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Wheat developmental stages and yield achievement i. Aerial development and yield achievement
Wheat development is constituted by successive and partially overlapping developmental phases. Wheat development is the result of exogenous factors such as temperature, vernalization, and photoperiod needs. Bread wheat development is mainly dependent on temperature due to its insensitivity to vernalization (Prasad et al., 2008). Therefore, the plant development is usually expressed in terms of thermal time unit, or growing degree days. It corresponds to a measure of the heat accumulation by plants along its development (McMaster and Wilhelm, 1997; Acevedo et al., 2002). Organ differentiation occurs during the various stages of wheat development. In 1974, Zadoks et al. published a scale for cereal development. Such scale is used to ‘quantify’ the wheat development. The Zadoks scale starts with the germination and then the emergence with the leaf production, the tillering with the tiller production, the stem elongation with the “node” production, the booting, the heading, the anthesis, and the physiological maturity (Figure II-4 and II-5).
The wheat crop cycle is divided into three periods: (i) the vegetative period, from sowing to floral initiation, occurring during tillering stage, (ii) the reproductive period, from floral initiation to anthesis, and (ii) the grain filling period, from anthesis to physiological maturity (Slafer, 2012). Leaf appearance starts at emergence and ends before booting with emergence of the last leaf, named the flag leaf. Plant height is set from emergence to some days after anthesis. Most of plant height is achieved within the stem growth phase starting some days before terminal spikelet stage (i.e., stem elongation) and ending some days after anthesis with the end of the peduncle growth (Figure II-5) (Acevedo et al., 2002).
Wheat grain yield can be dissected into various components. Each one is established at a more or less specific given period of the crop cycle. We are going to detail it now.
Wheat grain yield is established during the whole crop cycle. It is the combination of two main components set during the growth cycle overlapping at anthesis; from emergence to a week after anthesis: the number of grains per square meter, and during the grain development and filling, from grain set stage to physiological maturity: the kernel weight (Figure II-5). At a constant sowing density, the number of grains per square meter can then be dissected into: (i) the number of spikes per square meter and (ii) the number of grains per spike. All tillers produced by a wheat plant will not lead into spikes. Some will abort before anthesis (Gallagher and Biscoe, 1978a; Gaudillère and Barcelo, 1990). Compensation mechanisms exist between the different yield components in wheat. If one is impacted, the other ones might compensate loss (Slafer et al., 1996). In wheat, meiosis coincides with the booting stage (Z4.0). It starts in the middle of the spike and is spread toward both the base and the tip of the spike (Zadoks et al., 1974). In corn (Zea mays L.), Jones et al. (1985) reported at anthesis, after fecundation, a rapid cell division period leading to the appearance of endosperm cells and amyloplast of the future grains. Then, these cells grow, are differentiated and the starch deposition starts. This is the beginning of the grain filling phase (Figure II-5).
Figure I-7:Schematic diagram of wheat growth and development adapted from fe d so 199 so d me he so 000 d fe 01 , showing the main developmental stages of wheat growth, their correspondences within the Z doks’ s e (Zadoks et al., 1974; Tottman, 1987) and the timing of initiation of crop yield components. Periods of initiation of growth (or death) of specific organs and those of when different components of grain yield are produced are represented in bottom boxes.
There are two main flowering types in wheat due to their response to vernalization3 (Flood and Halloran, 1986): (i) winter wheat and (ii) spring wheat. The former one shows a strong response to vernalization and requires a period of cold wheather to initiate flower development. In its early stages, winter wheat is highly resistant to frost (-20°C). The latter one has a very mild response or no response to vernalization. It is sensitive to frost (Acevedo et al., 2002). To acquire the ability to flower, some wheat genotypes may require specific day-length: they are sensitive to photoperiod. Most of cultivated wheat genotypes are long-day plants, i.e., flowering is accelerated with day-length increase, but they do not really need specific length of day to initiate flowering (Major and Kiniry, 1991). The major vernalization and photoperiod genes have been identified, molecular studies have identified their interactions, and gene networks showing their inter-relationship have been proposed (Trevaskis et al., 2007; Distelfeld et al., 2009; Shimada et al., 2009; Trevaskis, 2010). Major genes will be presented on part V.
Roots establishment and growth in cereals
Cereal roots can reach 2m depth in field conditions by the end of anthesis (Lucas et al., 2000; King, 2003). Basic morphology of cereal root systems is well known. It grows following a consistent pattern and, as a consequence, has a relatively predictable architecture in uniform soils (Robinson, 1994). King (2003) reported that dynamic morphology of cereal root systems can be summarized with only a few variables without reducing significantly the resolution of the model.
