Optical methods for the non-destructive estimation of leaf area index in kohlrabi and lettuce

•Leaf area index is measured using direct and indirect methods with several setups.•Direct methods are destructive, expensive, laborious and time-consuming.•Less laborious indirect methods yielded precise values in kohlrabi and lettuce.•Digital photography is a good alternative to destructive methods in these crops.

The dimensionless leaf area index (LAI) is a fundamental crop characteristic. Since the direct measurement of LAI or leaf area is labour intensive and destructive, fast and reliable indirect methods have been devised to estimate LAI of different crops. The objective of this work was to test indirect methods for the non-destructive estimation of LAI in kohlrabi (Brassica oleracea var. gongylodes L.) and lettuce (Lactuca sativa var. capitata L.). Focusing on the gap fraction methodology, digital photographs and simultaneous radiation interception measurements were taken using a Li-Cor plant canopy analyser (LAI-2200) on 12 sampling dates from planting to harvest, with concurrent destructive estimations of the leaf area. Several geometric protocols of the LAI-2200 and inversion algorithms of the accompanying software were evaluated. Very good indirect-direct LAI relationships were obtained for kohlrabi (R2 > 0.97, n = 12) and lettuce (R2 > 0.99, n = 9) for the most suitable protocols and algorithms.