IDENTIFYING HEAT-RESISTANT RECOMBINANT INBRED LINES (RILS) OF LETTUCE IN THE TROPICS: PRODUCTIVITY AND ROOT PHENOTYPING

Previous studies have shown that temperate lettuce can be successfully grown in the tropics, aeroponically, as long as the rootzone was sprayed with cooled (20-25°C) nutrient solution. Due to ever increasing costs of fossil fuels, obviating or minimising cooling would be commercially advantageous. In this study, a lettuce RIL population, obtained by crossing heat-sensitive (HS) Lactuca sativa L. ‘Salinas’ and heat-resistant (HR) L. serriola accession UC96US23 (Argyris et al., 2005), was grown in an aeroponic system in a tropical greenhouse under fluctuating ambient conditions of 25-39°C, using full strength Netherlands Standard Nutrient Solution. The RIL population was then screened for HR by measuring shoot and root productivity, and root morphology. HR-RILs were found to have high shoot and root fresh weights (FW) and relatively large root systems, whilst HS-RILs showed converse results. High mortality was also observed for HS-RILs.
To study effects of rootzone temperature (RZT) on the shoot and root productivity, as well as root morphology, selected RILs and their parental lines were grown in ambient-RZT (A-RZT) and cool-RZT (C-RZT) of 18-25°C. Half the plants within each RZT treatment were reciprocally transferred, 18 days after transplanting, giving four RZT treatments: A-RZT, C-RZT, A→C-RZT, and C→A-RZT. Across all RILs and their parental lines, lettuce grown at C-RZT had the highest shoot and root FW, and leaf area, compared to other RZT treatments. Phenotyping root morphology, HR-RILs had longer roots with greater number of root tips, allowing for more efficient water uptake. The use of productivity and root phenotyping is thus a viable approach in identifying RILs for the consequent production of new cultivars.