Yields

Vegetable yields should show at least moderate increases with elevated [CO2]. Responses of lettuce, carrot and potato are described in more detail in section 10.3. In addition, responses of four glasshouse crops - cucumber (Cucumis sativus L.), pepper, tomato and eggplant (Solanum melongena L.) - to CO2 enrichment have been studied by Nederhoff (1994) in the Netherlands. In cucumber given [CO2] of 620 |mmol mol-1, fresh-weight fruit harvest increased by 34% compared with a crop grown at 364 |mmolmol-1. Two-thirds of this increase was caused by a greater number of harvested fruits and one-third by an increased average fruit weight. In pepper, increasing [CO2] from 300 to 450 |mmol mol-1 increased fruit production by 46%. One-third of this increase was caused by a shift in allocation, and the remaining two-thirds was caused by increased CO2 assimilation rate.

Disorders of leaf and shoot growth occurred in CO2-enriched tomato and eggplant. Depending on the incidence of the short-leaf syndrome in tomatoes, yield increases varied from 0 to 31%. In eggplant, yield increases were 24% even though active leaf area was reduced. Fruit quality was not affected by [CO2] enrichment in any of the greenhouse crops in the study. In a greenhouse tomato study in Raleigh, North Carolina, USA, cooling was used to increase the length of time the tomatoes could be [CO2] enriched. While yield increases of up to 35% were seen, they occurred only if the duration of enrichment was at least 90% of the daylight hours (Willits and Peet, 1989). In two onion (Allium cepa L. Cepa group) cultivars (Daymond et al., 1997), an increase in [CO2]

shortened time to bulbing, but the time from bulbing to bulb maturity was delayed. For the two cultivars at elevated [CO2], increases in bulb yield of 28.9-51% were due to an increase in the rate of leaf area expansion and the rate of photosynthesis during the pre-bulbing period as well as (or in addition to) the longer duration of bulbing.

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