Nine potato cultivars were field-grown to determine relative growth rate (RGR), crop growth rate (CGR) and tuber growth rate (TGR) in order to ascertain which of these parameters, namely leaf area ratio (LAR), specific leaf area (SLA), leaf mass ratio (LMR), leaf area index (LAI) and net assimilation rate (NAR), are most closely correlated with the differences among the cultivars in RGR, CGR and TGR. Nine samplings were carried, for short-cycle, and 11 for long-cycle cultivars, every seven days. The relations between all the indexes considered for all cultivars were analysed with simple linear correlation, separately for weekly data, every four and eight weeks. The data were also subjected to multivariate analysis with the three-stage least squares estimation method. Relations between RGR, CGR and TGR, and LAR, LMR, SLA, LAI and NAR, varied according to the length of the measurement interval and analytical method employed. Based on simple correlation analysis for seven-day periods, RGR CGR and TGR always proved closely and positively correlated with values of NAR, less so with LAR and LMR, and still less with LAI, and only slightly correlated with SLA. For four-week intervals, correlation with NAR appeared weak and for eight-week intervals even negative, while the relation with LAR and LAI increased. From multivariate analysis it emerged that the predominant factor affecting RGR was LAR which, together with LAI, negatively conditions NAR. Upon CGR and TGR an important role is played by LAI which directly and positively affects CGR and TGR, and negatively NAR. Time always showed a negative effect on NAR and positive on RGR and CGR, while LAR had a negative effect on NAR, CGR and TGR.In conclusion, for short periods the differences between cultivars in RGR, CGR and TGR appear chiefly explained by variations in NAR. For long periods and considering the relations among the various traits evidenced by multivariate analysis, the importance of NAR appears less marked, while the morphological traits, namely LAR in determining RGR and LAI for CGR and TGR, assume greater importance
