Further improvements in wheat yields are critical, for which increases in grain number would be required. In the recent past, higher grain number was achieved through increased growth of the juvenile spikes before anthesis, due to the reduction in stem growth. As current cultivars have already an optimum height, alternatives must be identified for further increasing grain number. One of them is increasing fruiting efficiency (grains set per unit of spike dry weight at anthesis). Fruiting efficiency is the final outcome of the fate of floret development and differences in this trait within modern cultivars would be related to higher survival of floret primordia. Then there are two alternative physiological pathways to improve fruiting efficiency by allowing a normal development of most vulnerable floret primordia: an increased allocation of assimilates for the developing florets before anthesis, or reduced demand of the florets for maintaining their normal development. Both alternatives may be possible, and it might be critical to recognize which of them is the actual cause of differences in fruiting efficiency. When considering this trait in breeding we must be aware of potential trade‐offs and therefore it must be avoided that increases in fruiting efficiency be constitutively related to decreases in either spike dry weight at anthesis or grain weight. In this review we described fruiting efficiency and its physiological bases, analyzing genetic variation and considering potential drawbacks that must be taken into account to avoid increases in fruiting efficiency being compensated by other traits.

The identification of physiological traits that determine grain number (and yield) in modern cultivars, and the possible tradeoffs among them, may help to detect promising traits for breeding to increase yield potential. High-yielding Argentinean wheat {Triticum aestivum L.) cultivars were grown under irrigated and highinput conditions during two seasons to study (i) yield and grain number m"^ (GN) as a result of stover biomass at harvest (SH) and harvest index (HI); and (ii) spike dry weight at anthesis (SDWgi g m"2) and spike fertility index (SFI; grain number per gram of spike chaff or non-grain spike biomass). Duration of the stem elongation phase (SEP; first node detectable to anthesis) was analyzed as an alternative to increase SDWg. The HI was highly associated with yield differences between cultivars for both years, independently of stem height. In contrast, SH did not vary among cultivars in either season (P > 0.05). The GN determined yield differences between cultivars and was highly associated with SFi and SDW^ for both years and cycles. Some cultivars with the same anthesis date showed stable variation for duration of the SEP between years (despite the year x cultivar interaction observed). As the SFI and duration of the SEP were not related, it seems promising to increase GN in a cultivar with high SFI through an increased duration of the SEP (which may yield higher SWDJ or vice versa.

To accelerate genetic gains in breeding, physiological trait (PT) characterization of candidate parents can help make more strategic crosses, increasing the probability of accumulating favorable alleles compared to crossing relatively uncharacterized lines. In this study, crosses were designed to complement

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Zero tillage oft en leads to wheat (Triticum aestivum L.) yield losses from diseases caused by necrotrophic foliar pathogens. Th e aim of this work was to evaluate the combined eff ect of tillage, N fertilization, fungicides, and resistant cultivars in reducing foliar disease severity to prevent signifi cant yield losses. A 2-yr study including combinations of (i) conventional and zero tillage; (ii) N fertilization rates 0, 80, or 160 kg ha -1 N; (iii) two fungicide treatments (with and without a fungicide (1 L of metconazole, 9%) at growth stages (GS) 32 and 39; and (iv) three wheat cultivars was conducted in the Rolling Pampas region in Argentina. Th e most common foliar disease in the trial was tan spot [Pyrenophora tritici-repentis (Died.) Drechs.]. Conventional tillage reduced foliar disease severity at GS 23 by 46 and 56% and the area under disease progress curve (AUDPC) by 20 and 14% for each season, respectively compared with zero tillage. Th e cultivar Buck Bigua had signifi cantly lower AUDPC values than the others. Fungicide and N application reduced disease severity at GS 23 by 35 and 34% respectively, on average over both years. Disease was less severe in zero tillage plots which received a fungicide compared to conventional tillage plots that were not treated with fungicide. In 2002 yields were greater in conventional tillage plots with 160 kg ha -1 N and fungicide application than in all other treatments. In 2003 yields were greatest in zero tillage plots with 160 kg ha -1 N and fungicide. Th e results of this study indicate that in spite of the increase of necrotrophic diseases, developing no-till systems in wheat monoculture is possible without signifi cant yield losses if eff ective disease management practices are applied.

Improving wheat (Triticum aestivum L.) yield potential via higher grain number per unit area (GN) may reduce average grain weight and consequently the yield increment would be low. Thirty‐nine high‐yielding modern cultivars differing in spike fruiting efficiency (FE) (number of grains per gram of no‐grain spike) were grown under potential conditions during two temperature‐contrasting years to study the level of source limitation during grain filling and rank the main physiological determinants of potential grain weight (PGW) and actual grain weight. The response of grain weight to a 100% higher source ranged from 0 to 25%, depending on cultivar and year. No general relationship was observed between PGW and FE. The stabilized grain water content, which was partially correlated with the endosperm cell number per grain (r2 = 48%, p < 0.05), was the main variable associated to grain weight variation (r2 = 77%, p < 0.00001) in both temperate and warm years. Genetic improvement focused on increasing the sink strength through higher GN is still an alternative to increase yield potential in modern cultivars as no source limitation during grain filling was observed. The improvement of FE could be used to increase GN and yield as no general negative relationship was observed between PGW and FE.

Spike fruiting efficiency (FE), defined as grains per unit of spike dry weight at anthesis (SDWa) is a promising trait for improving grain number (GN) in wheat (Triticum aestivum L.). It is often estimated at maturity as the grains per unit of chaff or FE at maturity (FEm). The fertile floret efficiency (FFE), defined as fertile florets per unit of SDWa, and grain set (GST), or the number of grains per floret, were studied to better understand FE determination for the first time. Two double haploid populations designed by crossing modern cultivars contrasting for FE ['Baguette 19' and 'Baguette Premium 11'(high FE) × 'BioINTA2002' (low FE)] were sown in five environments. The FE and FEm showed an unstable correlation (low or high) among genotypes within environments (caused by variable SDWa-chaff associations), resulting in a worse correlation between GN and FEm than between GN and FE. Therefore, the use of FEm as a surrogate for FE to improve GN may yield lower gains than those expected if FE were used. The narrow-sense heritability of FFE was high but the variability in fertile florets per spike among genotypes within environments was correlated with FFE only in the environments with high SDWa. Despite the close association between FE and FFE, the former was not totally set at anthesis, as GST greatly affected FE and GN. Selecting for higher FFE and GST, where genotype × environment effects determine heavy spikes at anthesis, is an alternative to breeding for improved GN.

Key message This study identified and validated two QTL associated with spike fertile floret and fruiting efficiencies. They represent two new loci to use in MAS to improve wheat yield potential.