Effects of Determinate Growth and Obscure Veins on Water Use Efficiency In California Processing Tomatoes.

Determinate growth habit and synchronized fruit set are traits of interest in breeding for California processing tomato cultivars. A one-time destructive machine harvest requires a compact canopy and high percent of ripe fruit per plant in modern cultivars. Along with these morphological and phenological changes, evapotranspiration rates have kept constant although productivity has increased in more than 50% since the 1970’s (Hanson and May, 2006. Irrig Sci 24, 211-221). A paralog of the self-pruning gene, which controls the switch between determinate and indeterminate habit, is the SP5G located on chromosome 5 at or near a QTL for plant height and flowering time, and  near a gene conferring obscure veins, obscuravenosa (obv) (Jones et al., 2007. Am J Bot 94, 935-947). The obv gene increases chloroplast density near leaf veins and it is present in a majority of California processing tomato cultivars. We hypothesized that: 1) stronger expression of determinate growth allows for higher allocation of resources to fruits while decreasing leaf and stem biomass, and reducing canopy size; 2) obscure veins confer higher photosynthetic capacity, which in turn might increase photosynthetic carbon assimilation per unit of water transpired, indicating higher water use efficiency. A group of introgression lines with defined segments of the Solanum pennellii genome in the genetic background of cultivated tomato, S. lycopersicum cv. M82, were compared to understand the effect of obscure veins and determinate habit. Canopy growth, biomass, gas exchange measurements, leaf thickness, and stomatal density were measured in a greenhouse and field study. Preliminary results show that lines with determinate growth increase fruit biomass but reduce total aboveground biomass, and the obscure vein trait may be associated with a decrease in water use efficiency.