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The chlorophyll fluorescence parameter Fv /Fm reflects the maximum quantum efficiency of photosystem II (PSII) photochemistry and has been widely used for early stress detection in plants. Previously, we have used a three-tiered approach of phenotyping by Fv /Fm to identify naturally existing genetic variation for tolerance to severe heat stress (3 days at 40°C in controlled conditions) in wheat (Triticum aestivum L.). Here we investigated the performance of the previously selected cultivars (high and low group based on Fv /Fm value) in terms of growth and photosynthetic traits under moderate heat stress (1 week at 36/30°C day/night temperature in greenhouse) closer to natural heat waves in North-Western Europe. Dry matter accumulation after 7 days of heat stress was positively correlated to Fv /Fm . The high Fv /Fm group maintained significantly higher total chlorophyll and net photosynthetic rate (PN ) than the low group, accompanied by higher stomatal conductance (gs ), transpiration rate (E) and evaporative cooling of the leaf (ΔT). The difference in PN between the groups was not caused by differences in PSII capacity or gs as the variation in Fv /Fm and intracellular CO2 (Ci ) was non-significant under the given heat stress. This study validated that our three-tiered approach of phenotyping by Fv /Fm performed under increasing severity of heat was successful in identifying wheat cultivars differing in photosynthesis under moderate and agronomically more relevant heat stress. The identified cultivars may serve as a valuable resource for further studies to understand the physiological mechanisms underlying the genetic variability in heat sensitivity of photosynthesis.

Evolutionary theory can be applied to improve agricultural yields and/or sustainability, an approach we call Evolutionary Agroecology. The basic idea is that plant breeding is unlikely to improve attributes already favored by millions of years of natural selection, whereas there may be unutilized potential in selecting for attributes that increase total crop yield but reduce plants’ individual fitness. In other words, plant breeding should be based on group selection. We explore this approach in relation to crop-weed competition, and argue that it should be possible to develop high density cereals that can utilize their initial size advantage over weeds to suppress them much better than under current practices, thus reducing or eliminating the need for chemical or mechanical weed control. We emphasize the role of density in applying group selection to crops: it is competition among individuals that generates the ‘Tragedy of the Commons’, providing opportunities to improve plant production by selecting for attributes that natural selection would not favor. When there is competition for light, natural selection of individuals favors a defensive strategy of ‘shade avoidance’, but a collective, offensive ‘shading’ strategy could increase weed suppression and yield in the high density, high uniformity cropping systems we envision.

The purpose of this study was to estimate the heritabilities and the genetic correlations of osteochondrosis in different joints and to estimate genetic correlations between osteochondrosis, leg weakness and production traits. The humerus condyles, anconeal process, distal ulna growth line, femoral condyles, distal tibia and medial trochlear ridge of talus on both sides of the animals were examined radiologically for osteochondral lesions and scored from 1 to 5 with increasing severity. The boars were also scored for 13 different leg weakness traits on a scale from 1 (normal) to 5 (very severe changes). During a 3-year period, a total of 2042 Landrace boars from 256 sires and 1946 Yorkshire boars from 240 sires were examined at a mean weight of 93 kg. The data were analysed by animal models for each breed. The Landrace was found to be more severely affected than the Yorkshire regarding osteochondrosis in all localities expect the anconeal process. The genetic correlations between left and right side scores of the osteochondral trait were close to 1. This indicates that genetically the same trait is measured in the left and in the right side of the animal. The heritability of average osteochondrosis scores was low to moderate ranging from 0•08 to 0•39 (s.e. between 0•02 and 0•04). The genetic correlations between osteochondrosis in the different body localities were low, indicating that osteochondrosis is not a generalized disease. The Landrace was more severely affected than the Yorkshire regarding all leg weakness symptoms except buck-kneed forelegs, forelegs turned out and weak pasterns on forelegs. The heritability of leg weakness traits ranged from 0•01 to 0•35 (s.e. between 0•01 and 0•04). Genetic correlations between osteochondrosis and leg weakness showed a clear picture for the Yorkshire breed regarding the humeral condyles: positive correlations with buck-kneed forelegs, forelegs turned out, stiff in front and rear. In both breeds osteochondrosis in femoral condyles showed a high positive correlation with hind legs turned out and a lower positive correlation with stiff in front. The sum of osteochondrosis traits (sumost) was unfavourably genetically correlated with daily gain, whereas the sum of leg weakness traits (sumleg) was unfavourably correlated with lean meat percentage in both breeds. The correlation between the summary measures sumost and sumleg was 0•09 (s.e. 0•11) in the Landrace and –0•07 (s.e. 0•12) in the Yorkshire.