Photosynthetic activity of vegetable plants and its horticultural significance

II. The time course of photosynthesis in the tomato plant as influenced by some external and internal factors, especially by water and starch contents in the leaf

Abstract

Tomato plants were placed in a plexiglass chamber in order to determine the time course of photosynthetic activity both under natural and artificial-ambient conditions. External and internal factors which might affect the rate of photosynthesis were examined and discussed in this paper.
The rate of carbon dioxide exchange was linearly proportional to the light intensity. In the afternoon, however, the rate was slightly depressed as compared with the rate at the same light intensity in the morning. Photosynthetic rate is known to be greatly influenced by external factors of radiant flux density, ambient carbon dioxide concentration, leaf temperature, and wind speed over the leaf surface. When carbon dioxide content was depleted to 0.55mg/l in this study, the photosynthetic rate was evaluated to be 90 per cent of the normal.
In the successive experiment, tomato plants were placed in the glasshouse under natural conditions and sampled at hourly intervals to measure the photosynthetic activity under consistent external conditions. The fall in the photosynthetic activity during afternoon suggests that the rate of photosynthesis was dependent on internal factors.
One of the internal factors was water content in the leaf. Without any symptom of wilting, water content dropped from 85 to 81 per cent of fresh weight, showing the lowest value at noon. The plant with water deficit leaves always gave lower carbon dioxide absorption value, approximately 80 to 90 per cent of the normal. Since the water deficit causes the hydro-passive and hydro-active stomatal closing, higher stomatal resistance may restrict the rate of photosynthesis during afternoon.
The accumulation of assimilates could be assumed to be one of the internal factors affecting the photosynthetic activity in the afternoon. Starch grain especially, highly accumulated in the leaf sampled at 13.00 and 15.00, had high degree of negative correlation with the rate of photosynthesis. The possible mechanism for reduction in the rate of photosynthesis by accumulated starch grains might be through an increase in the mesophyll resistance for the carbon dioxide diffusion to chloroplast, and a decrease in the light absorption by chloroplast.