Evaluation of fine root length and diameter measurements obtained using RHIZO image analysis

Image analysis systems facilitate rapid measurement of root length and diam eter, but their accuracy is not easily determined. The objective of this st udy was to develop a set of simple experiments for evaluating the accuracy of fine root measurements obtained using image analysis, Using the system R HIZO (trademark of Regent Instruments, Quebec), we tested the accuracy of ( i) length measurements made over a range of root lengths per unit area, (ii ) average diameter measurements and length per diameter distributions in st ring, wire, and fine root samples of varying diameter, and (iii) diameter m easurements on short segments of diagonally oriented objects. Our results s uggest that preliminary testing of image analysis systems is absolutely nec essary for producing reliable root measurements. Total length was accuratel y determined for typically encountered length per unit areas of < 1.5 cm cm (-1). For samples with higher values, however, the method underestimated to tal length by >5%. It is therefore recommended that users of image analysis systems determine this maximum length per unit area for accurate determina tions of total root length, In samples that contained different string diam eters, the total sample length and average string diameter could accurately be measured. However, the length per diameter class was underestimated by >20% when the string diameter was less than one pixel smaller than the uppe r limit of the diameter class. Adjustment of diameter intervals and increas ing the scanner resolution are required to reduce this underestimation. Bot h the length and the angle of the short segments analyzed were found to inf luence diameter measurements. Similar sets of experiments are proposed for a rigorous evaluation of the performance of other image analysis systems on root measurements.