13.4.1  Greenhouse cropping

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CO2 enrichment based on fuel combustion has had a chequered history in commercial greenhouses due to adverse side effects of polluting gases. Prior to the 1980s there were considerable problems associated with burning kerosene as a source of CO2 and heat. Often SO2 and ethylene were generated during combustion, both of which can have potent negative effects upon plant growth and development. These gases were produced because of incomplete combustion. However, with the introduction of low-sulphur kerosene, SO2 generation has ceased to be a problem. Propane as a fuel can also pose problems if improperly combusted or if leaks occur because of the presence of propylene, an active analogue of ethylene (Section 11.5.6). Ethylene itself is rather more potent, and can cause substantial crop loss in poorly ventilated greenhouses.

During fuel combustion, nitrogen oxides (NOx) also arise from a reaction between O2 and N2 in air at high tem-peratures. Nitrogen oxide (NO) is usually the predominant pollutant generated during high-temperature combustion, leading to oxidative damage of photosynthetic membranes and eventually leaf necrosis.


Figure 13.13  CO2 depletion due to photosynthetic activity measured in a non-ventilated cucumber greenhouse on a warm bright spring day. (Based on Hand 1989; reproduced with permission of Professional Horticulture)

Ironically, excess CO2 can also be injurious, with chloroplast disruption and chlorosis often observed above 1000 µmol CO2 mol–1. By contrast, CO2 depletion within enclosed structures is a particular problem where ventilation is regulated primarily to conserve heat (Figure 13.13). CO2 levels can deplete rapidly to 100–150 µmol CO2 mol–1. These concentrations are below free atmospheric levels on bright days and limit carbon assimilation.

Beneficial effects of CO2 enrichment for horticultural production can in many instances be interpreted in terms of a response of photosynthetic gas exchange by leaves and in terms of interactions involving high CO2, sunlight and dark respiration. Effects of increased CO2, including stimulation of carboxylation of Rubisco and reduction of oxygenation which suppresses photorespiratory CO2 loss, have been discussed earlier. CO2 enrichment increases quantum yield of photosynthesis for C3 species and lowers the light compensation point. This effect is particularly advantageous in high-latitude environments for winter/spring glasshouse cropping when light levels are low. CO2 enrichment also raises the temperature optimum for photosynthesis and growth and this may permit less frequent venting of enclosures to control temperature. Respiration is often suppressed by CO2, although variable responses are reported.