Grain yield is a major goal for the improvement of durum wheat, particularly in drought-prone areas. In this study, the genetic basis of grain yield (GY), heading date (HD), and plant height (PH) was investigated in a durum wheat population of 249 recombinant inbred lines evaluated in 16 environments (10 rainfed and 6 irrigated) characterized by a broad range of water availability and GY (from 5.6 to 58.8 q ha À1 ). Among the 16 quantitative trait loci (QTL) that affected GY, two major QTL on chromosomes 2BL and 3BS showed significant effects in 8 and 7 environments, with R 2 values of 21.5 and 13.8% (mean data of all 16 environments), respectively. In both cases, extensive overlap was observed between the LOD profiles of GY and PH, but not with those for HD. QTL specific for PH were identified on chromosomes 1BS, 3AL, and 7AS. Additionally, three major QTL for HD on chromosomes 2AS, 2BL, and 7BS showed limited or no effects on GY. For both PH and GY, notable epistasis between the chromosome 2BL and 3BS QTL was detected across several environments.

No abstract

The ability to assess green biomass is of particular interest in a number of wheat breeding environments. However, the measurement of this and similar traits is either tedious and time-consuming or requires the use of expensive, sophisticated equipment, such as field-based spectroradiometers to measure vegetation indices (VIs). Here, conventional digital cameras are proposed as affordable and easy-touse tools for gathering field data in wheat breeding programmes. Using appropriate software, a large set of images can be automatically processed to calculate a number of VIs, based on the performance of simple colour operations on each picture. The purpose of this study was to identify a set of picture-derived vegetation indices (picVIs) and to evaluate their performance in durum wheat trials growing under rainfed and supplementary irrigation conditions. Here, zenithal pictures of each plot were obtained roughly 2 weeks after anthesis, and the picVIs that were calculated were compared with the normalised difference vegetation index (NDVI), an index derived from spectroradiometrical measurements, and with the grain yield (GY) from the same plots. The picVIs that performed best were the Hue, CIE-Lab a* and CIE-Luv u* components of the average colour of each picture, the relative green area (GA) and the 'greener area', similar to GA but excluding the more yellowish-green pixels. Our results showed a high correlation between all these picVIs and the NDVI. Moreover, in rainfed conditions, each picVI provided an estimation of GY similar to or slightly better than that provided by the NDVI. However, in irrigated conditions during anthesis, neither these picVIs nor the NDVI provided a good estimation of GY, apparently because of the saturation of the VI response in conditions of complete soil cover and high plant density.

The role of ear photosynthesis in grain filling was studied in a number of durum wheat (Triticum turgidum var durum L.) landraces and varieties from the Middle East, North Africa, and from the collections of ‘Institut National de la Recherche Agronomique’ (INRA, France) and ‘Centro International de Mejora de Maiz y Trigo’ (CIMMYT, Mexico). Plants were grown in the field in a Mediterranean climate. Flag leaves (blade plus sheath) and ears were kept in the dark from 1 week after anthesis to maturity which reduced grain weight by 22.4% and 59.0%, respectively. In a further experiment, the carbon isotope discrimination ratio (Δ) of ear bracts, awns and flag leaves was measured on samples taken at anthesis and on mature kernels. The mean value of Δ for the water soluble fraction of bracts (17.0‰) and awns (17.7‰) were lower than those of leaves (19.5‰) and fairly similar to those of kernels (17.4‰) averaged across all genotypes. Data indicate that most of the photosynthates in the grain come from ear parts and not from flag leaves. In addition, a higher water use efficiency (WUE) of ear parts than of the flag leaf is suggested by their lower Δ values. Gas exchange in ears and flag leaves was measured during grain filling. Averaged over all genotypes, CO2 diffusive conductance was about five times higher in the flag leaf than in the spike (with distal portions of awns outside the photosynthetic chamber) 2 weeks after anthesis. In absolute terms, the dark respiration rate (Rd) was greater than the net photosynthesis rate (Pn) by a factor of 1.74 in the spike, whereas Rd was much smaller, only 22.1, 65.7 and 24.8% of Pn in blade, sheath and awns, respectively. Data indicate that photosynthesis, and hence the water use efficiency (photosynthesis/transpiration), is greatly underestimated in ears because of the high rates of respiration which diminish the measured rates of net CO2 exchange. Results of 13C discrimination and gas exchange show that genotypes from North Africa have higher WUE than those from the Middle East. The high Rd values of ears as well as their low diffusive conductance suggest that CO2 from respiration may be used as source of carbon for ear photosynthesis. In the same way, the anatomy of glumes, for example, supports the role of bracts using internal CO2 as source of photosynthesis. In the first experiment, the Δ in mature grains from culms with darkened ears compared with control culms provided further evidence in support of this hypothesis. Thus, the Δ from kernels of control plants was 0.40 higher than that from ear‐darkened plants, probably because of some degree of refixation (recycling) of respired CO2 in the grains.

In this review, we will discuss physiological traits of C 3 cereals related to water use efficiency (WUE) in Mediterranean environments, from leaf (WUE instantaneous ) to crop level (WUE yield or 'water productivity'). First, we analyse the WUE instantaneous and the possible trade-off between improving this parameter and growth/yield performance. Ways to ameliorate WUE without penalties are discussed. We also analyse in what cases breeding by high or low WUE instantaneous is a suitable criterion to maintain grain yield under drought (Mediterranean) conditions. This question is approached in the framework of carbon isotope discrimination, (Á 13 C), the main indirect parameter used to integrate (at time and space scale) the WUE instantaneous in C 3 plants. A negative correlation between these two parameters has been confirmed by several studies. The relationship between Á 13 C and grain yield, however, is more complex, and may differ from one environment to another. In Mediterranean conditions with moderate or no water stress, a positive correlation between Á 13 C and grain yield is found in barley and wheat, whereas in 'stored-water' crops (such as in some regions of Australia), lower Á 13 C (i.e.higher WUE instantaneous ) is associated with higher grain yield, particularly in more stressful conditions. These apparent inconsistencies and their possible implications for plant breeding are discussed. One physiological trait that has received minor attention in attempts to improve WUE instantaneous is the role of ear photosynthesis. Ears of barley and durum wheat have a higher WUE instantaneous than the flag leaf, both in well-watered and in drought conditions. The underlying causes of the higher WUE instantaneous of ears are not fully understood, but their refixation capacity (i.e. the capacity to reassimilate respired carbon dioxide) could be important. Although the genotypic variability of this trait has not been extensively studied, some data support the idea that variation in refixation capacity may be attributable to genetic factors. At the crop level, decreasing soil evaporation is a crucial factor in efforts to improve the WUE yield in Mediterranean conditions, and fast initial growth of the crop (i.e. early vigour) seems to be relevant. In wheat, modern varieties with dwarfing genes (giberellic acid -insensitive) have higher yields but, concomitantly, they have lower initial growth performance. Recently, semi-dwarf cultivars (giberellic acid -sensitive) with high grain yield and simultaneously high early vigour were found, opening new avenues to increase WUE yield in wheat. The negative effects of futile water loss by cuticular and nocturnal transpiration are also commented. Finally, we discuss some agronomic practices (in particular, 'deficit irrigation' systems) linked to physiological traits that confer higher WUE yield, , in particular, in the cases of Mediterranean regions.