Commercial growers use glasshouses to control the conditions around crops so that the rate of photosynthesis, and so the yield, is as high as possible. They apply the principle of limiting factors: there is no point increasing one factor if another is limiting.
• Light intensity: artificial lighting increases light intensity in winter and can extend the day. Some lamps give mainly red and blue light, which chlorophyll absorbs best.
• Carbon dioxide concentration: burning paraffin or natural gas releases carbon dioxide, raising the concentration from about 0.04% to around 0.1%. Carbon dioxide can also be released from cylinders.
• Temperature: heaters keep the temperature near the optimum for the crop's enzymes. Burners also release heat, so they raise both carbon dioxide concentration and temperature. In summer, ventilation and shading prevent overheating.Water is supplied by irrigation and mineral ions by fertilisers, so these do not limit growth.Worked example: A grower lights a tomato crop at night, but the rate of photosynthesis barely rises. Light intensity is now high, so another factor must be limiting. If the glasshouse is cold, temperature is limiting, so heating (or a paraffin burner) is likely to be more effective than more light.Every change has a cost. Growers only raise a factor if the value of the extra crop is greater than the cost of the extra lighting, fuel or carbon dioxide. The best conditions are the ones that give the greatest profit, which may not be the fastest possible rate.
• Light intensity: artificial lighting increases light intensity in winter and can extend the day. Some lamps give mainly red and blue light, which chlorophyll absorbs best.• Carbon dioxide concentration: burning paraffin or natural gas releases carbon dioxide, raising the concentration from about 0.04% to around 0.1%. Carbon dioxide can also be released from cylinders.
• Temperature: heaters keep the temperature near the optimum for the crop's enzymes. Burners also release heat, so they raise both carbon dioxide concentration and temperature. In summer, ventilation and shading prevent overheating.Water is supplied by irrigation and mineral ions by fertilisers, so these do not limit growth.Worked example: A grower lights a tomato crop at night, but the rate of photosynthesis barely rises. Light intensity is now high, so another factor must be limiting. If the glasshouse is cold, temperature is limiting, so heating (or a paraffin burner) is likely to be more effective than more light.Every change has a cost. Growers only raise a factor if the value of the extra crop is greater than the cost of the extra lighting, fuel or carbon dioxide. The best conditions are the ones that give the greatest profit, which may not be the fastest possible rate.