It has been assumed that durum wheat yields mre under stress than bread wheat (geographical distribution and statements in the literature). However, this is not known as only few research papers compared the performance of both crops, and in no case under a wide range of environments in field conditions. This study did precisely this sort of comparison, along 3 experimental years in which a range of fertilization and irrigation treatments were applied. In addition, a comprehensive database with data from field experiments was gathered and meta-analysed. We found from both approaches a cross-over interaction of yield of bread and durum wheat, but rejecting the hypothesis: bread outyielded durum wheat in the low-yielding conditions while durum wheat tended to have higher yield potential as a consequence of differences in sink strength in post-anthesis. The causes under the yield interaction and its consequences in efficiency terms are discussed.

Wheat (different species of Triticum), among other cereals, was consumed by humans before the beginning of agriculture. Nonetheless, from that moment on, the intuitive process of picking the biggest grains for sowing the next crop progressively changed those regular grasses to suit human needs, starting an approach to a plant ideotype with better agronomic characteristics, including higher yields (Fig. 1). That first “green revolution” for wheat started along the Fertile Crescent c. 10,000 BP (Hillman and Davies, 1990) with the progressive domestication of some species of wheat, remarkably the eikorn (Triticum monococum) and the emmer wheats (Triticum turgidum ssp. Dicoccum) which were widely used in the ancient times (Mac Key, 2005) and from which in some moment the modern bread and durum wheats derivate. The cultivated emmer has a lower TKW (thousand kernel weight) than durum (Faris et al., 2014) and bread wheat (Konvalina et al., 2008) is hulled and do not contain the Q “domestication” gene related to the pleiotropic effects of free threshability, rachis stiffness, and glumes tenacity. Durum wheat, (Triticum turgidum L. ssp. durum) shares the AABB genome with its ancestor, while the hexaploid bread wheat (Triticum aestivum L.) with the AABBDD genome derivates by a process of allopolyploidization from the tetraploid emmer and the D genome from the wild diploid Triticum tauschii (Coss.) (Huang et al., 2002; Marcussen et al., 2014) (Fig. 1), an annual grass which usually produces numerous tillers, thin stems, small and narrow leaves, and small grains (Ehdaie and Waines, 2013). Despite that it is controversial the exact timing when bread wheat allopolyploidization occurred, it must have been relatively recent, since the Q gene found in both wheat was originated from a single event in the past (Simons et al., 2006). That additional ploidy level conferred to bread wheat a greater adaptability and numerous loci for constrain resistance have been found on it (Mujeeb-Kazi and Hettel, 1995).

Wheat is one of the most important cereals feeding humanity (Shewry, 2009), nowadays 7000 millions of people. Nonetheless, with the latest estimations concerns are growing about the possibility that food production will not meet demand in some near future (Pinstrup- Andersen et al., 1999; Tweeten and Thompson, 2009; FAO, 2009; Godfray et al., 2010; Reynolds et al., 2012; Keating et al. 2014), since by the 2050 the humanity will have reached 9.1 billion, the food production must increase about a 70 % (FAO, 2009) or up to 98% considering the higher socioeconomic development expected (Valin et al., 2014). Following that considerations Borlaug, 2002 calculated c. a 57% by the year 2025; which for wheat means that world average yield must increase at a rate of c. 70 kg/ha per year (Fig. 2). However, increases in food production (Alexandratos, 1999; Borlaug, 2002) and in wheat yield potential are diminishing (Traxler et al., 1995; Acreche et al., 2008; \

• Bread and durum wheat relative importance and end uses
World wheat production is c. 644 millions of tones, most of them from bread wheat, which is one of the most important staple foods of the human diet, providing c. 19% of the carbohydrates and 20% of proteins (Braun et al., 2010) by direct consumption. What is more, in some countries wheat consumption can reach the 63% of the total ingested calories (http://faostat.fao.org/). decreased c. 70 % in the last half of the century (Cussó and Garrabou, 2007). Inversely, the amount consumed indirectly as feed for livestock (c. 25% of total world wheat production) firmly increased in many developed countries during the last decades (Fig. 3c) as a consequence of an important increase of meat consumption; but expectedly, other end products will particularly be important future sinks of wheat production, e.g. biofuel (Shewry, 2009) or bioplastics (Domenek et al., 2004).

Durum wheat, in contrary to bread wheat, is a relatively minor crop c. 6% of total wheat production (International Wheat Council, 2010), mainly distributed in the Mediterranean basin, which is mostly used for human direct consumption as pasta products, couscous, and flat bread. However, durum wheat consumption has a small but constant tendency to increase in many countries (Fig. 3d), (Taylor and Koo, 2011); as a consequence of increases in pasta products, e.g. pasta consumption per capita in 2000 (8.8 kg) almost doubled the consumption in 1975 in the U.S., (Elias and Manthey, 2005), and because the popularization of new durum products (e.g. couscus, flat bread and kebab double-sided bread) (Elias and Manthey, 2005; Sissons, 2008).

Overall, it is important to remark that wheat consumption habits and uses are changing and despite most wheat production is used for human direct consumption, an important part of the wheat uses can be indistinctly satisfied by both wheat species.

