Abstract
In effort to improve resource-use efficiency of indoor specialty-crop production, this study examined potential interactive effects of a range of photosynthetic photon flux densities (PPFDs) and carbon dioxide (CO
2
) concentrations on growth and quality attributes of densely seeded baby-stage red lettuce (
Lactuca sativa
cv. Rouxai). Growth PPFDs tested included 200, 300, 400, and 500 µmol·m
−2
·s
−1
. Growth CO
2
concentrations tested included 400, 800, 1200, and 1600 µmol·mol
−1
. Growth parameters including shoot fresh mass, shoot dry mass, and leaf area were measured after a 17-day cropping cycle. Quality attributes such as red pigmentation and chlorophyll concentration were quantified nondestructively. Energy consumption for lighting (kWh) was measured over the entire cropping cycle for each experimental treatment, and energy-use efficiency (EUE) was calculated as a function of shoot fresh mass produced per kWh of electricity expended for lighting. Results indicated significant PPFD × CO
2
interaction effects for all measured productivity parameters except chlorophyll concentration. CO
2
was the less-limiting factor, with crop-productivity responses maximizing at 1200 µmol·mol
−1
. PPFD was the more-limiting factor, with no evidence of light saturation at any PPFD/CO
2
combination tested. Although both PPFD and CO
2
influenced pigmentation, chlorophyll concentration was more strongly affected by PPFD. EUE was highest at the lowest PPFD tested across all CO
2
concentrations and declined with increasing PPFD. In addition to establishing a PPFD × CO
2
profile for baby-stage lettuce production, our findings suggest that from the industry standards of 800 µmol·mol
−1
CO
2
and 200 µmol·m
−2
·s
−1
PPFD, increasing PPFD further can lead to four to nine more cropping cycles per year, which possibly could offset increased electricity costs, particularly with premium market pricing for baby-stage lettuce. The choice of PPFD dictating CO
2
concentration to achieve production goals may be determined by incorporating the findings of this study into comprehensive cost-benefit analyses.