SHOOT AND ROOT PHYSIOLOGICAL-RESPONSES TO LOCALIZED ZONES OF SOIL-MOISTURE IN CULTIVATED AND WILD LETTUCE (LACTUCA SPP)

Cultivated crisphead lettuce (Lactuca sativa L.) has a shallower root system than its wild relative, Lactuca serriola L. The effects of loca lized soil water, at depth, on plant water relations, gas exchange and root distribution were examined in the two species using soil columns with the soil hydraulically separated into two layers, at 0-20 cm and 20-80 cm, but permitting root growth between the layers. Three treatm ents were imposed on 7-week-old plants, and maintained for 4 weeks: (i ) watering both layers to field capacity; (ii) drying the upper layer while watering the lower layer to field capacity, and (iii) drying bot h layers. Drying only 0-20 cm of soil had no effect on leaf water stat us, net photosynthesis, stomatal conductance or biomass production in L. serriola compared to a well-watered control, but caused a short-ter m reduction (10 d) in leaf water status and photosynthesis in L. sativ a that reduced final shoot production. The different responses may be explained by differences in root distribution, Just before the treatme nts commenced, L. serriola had 50% of total root length at 20-80 cm co mpared to 35% int. sativa, Allocation of total biomass to roots in L. serriola was approximately double that in L. sativa. The wild species could provide germplasm for cultivated lettuces to extract more soil w ater from depth, which may improve irrigation efficiency.