In a laboratory, one factor is changed while the others are held constant. In the wild, light intensity, carbon dioxide concentration and temperature all change through the day and the year, so the limiting factor changes too.At dawn, light intensity is low and it is the limiting factor. As the Sun rises, light intensity increases. By midday on a bright summer day in the UK, light is usually no longer limiting and the low atmospheric carbon dioxide concentration (about 0.04%) often limits the rate instead. On a cold, bright winter morning, temperature may be limiting. Plants on a woodland floor are shaded by the canopy, so light limits their photosynthesis for most of the day. At night there is no photosynthesis at all.Hot, dry conditions add another limit: plants close their stomata to save water, so carbon dioxide cannot enter the leaf even though light is plentiful.Growers use limiting factors to raise yields in glasshouses. They can:
• add artificial lighting to extend the day or raise light intensity;
• burn paraffin or propane, which releases carbon dioxide and heat, raising carbon dioxide to about 0.1%;
• heat the glasshouse to keep the temperature near the optimum.Increasing one factor only helps until another becomes limiting, so growers need to raise the factors together. Each change costs money for fuel or electricity, so a grower should increase a factor only if the value of the extra yield is greater than the extra cost.
• add artificial lighting to extend the day or raise light intensity;
• burn paraffin or propane, which releases carbon dioxide and heat, raising carbon dioxide to about 0.1%;
• heat the glasshouse to keep the temperature near the optimum.Increasing one factor only helps until another becomes limiting, so growers need to raise the factors together. Each change costs money for fuel or electricity, so a grower should increase a factor only if the value of the extra yield is greater than the extra cost.