evaluation of effects of some important agronomic traits on potato yield and possibility determining of required time of potato crop to nitrogen fertilizer levels by using chlorophyll meter

in order to investigate the effects of nitrogen fertilizer on yield and yield component of potato, (solanum tuberosum l.) and researching for direct and indirect effects of some important agronomic traits, an experiment was conducted based on randomized complete blocks design with three replications at the agricultural research station, ferdowsi university of mashhad, iran, during 2008. chlorophyll meter was used to identify the required precise time of potato to nitrogen fertilizer application. nitrogen topdress fertilizer was applied at two levels (of 100 and 150 kgn.ha-1) and during two nitrogen index of 90 and 95 percent. results of analysis of variance showed that 95 percent nitrogen index versus 90 percent nitrogen index has a better efficiency in increment of potato crop yield, but in 95 percent index, nitrogen fertilizer levels of 100 and 150 kgn.ha-1 did not showed significant difference in tuber yield. results of path analysis indicated the traits of leaf nitrogen and large tuber percentage had the most positive direct effect on tuber yield and the traits of tuber specific gravity and tiny tuber percentage had the most negative direct effect on tuber yield. supplying of nitrogen despite decrement of tuber specific gravity, increased tuber yield by increment of large tuber percentage. assessment of correlation coefficient indicated that there are high correlations between spad reading, leaf nitrogen percentage and large tuber percentage and also there are high correlations between these traits and yield of potato. in general, it appeared that nitrogen fertilizer application by using chlorophyll meter, in addition to producing desirable yield of tuber, leads to make desirable quality of tuber and economy in nitrogen fertilizers application.