The whole root system of a plant can be organized in three main schemes: (i) the taproot system found in most of Dicotyledonous and Gymnosperm, (ii) the fascicular root system characteristic of most of Monocotyledonous, and (iii) adventitious roots system (Prat and Rubinstein, 2005). Root system of many cereals like wheat, barley, and oats is classified as fascicular root system. However, it consists in two different root systems occurring successively. First, seminal roots grow from the seeds and then, starting at tillering (Z2.0, Zadoks et al., 1974; Tottman, 1987), nodal roots, also known as adventitious roots, appear at the base of the main stem and tillers, and develops abundant root hair. Each tiller develops its own roots allowing it to be independent of the plant (Lucas et al., 2000). Authors reported a root extension rate, sensitive to temperature and environment, ranging from around 5 mm d-1 for cereals sown in autumn to 15-25 mm d-1 in spring. At full emergence and maximum canopy size, with a root depth reaching 1.5 to 2.0 m depth, maximum root weight is around 1 t ha-1 and total root length range between 16 and 32 km m-2.
Grain yield progress from the XVIIIe to the 1990’s
A recurrent purpose in agronomic science is the improvement of crop yield. Many studies focused on bread wheat yield evolution due to its importance in both economy and in human food supply: for the UK winter wheat (Austin et al., 1980, 1989), for the Canadian Prairies spring wheat (Stewart and Dwyer, 1990), for the north western mexican bread wheat (Bell et al., 1995), for the French winter wheat (Brancourt-Hulmel et al., 2003; Brisson et al., 2010; Oury et al., 2012) and for many other countries (Calderini and Slafer, 1998).
In France, first bread wheat varieties cultivated were landraces. One of the first traces of bread wheat variety recorded is ‘Rouge d’Alsace’ and ‘Noé’ around 1826 (Doré et al., 2006). At the beginning of 19th century, yield was around 0.9 t ha-1. In 1950, 150 years later yield had just doubled to reach 2.0 t ha-1. The global grain yield increase was really slow with around +0.01 t ha-1 year-1 (Bonjean et al., 2001; Brancourt-Hulmel et al., 2003). A the end of 19th century, the first variety bred by Henry De Vilmorin, so called ‘Dattel’, from a cross between two English wheats, was the result of a kind of pedigree breeding. At this time, English varieties were late, displayed good resistance to yellow rust (Puccinia striiformis) and to lodging, but had a really poor bread quality. Such quality was brought by “Aquitaine wheat”, originating from Russia. The ‘Bordier’ variety, released in 1889, sign the start of variety combining both habits from Aquitaine and English wheats (Doré et al., 2006). With 1920s came the development of public and private breeding stations that represent an important change in plant breeding. Just before the Second World War, in 1938, with the progresses achieved, France became temporarily self-sufficient in wheat (Bonjean et al., 2001). From 1950 to 1990, yield more than tripled to reach 7.3 t ha-1, corresponding to a progress of +0.13 t ha-1 year-1 (Bonjean et al., 2001). This progress came from the improvement both of agronomic crop management (higher level of input such as nitrogen and pesticide, use of certified seeds, etc.) and of genetic of cultivated varieties. Genetic progress can be dissociated from yield progress due to crop management modernization. In 2003, Brancourt-Hulmel et al. estimated the genetic progress between 1950 and 1990 at +0.063 t ha-1 year-1. Genetic progress was mainly due to the introgression of dwarfing genes, known as the Green Revolution4, which strongly improved harvest index and enabled higher imput levels with reduced lodging risks. It is also the consequence of a better disease resistance (Bonjean et al., 2001).
4 Initiated by Norman Borlaug, the Green Revolution is the result of a series of investigations and technology transfer initiated in the 1940’s and lasted until 1960’s. It leaded to an dramatic increased of the worldwide agricultural production by the development of high yielded cereal varieties, irrigation, modernization of agronomic practices, and the wider use of improved seeds and chemical products (Wikipedia, 2014b).
Most of the breeding efforts to improve wheat grain yield resulted in an increase in the number of grains per square meter, by through the number of grains per spike. In 1989, Austin et al. compared older and ‘modern’ US winter wheat varieties, with higher grain yield. He showed that the grain per square meter increased by more than 59 %, with 14 % more spikes per square meter and 30 % more grains per spike, and with a relatively constant grain weight. Similar conclusions were also reached by other studies (Perry and D’Antuono, 1989). A well-known hierarchy of yield components in yield achievement is that the number of grains per square meter is much more important than the grain size, i.e., the number of grains per square meter is the coarse-regulation mechanism and the grain size, only a fine-tuning mechanism (Slafer et al., 2014). As a consequence, modern wheat cultivars are able to sustain grain filling of much more grains per square meter.
