Ahmad M. Alqudah

Genetic dissection of photoperiod response based on GWAS of pre-anthesis phase duration in spring barley.

Heading time is a complex trait, and natural variation in photoperiod responses is a major factor controlling time to heading, adaptation and grain yield. In barley, previous heading time studies have been mainly conducted under field conditions to measure total days to heading. We followed a novel approach and studied the natural variation of time to heading in a world-wide spring barley collection (218 accessions), comprising of 95 photoperiod-sensitive (Ppd-H1) and 123 accessions with reduced photoperiod sensitivity (ppd-H1) to long-day (LD) through dissecting pre-anthesis development into four major stages and sub-phases. The study was conducted under greenhouse (GH) conditions (LD; 16/8 h; ∼20/∼16°C day/night). Genotyping was performed using a genome-wide high density 9K single nucleotide polymorphisms (SNPs) chip which assayed 7842 SNPs. We used the barley physical map to identify candidate genes underlying genome-wide association scans (GWAS). GWAS for pre-anthesis stages/sub-phases in each photoperiod group provided great power for partitioning genetic effects on floral initiation and heading time. In addition to major genes known to regulate heading time under field conditions, several novel QTL with medium to high effects, including new QTL having major effects on developmental stages/sub-phases were found to be associated in this study. For example, highly associated SNPs tagged the physical regions around HvCO1 (barley CONSTANS1) and BFL (BARLEY FLORICAULA/LEAFY) genes. Based upon our GWAS analysis, we propose a new genetic network model for each photoperiod group, which includes several newly identified genes, such as several HvCO-like genes, belonging to different heading time pathways in barley.

Drought Stress Effect on Crop Pollination, Seed Set, Yield and Quality

The effect of drought stress on crop growth and yield has become more common worldwide in the last two decades. The reproductive stage is the most critical stage for drought stress during crop growth, because it strongly impacts yield and seed quality. Improving crop growth and yield under drought is thus a major goal of plant breeding. Drought stress negatively affects flower pollination by decreasing the amount of viable pollen grain, increasing the unattractiveness of flowers to pollinators, and decreasing the amount of nectar produced by flowers. Consequently crop seed set is lowered. Moreover, drought stress affects crop yield by reducing grain yield and all yield components. The correlation is clear between crop pollination, seed set and yield. Drought stress not only affects seed production, but also affects seed quality such as germination and vigor tests. In this chapter we review the currently available information on pollination, yield, and yield components and seed quality under drought. We give an outlook towards the physiological and biochemical processes involved in the reduction of crop yield in response to drought stress at the reproductive stage. We focus on physiological processes of plant reproductive organs in response to drought stress at anthesis and the attractiveness of the flowers to pollinators. Here we help plant breeders to select drought tolerant traits by understanding the correlations between pollination, yield, yield components and seed quality under drought stress at reproductive stage and to explain how drought stress effects final yield and seed quality during this stage.

ENHANCING SEED GERMINATION OF FOUR CROP SPECIES USING AN ULTRASONIC TECHNIQUE

SUMMARY A laboratory experiment was conducted to determine the effect of ultrasound (US) treatment on seed germination of chickpea, wheat, pepper and watermelon. All tests were carried out at 40 kHz in a water bath ultrasonic device varying two factors, treatment duration (5, 10, 15, 30, 45 or 60 min) and germination temperature (15 or 20 ◦ C). Parallel tests were run in which seeds were soaked in water without sonication in order to eliminate the effect of water from US test results. The effects of US on seed germination varied between crops and were more obvious on germination speed, expressed as germination rate index (GRI), rather than on germination percentage (GP). In particular, US treatment significantly increased the GRI of chickpeas, wheat and watermelon, resulting in a maximum increase of 133% (at 45 min), 95% (30 min) and 45% (5 min), respectively, above control seeds. The beneficial effects of US on the GRI of these crops were observed at both 15 and 20 ◦ C, suggesting that US treatment offers a practical priming method to overcome the slow germination that may occur at low temperatures. Water-soaking treatment improved the GP of both chickpea and pepper seeds by 59 and 24%, respectively, compared to the control but neither water nor US had any positive effect on pepper GRI. Post-treatment measurement of moisture content of these seeds produced variable results depending on crop species and US treatment duration. Results of this research indicated that US treatment effectively enhanced speed of germination of chickpea, wheat and watermelon seeds. This increase in speed of germination may improve early field establishment of these crops in the semiarid Mediterranean region and thus needs further investigation. The US technique may also be very useful for plant propagators in nurseries to achieve fast seedling establishment of watermelon.

VRS2 regulates hormone-mediated inflorescence patterning in barley

Plant architecture has clear agronomic and economic implications for crops such as wheat and barley, as it is a critical factor for determining grain yield. Despite this, only limited molecular information is available about how grain-bearing inflorescences, called spikes, are formed and maintain their regular, distichous pattern. Here we elucidate the molecular and hormonal role of Six-rowed spike 2 (Vrs2), which encodes a SHORT INTERNODES (SHI) transcriptional regulator during barley inflorescence and shoot development. We show that Vrs2 is specifically involved in floral organ patterning and phase duration by maintaining hormonal homeostasis and gradients during normal spike development and similarly influences plant stature traits. Furthermore, we establish a link between the SHI protein family and sucrose metabolism during organ growth and development that may have implications for deeper molecular insights into inflorescence and plant architecture in crops.

