,51,52,53,54]. Its significance increases even more considering the reality of climate change, affecting differentially various areas in the globe, and the necessity to effectively keep up with its accelerating genetic progress.
The results point to some important inferences with wider significance for breeding programs: The potential genetic variation for productivity and stability within cultivars seems to be large and inexhaustible, but goes undetected without the appropriate resolving tools. In order to effectively exploit this variation, avoid degeneration, and secure constant genetic gain, the nonstop intracultivar selection emerges as an essential process. The new selection and whole-plant field phenotyping equations offer unique properties for predicting the crop yield and stability of individual plants. This also involves the nonstop exploitation and incorporation of genes for resistance to biotic and abiotic stresses, permitting to effectively face pathogen selection pressure.
3.2. Intra-Cultivar Selection for Yield and Resistance to Cotton Verticillium Wilt
In the second set of experimental data, the application of the honeycomb selection for three consecutive years within Sindos-80 led to the development of the experimental variety Macedonia, which outyielded Sindos-80 by an average of 10% when tested at sixteen locations across Greece in standard RCB trials for two additional years . Although the variety Macedonia was developed using very high selection pressures (1.5%), its progeny lines retained the ability to further generate high genetic diversity. This was evident when honeycomb selection for yield potential per plant was simultaneously applied for one year in a site heavily infected with Verticillium and in a non-infected field. The process allowed the isolation of two lines with similar degree of Verticillium resistance as the resistant controls Acala S.J.5 and S.J.2. The two lines outyielded all controls (Acala S.J.5 and S.J.2, Macedonia, and Sindos-80) in both contrasting environments. In particular, they outyielded the more resistant Acala control (Acala S.J.5) by 140% in the non-infected field and by 40% in the infected field. The original mother variety, Sindos-80, is susceptible to Verticillium wilt, while Macedonia exhibited resistance similar to that of the Acala S.J.2. Importantly, the one year of selection in the infected site also revealed the existence of two low-yielding lines within the variety Macedonia, whose yield per plant was inferior to Acalas by 65%. These lines were even more susceptible to Verticillium than the original mother variety Sindos-80.
4. Experimental Section
4.1. Intra-Cultivar Selection for Yield Stability and Crop Yield Potential
In this set of experiments, the material used for applying selection was a leading, elite cotton cultivar, Celia. The study lasted four years, the first two were devoted to single-plant and progeny line selection within Celia and the subsequent two to the implementation of standard Randomized Complete Block (RCB) trials to assess the efficiency of selection in comparison to the original cultivar. All trials were grown at the Farm of the Aristotelian University at Thessalonica  under standard agronomic practices for the region and appropriate weed management.
First growing season-2004: Seven cotton cultivars, coded from 1 to 7, were grown in a replicated-7 honeycomb trial [Figure 1, code 13] with 133 plant positions per cultivar and 100 cm plant-to-plant spacing to exclude interplant competition. Seedcotton from all plants was picked up individually, weighed, and the individual plant seedcotton yields were transformed into unitless values based on the Plant Yield Index component of Equation A. Thirty plants from cultivar coded 3, i.e., Celia, were selected and advanced to the next year’s trials.
Second growing season-2005: Thirty progeny lines belonging to the 30 plants obtained after intracultivar selection and the original cultivar as control, were grown in a replicated-31 honeycomb trial [Figure 2, codes 1 to 31, 13] with 28 plant positio