After the Green Revolution, the Mexican wheat programme was led by the INIFAP5 and the CIMMYT6. It mainly focused on the creation of varieties adapted to the northwestern Mexican irrigated conditions. Since 1969, three different environments were targeted in Mexico: the northwestern irrigated areas, the Bajio and central Mexico irrigated areas, and the central highlands rainfed areas (Rajaram and Van Ginkel, 2001). Between 1966 and 2001, in northern Mexican and in the Bajio and Central Mexico irrigated areas, a yield increase of +0.07 t ha-1 year-1 and +0.058 t ha-1 year-1 was reached, respectively. However, in the central highlands rainfed areas, the grain yield progress was lower than in irrigated conditions and reached only +0.024 t ha-1 year-1. Historical genetic gain in absolute values is almost always lower under stressed environments than in unstressed conditions (Rajaram and Van Ginkel, 2001). However, Blum (2006) shown that when the genetic gain is regarded as a percentage of average yield, whatever the environment considered, genetic gains are quite close. Austin et al. (1989) observed a gain from 0.6 to 0.7 % in unstressed conditions and from 0.4 to 0.6 % under stressed environments.
Table of contents :
CHAPTER I: Literature review
I. Bread wheat features
a. Economic importance
b. Nutritional importance and industrial uses
c. Origin, domestication and geographical distribution of wheat
d. Wheat genetic resources
e. Wheat developmental stages and yield achievement
i. Aerial development and yield achievement
ii. Roots establishment and growth in cereals
f. Grain yield progress from the XVIIIe to the 1990’s
II. From the concept of stress to the characterization of the environment
a. Definition and description of water deficit and high temperature stresses
b. Distribution of water deficit and heat stresses worldwide on wheat cultivated areas
c. Water movements from the soil to the atmosphere through the plant
i. Theory and concepts of water potential (Taiz and Zeiger, 2010b)
ii. Water in the soil
iii. The gradient of water potential drives the water through the soil-plantatmosphere continuum
d. Characterization of the environment
i. An agronomic diagnostic using probe genotypes
ii. Characterizing the water deficit along the crop cycle
III. Impact of drought and heat stress on wheat
a. Differential sensitivity to drought and heat stress along the crop cycle
b. Grain yield achievement
c. Physiological effects of drought and heat stress on plant development and growth
i. Aerial development and growth
ii. Impact on the rooting system
iii. Drought and heat stress impact the photosynthesis process
IV. Adaptive responses of cereals to water deficit and heat stresses
a. Concepts of tolerance and resistance to drought and heat stress
b. Conceptual models for traits associated with adaptation to drought and heat stress
prone environments
c. Traits to improve tolerance to drought and heat stress
i. Water uptake, WU
ii. Water and radiation use efficiency, WUE/RUE
iii. Harvest index, HI
d. Bread wheat breeding: improvement of tolerance to drought and heat stress
V. Highlighting and assessing the huge available natural genetic diversity of bread wheat for drought and heat stress tolerance
a. Genetic variability for yield driving traits under drought and heat stress conditions
b. Genetic elements influencing the genetic variability: Ppd, Rht, 1BL-1RS
c. Study of the GEI, or how better benefit can be taken from the understanding of how the genetic variability interacts with the environment
i. Importance of the GEI
ii. Presentation of the GEI
iii. Analytical tools to study the GEI
VI. Assessing the genetic determinism of tolerance to drought and heat stress
a. From genetic and physical maps to whole wheat genome sequencing
b. Quantitative trait loci analyses
c. Impact of the segregation distortion on QTL analysis
d. QEI: dissection of the genetic component of the GEI
e. Synthesis of the previously reported QTL for traits associated with drought and heat stress tolerance in wheat
CHAPTER II: Objectives and strategy developed during the thesis
I. Frame and objective of the thesis
II. Research questions and strategy developed
III. Plant material and experimental design of the study
a. The plant material
b. Experimental trial network design
CHAPTER III: E, environmental characterization of the trial network
CHAPTER IV: GEI, study of the genotype-by-environment interaction
CHAPTER V: G, study of the genetic component
CHAPTER VI: General discussion and conclusion
I. General conclusions
a. Importance of the environmental characterization in MET network
b. Importance of the physiological phenotyping approach in the understanding of the drought and heat stress tolerance in wheat
c. Dissection of the genotype-by-environment interaction using environmental covariates revealed the stress sensitivity of the germplasm
d. Genetic dissection of the traits involved in the control of drought and heat stress tolerance in wheat can lead to a wider use of the QTL-by-environment interaction
II. General discussion
a. Need for standard protocols
b. New phenotyping methods
c. Interest of the genetic material studied
d. Relevant genomic regions directly usable in breeding
III. Perspectives
a. A new experimental design to tackle the long term objective of the thesis
b. Use of the environmental characterization methodology developed to characterize the bread wheat trial network
c. Winter x spring wheat crosses
d. To go further on genetics
