Field plot size and drought phenotyping

The field experimental unit in breeding programs can vary from small hill plots to very large plots harvested by combine. Plot size varies mainly due to the available amount of seed per genotype or the available amount of land and work force. In the long history of plant breeding it has been well established which plot size can be acceptable for each population, generation or task. Even hill plots and single rows (“rod rows”) are allowed, despite the fact that they have no border and can be seriously affected by inter-plot and inter-genotypic competition. These are commonly used for very heritable single plant traits, such as flowering date, disease reaction, etc’. When preliminary screening of very large populations is the purpose and when repeated evaluation of selected materials is to be performed in the next generation, such small plots are acceptable for initial screening. Furthermore, in most early stage screening where very small plots are used, growing conditions are favorable and therefore the risk of inter-plot competition is reduced although not eliminated.

Enter molecular mapping for drought resistance. It is not uncommon that phenotyping of mapping populations for drought resistance is performed with very small plots of several plants or very short single rows. A common explanation (if offered) is that these are standard plots used by breeders or that there was not enough space in the field or under the rainout shelter. It should be made very clear that very small plots for testing a very diverse population under drought stress is most likely to cause serious errors in assessing the real response to stress of individual genotypes. A large part of the genotypic response to drought stress is a plant community and a leaf canopy phenomenon, which is lacking in a single row of few plants. So here we go again complaining about reports on mapping exercises of drought resistance traits where the molecular part of the work is super but the phenotyping part can be rated between unacceptable to ridiculous. Still, more than few of these reports find their way to publication in serious plant science journals, most probably because they were subjected to peer reviews by molecular biologists.

Despite all of the above single rows or even single plants may sometimes be acceptable, depending on the crop, method/timing of stress application and the specific traits measured*. An educated decision should be made regarding plot size by consultation with a breeder and a crop physiologist.

(Plantstress Curator is grateful to Dr. Greg Rebetzke, CSIRO Australia for raising this issue)

(*) Example: survival of single F₂ plants under an extreme/killing drought stress, with verification in F₃.