An alternative method for growth analysis of lettuce grown in a plant factory with artificial lighting using projected canopy area and fresh mass

Relative growth rate (RGR), net assimilation rate (NAR), and leaf area ratio (LAR) are indexes used for plant growth analysis. RGR is the product of NAR and LAR, and thus low RGR can either be attributed to low photosynthetic capacity per unit leaf area indicated by NAR or to the suppression of the leaf area expansion indicated by LAR. Therefore, growth analysis is useful for efficient environmental control. In this study, leaf area, a parameter used to determine NAR and LAR, was replaced with projected canopy area measured using Kinect sensor (NAR’ and LAR’). Furthermore, dry mass, a parameter used to determine RGR, NAR, and LAR, was replaced with fresh mass (RGR”, NAR”, and LAR”) to analyze plant growth in a non-destructive manner. The dry mass, fresh mass, leaf area, and projected canopy area of leaf lettuce (Lactuca sativa var. crispa ‘Green jacket’) plants grown in a plant factory with artificial lighting were measured on the day of planting and 14 days after planting. The results showed that the correlation coefficient (r) between NAR and NAR' and LAR and LAR' were 0.957 and 0.966, respectively. Furthermore, the r values between RGR and RGR”, NAR and NAR”, and LAR and LAR” were 0.998, 0.964, and 0.947, respectively. These results suggest that plant growth can be analyzed in a non-destructive manner by using measurements of the projected canopy area and fresh mass.