ECOLOGY AND GENETICS OF ROOT ARCHITECTURE AND SOIL WATER EXTRACTION
2. PROCEDURES AND RESULTS
2.1. Confirmation of QTL for deep rooting traits and soil water extraction in lettuce To confirm that the QTL regions consistently affected soil water extraction, we
examined F7 recombinant inbred lines (RILs) that were derived from the original cross of cultivated lettuce (L. sativa cv. 'Salinas') by wild lettuce (L. serriola). The F7 RILs are very different in size and form from one another, and provide a good opportunity to test the effect of a given gene on growth and water acquisition. F7 RILs were identified that were homozygous for cultivated and wild parent alleles at the two map regions of interest (2d-f and 4c-f) based on AFLP markers. These were grown in a field experiment at UC Davis on silt loam soil. The plants were grown with adequate moisture for 6 weeks, and then drought was imposed for 11 days before sampling. This level of drought is typical for plants in commercial fields between furrow irrigations. Shoot dry weight and soil cores (0–10 cm, 10–25 cm, 25–
136
50 cm, 50–75 cm, and 75–100 cm) were taken within a 30-hour period. In each plot, four plants were taken for aboveground dry weight, and one 3.5 cm diameter core was taken and separated into the five depth increments for gravimetric moisture analysis at each depth.
The goal was to determine if F7 RILs for L. serriola alleles for 2d-f and 4c-e were associated with greater moisture extraction at depth, and had greater yield compared to F7
RILs with L. sativa genes. If a deeper root system requires more belowground allocation of carbon, then shoot production may suffer, causing slower growth rates of lettuce.
When plants had the wild lettuce (L. serriola) alleles for the QTL associated with deep lateral roots in the lowest 5 cm of the taproot and deep soil water extraction at 75–100 cm depth (4c-f) [5], shoot biomass was greater, and more water was extracted deep in the profile per amount of shoot biomass compared to plants with the cultivated lettuce (L. sativa) alleles (Fig. 1). Also, they had decreased water use efficiency, based on d13C measurements, implying higher transpiration rates, and thus greater water availability (Table I). Deep laterals at the tip of the taproot may increase water use from the deep profile at relatively low construction costs, which if introgressed into cultivated lettuce could yield higher shoot biomass production under typical irrigation regimes.
TABLE I. CARBON ISOTOPE DISCRIMINATION IN F7 RILS WITH EITHER L. serriola OR L.
sativa ALLELES IN THE QTL REGIONS FOR ROOT ARCHITECTURAL TRAITS AND DEEP SOIL WATER EXTRACTION. THE PARENTAL VALUES WERE 27.4 FOR L. serriola AND -26.4 FOR L. sativa
Unexpectedly, the deep taproot alleles from L. serriola at QTL 4c-f did not give similar results. Plants with wild lettuce alleles for QTL 2d-f (deep taproot/ biomass & deep soil water extraction at 50–100 cm depth) [5] had decreased shoot biomass, increased soil water present per amount of shoot biomass deep in the profile, and no difference in d13C, compared to plants with L. sativa alleles at this QTL region (Fig. 2, Table I). Under these growing conditions, L. serriola alleles for this QTL resulted in lower shoot growth, and thus, reduced shoot water use. An explanation may be that allocation of root biomass to a storage taproot may be more costly in terms of shoot growth than the construction of fine lateral roots at the tip of the taproot. Construction of a deep taproot per amount of shoot biomass may only increase uptake of deep soil water in some circumstances, e.g., more severe drought than imposed by the typical irrigation regime for lettuce in California.
A marker-assisted selection program was initiated to introgress the L. serriola alleles in the 4c-f QTL region into cultivated lettuce to produce near-isogenic lines of cultivated lettuce with the trait for many laterals at the tip of the taproot, and deep soil water extraction.
0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 5 0 0
L. sativa parent L. serriola parent
Aboveground Biomass (g m-2 )
QTL 2d
-f, L. sativa QTL 2d-f, L. serriola
P = 0 .0 3
AFLP genetic map that was originally produced for this cross [5]. This is now at the BC3
stage. Since data from the RILs experiment suggest that introgression of the L. serriola QTL for many lateral roots near the tip of the taproot could increase lettuce yield and increase the uptake of water from deep in the soil profile, this could potentially allow growers to irrigate lettuce less frequently and with less water without negative impact on yield.
Fig. 1. F7 RILS: Verifying QTL 4c-f (deep laterals/root biomass & depth of soil water extraction). A comparison was made between F7 RILS with either L.
serriola or L. sativa alleles in the QTL region (n = 93). The field was well-watered until 11 days before harvest. The L. sativa parent is cultivated lettuce and the L. serriola parent is wild lettuce.
0 50 100 150 200 250
L. sativaparent
L. serriola parent Water in 50-75 cm Layer (g m-2) Aboveground Biomass (g m-2)
QTL 4c-f, L. sat
iva
QTL 4c-f, L. ser riola P = 0.08
138
0 50 100 150 200 250
L. sativa parent
L. serriolaparent Water in 50-75 cm Layer (g m-2) Aboveground Biomass (g m-2)
QTL 2d-f, L. sativa QTL
2d-f, L. serriola P = 0.07
0 50 100 150 200 250 300 350 400 450 500
L. sativa parent L. serriola
parent Aboveground Biomass (g m-2 )
QTL 2d-f
, L. sativa QTL
2d-f,L. serriola P = 0.03
Fig.2. F7 RILS: Verifying QTL 2d-f (deep taproot/biomass & depth of soil water extraction). A comparison was made between F7 RILS with either L.
serriola or L. sativa alleles in the QTL region (n = 81). The field was well-watered until 11 days before harvest. The L. sativa parent is cultivated lettuce and the L. serriola parent is wild lettuce.