Yield is the result of complex interactions throughout the growing season with the direct or indirect expression from most of the genes (Slafer, 2003). It is a complex character and any attempt to increase it would be more likely if based on a deep understanding of its generation. Commonly, yield can be divided into its components (Fig. 5), i.e. plants m-2, spikes plant-1, spikelets spike-1, grains spikelet-1, and grain weight, we can find evidences that both wheat species differ in some of those variables in the literature (Fischer and Wood, 1979; López-Castañeda and Richards, 1994; Zubaidi et al., 1999; Trethowan et al., 2001; Reynolds et al., 2002).

Unfortunately, differences in some variables, between the studied species or cultivars, are not directly translated to yield, because the existence of competence between plants for resources and because the yield components present relationships of competence between them determining yield components compensations (Fischer, 2001; Slafer, 2003), e.g. more spikes m-2 implies a lower number of grains spike-1. However, differences in physiological attributes, such as the duration of the crop growth phases, the efficiency in capturing and using resources or in response to stress, can result in actual yield differences, disregarding the compensations that may exist.

1.2 Statement of problem
Some of the difficulties to find general assumptions in the relative performance of both species are the complexity of the factors involved in yield formation, the limited number of variables which can be determined in each experiment, and the difficulty to accommodate a large degree of variability within particular experiments. In fact, most of the experimental assays comparing bread and durum wheat in the same environmental conditions were carried out under a very limited range of environments, many times limited to yield potential conditions (e.g. Fischer and Maurer, 1978; Aggarwal et al., 1986; Acevedo, 1991; Josephides, 1992; López-Castañeda and Richards, 1994; Palumbo and Boggini, 1994; Zubaidi et al., 1999; Calderini et al., 2006) and unfortunately, little is known based on solid experimental bases about the possible differential adaptation (presumed by the consistent pattern of distribution of these crops with respect to the yielding conditions) of these species to the environment, and the expected G X E interactions, and even less from the causes producing them. Furthermore, the scattered results from these limited attempts commonly show opposite conclusions or are inconclusive and in general, a direct extrapolation of knowledge from one species to another, as is commonly done in the literature, prevents the understanding of their specific differences. For this reason it is highly relevant to understand the environmental adaptation of the major physiological determinants with the aim of developing new selection criteria for breeding and to use the genotypes more efficient or productive for a given environment and ultimately rationalize land use to increase field crops productivity. Therefore, it would be important to test scientifically the assumptions derived from the pattern of land allocation suggesting that bread wheat would be higher yielding than durum wheat under relatively aleviated stress conditions and vice-versa.

• Objectives
The overall aim of this thesis was to compare the bases of the differences in productivity between bread and durum wheat under a wide range of environmental conditions, particularly with important variations in water and N availability in contrasting growing seasons.

To address the overall aim, I pursued the following specific objectives:

• To quantify the differences in performance, in productive terms, between bread and durum wheat in a wide range of environmental conditions (Chapter II).

• To identify differences in the species yield components and biomass growth, partition and dynamics along the environments (Chapter III).

• To study the origin of the possible species differences in performance by analyzing yield and biomass formation and partition before and after anthesis (Chapter IV).

• To identify differences in the species morphology (leaves, tillers, roots) and growth dynamics in relationship with the adaptation to the variations of the environments (Chapter V).

• To analyze the uptake and use efficiency of resources, in particular for the radiation, water and N, under the studied wide range of experimental conditions (Chapter VI).

The main approach used to achieve the aims was to run a set of field experiments in which both types of wheat were grown side-by-side throughout three consecutive growing seasons under contrasting water x N conditions in each case. Furthermore, I did review the literature, collecting data fragmentary available and meta-analysing these dispersed data in a single framework. In addition, as a subordinate objective I analyzed as a by-product the possibility of realizing good assessments of yield in earlier plant developmental moments by using the normalized difference vegetation index (NDVI), overall a group of bread and durum cultivars and water and N treatments and included this analysis as an Annex to the thesis.

• Organization of study
The present thesis is constituted by seven chapters and an annex. It includes this general introduction (Chapter I), six experimental research chapters (Chapters II, III, IV, V, VI), a general discussion with conclusions of the whole work (Chapter VII), and an annex reporting on the relationships between yield and NDVI measurements taken in pre-anthesis. In addition to unpublished results, this thesis includes data from two research papers published

in SCI journals (Marti et al., 2007; Marti and Slafer, 2014). However, for convenience with the traditional structure of a thesis, results have been divided into chapters following the different relevant issues of study (and not respecting the information gathered in the papers). As each chapter intends to be independently understood, all the chapters include the common regular sections of a scientific paper, but I tried to avoid repetitions, particularly with the materials and methods and in the introductions, leaving very concise introductions for each experimental chapter.

Chapter II includes experimental evidences to contrast the species relative performance in yield, comparing our experimental data with most of the existing literature data; Chapter III presents experimental evidences of the yield components generating yield differences, considering both numeric components and growth components; in Chapter IV I studied in further detail the generation of differences in yield and biomass mainly as a consequence of differences in sink-strength; Chapter V presents a descriptive research of some morphological differences between wheat species; and Chapter VI shows experimental evidences of capture and use of resources. Chapter VII offers a brief general discussion and conclusions. Finally, Annex I contains an analysis of the possibility to assess yield from indirect measurements done in early stages of development.

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