Phytonutrients and Genomics of Organic Tomatoes: Soil Fertility and/or Plant Defense

In previous studies, we showed that apples, strawberries, and tomatoes grown with organic nitrogen fertilizer sources had higher phytochemical contents than those fertilized with readily available sources of nitrogen. It is also possible that increased pest herbivory in the field stimulates plant defenses that elevate these phytochemicals in organic crops. To test the relative contributions of soil fertility management and herbivory to phytochemical accumulation we conducted a greenhouse study of tomato plants grown in exclusion cages and either fertilized with organic or readily available nitrogen sources, and exposed or not to herbivory by aphids. We have harvested the fruit and vegetative tissues of these plants, and are in the process of measuring the phytochemical concentrations/contents of a range of fruit sizes. The most significant findings to date are that the yields of green immature fruit and total vine weights were slightly greater for the conventionally fertilized tomato plants than for the organic fertilized. Nevertheless, these findings in favor of the conventional treatment are overshadowed by the higher fruit soluble solids (sugars) content and reduced aphid infestation of the organically fertilized plants. We will also measure gene expression in leaves and fruit of selected samples. These experiments will enable us to determine relative contributions of soil carbon/nitrogen and herbivory to phytonutrient accumulation in fruit and how differential gene activity may guide these two mechanisms in organic cropping systems. This project supports the 2010 BIOAg priority that sustainable farming practices increase food quality and nutrition.