I investigated the interaction of a tall grown cereal crop, grain Sorghum (Sorghum bicolor), with each of three weeds during early growth under a moisture gradient. The weeds Parthenium hysterophorus, Tagetes minuta and Verbesina encelioides, all belonging to Asteraceae, are native to South America and common as annual weeds in warmer parts of the world.

I aimed to answer the following questions:

- Do the crop and weeds affect each other?

- Do the weedy species differ in competitive effect/response on/to Sorghum?

-Can differences in reactions to water stress explain the difference between the weeds variation in competitive effect and response?

The moisture gradient consisted of five levels, one representing adequate moisture and the others different levels of moisture stress. Seedlings were harvested after six weeks of growth and total biomass (dry weight) and a number of morphologic ratios were measured:

-shoot weight/shoot lenght  

-root weight/root lenght

-shoot lenght/number of nodes

-shoot weight/leaf area

-root weight/shoot weight