Photosynthetic activity of vegetable plants and its horticultural significance

Abstract

In the first experiment, tomato plants were grown under the plastic-house in which the air was ventilated by operating the ventilating fans through the mouth piece fitted at the opposite side of the fan. Thus, there were considerable horizontal gradients for temperature, wind velocity and CO2 concentration from the inlet to the outlet of ventilating air over 46 meters in depth.When the wind velocity decreased to 20-30cm•sec-1, remarkable CO2 depletion was observed within tomato canopies. An air temperature increased noticeably with increasing the distance from the inlet. And maximum temperature gradient was in the order of 5°C at incident solar radiation of 8 kcal. m-2 min-1 at the end of May.Early yield of tomatoes was considerably decreased by heavy wind velocity near the inlet of ventilating air. Tomato fruits harvested from the poor-ventilated plants, however, had less titratable acidity, soluble solids and ascorbic acid. Moreover, these fruits had high respiratory rates at their post-harvest stage. In general, the titratable acidity and soluble sugar contents in the tomato fruits continued to decrease over whole harvesting period. Such a decrease of acid and sugar contents was more remarkable in the fruits harvested from the poor-ventilated plants.On the other hand, well-ventilated plants showed higher values of leaf dry weight per area, fresh- and dry-weight percentage and lower values of internodes length. Photosynthetic activity of the leaf sampled from well-ventilated plants was significantly higher than that of poor-ventilated. While, respiratory rate of the leaf was higher in the poor-ventilated plants, although Q10 value seemed to decline for the plants grown under condition of higher day temperature. Chlorophyll content was also largely affected by the ventilation. Poor-ventilated plants had leaves with lower chlorophyll a and higher chlorophyll b contents. So, the ratio chlorophyll a to chlorophyll b was considerably decreased in the poor-ventilated plants.The second experiment was carried out to estimate the favourable air volume for the ventilation from practical point of view, as much as to supply CO2 for sufficient photosynthesis and to decrease the temperature gradient.Efficiency of photosynthetic activity for tomato plants increased linearly with increasing ventilation rates, then passed through transition ranges and finally became almost independent of the ventilation rates. This critical rate seemed to be in the order of 40 times for room space volume per hour. The efficiency of photosynthetic activity was also affected by leaf area density, showing lower efficiency at higher densities. Photosynthetic activity of tomato leaves was increased with increasing wind velocity up to the order of 50-90cm•sec-1, and further increases in the wind velocity decreased slightly the photosynthetic rates, Temperature gradient between outside and inside of plastic-house was measured by changing the ventilating air at three light intensities. These results suggest that air volume 140m3. 100m-2. min-1, wind velocity 30cm•sec-1 inside the canopies or 50cm•sec-1 over the plant top and ventilation rates of 30-40 times per hour were necessary for sufficient ventilation, both for C02-transport and temperature decrease.