ROOT ARCHITECTURE IN CULTIVATED AND WILD LETTUCE (LACTUCA SPP)

Root architecture is described for intact root systems of cultivated ( Lactuca sativa L.) and wild (L. serriola L.) lettuce, grown for 5 to 6 weeks in greenhouse pot and cylinder experiments in coarse-textured s oil, L. sativa cv, Salinas and a selfed line of L. serriola attained t he same biomass at 4 to 6 weeks after planting, Root biomass allocatio n was also similar, but root architecture differed, In the top zone al ong the tap root (0 to 5 cm), Salinas tended to produce more laterals, a greater total root length, and more external links (segments that o riginate at a branch point and end in a meristem) than wild lettuce, I n the 5 to 55 cm zone of the tap root, these measures were greater in the wild species, These patterns of root structure were generally corr oborated by a second cylinder study with a different pair of L. sativa and L. serriola. Regressions of root structural traits were made agai nst total root dry weight as a means to compare root architecture inde pendently of the size of the root system, Regressions demonstrated tha t production of root segments differed between the two species; for ex ample the slope for the regression of summed external link length in t he top 0 to 5 cm with total root dry weight was significantly higher f or Salinas, indicating that the rate of construction in the top 0 to 5 cm was greater for cultivated than for wild lettuce, Yet, from 5 cm d epth to the tap root tip, the rate of construction of external link le ngth was greater in L. serriola, For many of these types of regression , r(2) and mean slope +/- SE suggested that more variation occurred in cultivated than wild lettuce, yet genetic heterogeneity was probably low within the studied taxa, Inadvertent selection may have occurred i n the breeding of cultivated lettuce varieties for increased root grow th in the surface zone where water and fertilizer are applied, and for greater plasticity in construction of root segments, which might maxi mize the efficiency of exploitation of soil moisture and nutrients.