Plant scientists at the University of California, Davis are breeding lettuce plants whose roots will save irrigation water by sipping deeper from soil moisture.

Louise Jackson, plant physiologist in the Vegetable Crops Department, is leading the study of roots with funding from the California Lettuce Research Board.

The approach, she told the board recently in Sacramento, is to find ways to manipulate root architecture for greater uptake of moisture without altering the dry weight of shoots, or heads, of commercial iceberg lettuce.

“With a deeper root system, we think plants could access water and nutrients deeper in the soil profile and therefore reduce the amount and frequency of irrigation water applied, and possibly decrease the amount of nutrients applied as well,” she said.

Data from the trials indicate that a plant can reach more moisture either with a deeper taproot or with lateral roots concentrated at the tip of the taproot.

“The more beneficial route for keeping shoot weight constant,” Jackson said, “is for the plant to build lots of laterals at the tip of the taproot. When the plant has a deeper taproot, it is at a cost to the shoot dry weight.”

The taproot, she explained, is chock full of stored nutrients and may not be economical for production of lettuce heads.

Ideal combination

Thus, she said, pending final data, the apparent ideal combination, for production of heads and for deep water extraction, would be a cultivated lettuce gene for a shallow taproot and a wild lettuce gene for numerous lateral roots at the tip of the taproot.

The team is drawing from a store of crosses and backcrosses of cultivated Lactuca sativa and wild Lactuca serriola lettuces. They have tentatively identified wild lettuce genes that govern traits for deep rooting, long taproot length, and lateral roots at the bottom of the taproot. They are currently being confirmed.

Isolating these genes was easier, Jackson said, than the tasks ahead to cross the screened plants with cultivated lettuce to produce plants having the familiar traits of commercial lettuce. They will use molecular markers to determine traits.

Early crosses produced plants with scrambled combinations of genes, both desired and undesired, and many plants when grown in the field showed an array of odd sizes and shapes far from resembling commercial lettuce.

In field plots at UC, Davis the plants were grown with sufficient moisture for six weeks and then irrigation was halted for either eight days (mild drought) or 11 days (more severe drought). Measurements of above-ground biomass were taken, along with soil core samples to about a three-foot depth within a 30-hour period.

Potential advantages of the more efficient root system might be more rapid stand establishment and stronger competition with weeds.

Another research leader, Robert L. Gilbertson, plant pathologist at UC, Davis, reported to the board on his studies on lettuce mosaic virus (LMV), a potentially serious disease of the crop.

He said the ELISA test used on seed lots is sound, although it requires skilled personnel to perform, and alternate hosts, including the ornamental, gazania, continue to be reservoirs for the virus.

Although diversity within the virus may cause minor differences in symptoms, he said, “In my opinion, no monster strain of the virus is out there threatening the industry.”

While another new, potential approach to dealing with the virus is with resistant, transgenic lettuce cultivars, Gilbertson noted the issue is on hold in view of the controversy surrounding transgenics.

Although LMV has not been a significant problem since seed testing and other steps have been in place, during the mid-1980s infections occurred in the Salinas Valley and elsewhere, leading to conjecture about new strains of the disease and the validity of the seed test.

E-mail: dan_bryant@intertec.com