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 soil, L. sativa cv, Salinas and a selfed line of L. serriola attained the same biomass at 4 to 6 weeks after planting, Root biomass allocation was also similar, but root architecture differed, In the top zone along the tap root (0 to 5 cm), Salinas tended to produce more laterals, a greater total root length, and more external links (segments that originate at a branch point and end in a meristem) than wild lettuce, In 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 corroborated by a second cylinder study with a different pair of L. sativa and L. serriola. Regressions of root structural traits were made against total root dry weight as a means to compare root architecture independently of the size of the root system, Regressions demonstrated that production of root segments differed between the two species; for example the slope for the regression of summed external link length in the top 0 to 5 cm with total root dry weight was significantly higher for 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 depth to the tap root tip, the rate of construction of external link length 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 in the breeding of cultivated lettuce varieties for increased root growth in the surface zone where water and fertilizer are applied, and for greater plasticity in construction of root segments, which might maximize the efficiency of exploitation of soil moisture and nutrients.