Articles
DIRECT DIGITAL CONTROL OF PLANT GROWTH III ANALYSIS OF THE GROWTH AND DEVELOPMENT OF TOMATO PLANTS
The system and the control logic used in the present experiment are the same as the preceding one.
As control variables for DDC, air temperature, carbon dioxide concentration and water vapor deficit have been used and each increment size is 2 degC for air temperature, 200 vpm for carbon dioxide concentration and 1 g/kg of dry air for saturation deficit.
The coefficient B is set at 1.5 for the first period and later at 1.9, and is used for the determination of the night temperature.
As a control section(AC), a conventional two 1 ‘el on-off controller has been used to maintain the AC section at 26°C in the daytime, and 16°C in the nighttime.
Seeds of tomato were sown on Sept. 8 and were grown in the conventional type cabinet until Oct. 21. Starting on Oct. 21, the control of both the DDC and the AC sections continued in parallel till Dec. 24.
Conventional destructive measurement of plant growth was conducted at the ends of the vegetative period (10/21–11/1) and the reproductive stage (11/1–12/24). In the former period, plant growth in the computer controlled section(DDC) is about 20% superior to that in the AC section.
Then in the latter period, it was only 9% superiority but the harvesting time was shifted two weeks earlier in the DDC section than in the AC. No cavity could be found in tomato fruits grown in the DDC section.
The night temperature selected by the computer was around 22°C which is considered rather higher than the commercial blue print.
The total amount of growth respiration is about 34.5% of the gross photosynthesis.
The morning and afternoon ratios of daily solar radiation and gross photosynthesis were 61.5% and 59.9%, respectively.
It is again apparent that solar radiation in physical amount is the dominant factor for the difference of photosynthesis in the morning and in the afternoon.
Pre-heating which is considered to promote plant photosynthesis in the early morning in the greenhouse is now predominant in some growers in Japan.
The effect of pre-heating up to 20°C for two hours right after sunrise on photosynthesis has been compared with that under the set points of 5°C and 15°C. It was found that as light intensity in the greenhouse is the limiting factor for photosynthesis, there were not appreciable differences among these temperature setpoints.