an atmosphere containing 80ppm CO2. Such an after-effect of low-CO2 experience for tomato could be brought out only when the plants had been placed under conditions of low CO2 concentration and a certain degree of light intensity. On the other hand, cucumber plants had no after-effect of low-CO2 experience, although total activity of 14C fixed into the plants was clearly decreased at very low CO2 concentration.A downward translocation of 14C-assimilates to the roots was restricted more significantly in cucumber than that in tomato at 80ppm CO2 in the light. A considerable restriction for translocation of assimilates from leaves into the roots was observed in cucumber plant during normal photosynthesis in the light.CO2 compensation concentration which is reached in a closed system containing the plants at 20, 000 lux was 38±2ppm CO2 for tomato and 65±4ppm CO2 for cucumber. The amounts of CO2 evoluted into CO2-free air through the respiration were also higher in cucumber.Tomato plants which had been kept at 80ppm CO2 for 2 hours had lower value of respiratory activity in the roots. In addition, low-CO2 experience caused a considerable decrease in the uptake of phosphorus through the tomato roots.Tomato plants placed in an atmosphere containing 80ppm CO2 for 1 hour in every morning during 2 weeks resulted in the marked decrease in dry weight of root, and considerable decrease in net assimilation rate.