13.4.2  Vegetables and fruit crops

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Leafy vegetables, fruit and ornamentals are commonly grown under CO2 enrichment. Enrichment of lettuce and celery is very effective since most of the total plant weight contributes to marketable harvest weight. Two-to three-fold enrichment of CO2 concentrations can result in midwinter lettuce that either is 25–40% heavier at harvest or takes 10–15 fewer days to attain a standard market weight (recall the ‘Head Start’ program mentioned in Section 6.2.3). Celery responds similarly by producing a heavier ‘stalk’ or requiring reduced time to reach marketable size.


Figure 13.14 Marketable yield of tomatoes and cucumbers increases during summer in response to CO2 concentration. (Based on Hand 1989; reproduced with permission of Professional Horticulture)

Tomato, the major crop grown under glass in Western Europe, is also the most studied horticultural crop for CO2-enrichment effects. Marketable fruit yield can be increased by 25–28 t ha–1 (10–11 tons per acre) for each 100 µmol CO2 mol–1 increase in mean daytime CO2 within the range 300–500 µmol CO2 mol–1 (Figure 13.14).

CO2 enrichment stimulates vegetative growth of tomato by increasing both net assimilation rate and expansion of leaf area. This is particularly important in winter when large numbers of plants are propagated commercially and CO2 enrichment can substitute for limiting light. The quality of planting stock is also improved. Once tomatoes reach their reproductive phase, CO2 enrichment induces earlier flowering, reduces flower abortion and thus increases fruit set. Overall, fruit development on early trusses is enhanced and commercial yields are boosted via an increase in both the number of trusses and the weight of fruit per truss.

Cucumber fruit yield is also stimulated by CO2 enrich-ment (Figure 13.14), increasing by 54 t ha–1 (22 tons per acre) for each 100 µmol CO2 mol–1 increase in daytime CO2 concentration during summer. Other benefits include earlier flowering, increased number of flowers, increased branching, a higher proportion of female flowers and earlier commencement of harvest for most varieties.


Table 13.2

In Japan, trees of mandarin oranges are often grown in containers within plastic greenhouses which results in high yields of earlier, blemish-free fruit. Similarly in Australia, Valencia orange trees grown in containers under CO2 enrichment from time of flowering until fruit harvest yielded 70% more fruit (Table 13.2). These fruit did not differ from unenriched controls in soluble solids content or acidity, indicating that CO2 enrichment did not reduce quality. Progression of fruit colouration was more rapid with CO2 enrichment. Considerable potential thus exists for protected cultivation of premium quality fruit in high CO2.