Awn primordium to tipping is the most decisive developmental phase for spikelet survival in barley

In small-grain cereals, grain yield is closely associated with grain number. Improved spikelet survival is an important trait for increasing grain yield. We investigated spikelet number, spikelet survival and yield-related traits under greenhouse conditions, and pot- and soil-grown field conditions. Thirty-two spring barley (Hordeum vulgare L.) accessions (14 two- and 18 six-rowed accessions) were manually dissected to determine spikelet/floret number on the main culm spike (SNS) at awn primordium (AP), tipping (TIP), heading and anther extrusion. We observed a significant difference between two- and six-rowed barley for SNS and spikelet survival at all stages and growing conditions. Both traits were highly genetically controlled, with repeatability and broad-sense heritability values of 0.74-0.93. The rate of spikelet survival from AP to harvest was higher in two- (~70%) than in six-rowed (~58%) barley. Spikelet abortion, starting immediately after AP, was negatively affected by increased SNS and the thermal time required to reach the AP stage. The largest proportion of spikelet reduction happened during the AP-TIP phase, which was the most critical period for spikelet survival. The duration between AP and the end of stem elongation correlated better with spikelet survival and yield-related characters than the estimated duration of stem elongation using leaf height measurements. Our observations indicate that the main spike plays an important role in single-plant grain yield. Extending the length of the critical AP-TIP phase is promising for improving yield through increased spikelet development and survival. The results also demonstrate that greenhouse conditions are appropriate for studying traits such as phase duration and spikelet survival in barley.

Genome‐wide association analyses of 54 traits identified multiple loci for the determination of floret fertility in wheat

Increasing grain yield is still the main target of wheat breeding; yet todays wheat plants utilize less than half of their yield potential. Owing to the difficulty of determining grain yield potential in a large population, few genetic factors regulating floret fertility (i.e. the difference between grain yield potential and grain number) have been reported to date. In this study, we conducted a genome-wide association study (GWAS) by quantifying 54 traits (16 floret fertility traits and 38 traits for assimilate partitioning and spike morphology) in 210 European winter wheat accessions. The results of this GWAS experiment suggested potential associations between floret fertility, assimilate partitioning and spike morphology revealed by shared quantitative trait loci (QTLs). Several candidate genes involved in carbohydrate metabolism, phytohormones or floral development colocalized with such QTLs, thereby providing potential targets for selection. Based on our GWAS results we propose a genetic network underlying floret fertility and related traits, nominating determinants for improved yield performance.

Effects of late-terminal drought stress on seed germination and vigor of barley (Hordeum vulgare L.)

Late-terminal drought stress during grain filling has recently become more common in the semi-arid Mediterranean region, where barley (Hordeum vulgare L.) is grown as an important winter cereal crop. Little information is available in the literature about the effect of late-terminal drought stress on seed germination and vigor of barley. The objective of this experiment was to study the effect of late-terminal drought stress on seed germination and vigor of barley as estimated by the germination after accelerated aging test. Drought stress reduced grain yield of barley. Grain yield was correlated positively with leaf gross photosynthetic rate and negatively with leaf osmotic potential. Late-drought stress had no effect on standard germination, but reduced the germination after the accelerated aging test. These data suggested that late-terminal drought stress had a greater effect on seed vigor than standard germination in barley.

Leaf primordium size specifies leaf width and vein number among row-type classes in barley

Exploring genes with impact on yield-related phenotypes is the preceding step to accomplishing crop improvements while facing a growing world population. A genome-wide association scan on leaf blade area (LA) in a worldwide spring barley collection (Hordeum vulgare L.), including 125 two- and 93 six-rowed accessions, identified a gene encoding the homeobox transcription factor, Six-rowed spike 1 (VRS1). VRS1 was previously described as a key domestication gene affecting spike development. Its mutation converts two-rowed (wild-type VRS1, only central fertile spikelets) into six-rowed spikes (mutant vrs1, fully developed fertile central and lateral spikelets). Phenotypic analyses of mutant and wild-type leaves revealed that mutants had an increased leaf width with more longitudinal veins. The observed significant increase of LA and leaf nitrogen (%) during pre-anthesis development in vrs1 mutants also implies a link between wider leaf and grain number, which was validated from the association of vrs1 locus with wider leaf and grain number. Histological and gene expression analyses indicated that VRS1 might influence the size of leaf primordia by affecting cell proliferation of leaf primordial cells. This finding was supported by the transcriptome analysis of mutant and wild-type leaf primordia where in the mutant transcriptional activation of genes related to cell proliferation was detectable. Here we show that VRS1 has an independent role on barley leaf development which might influence the grain number.

Heading Date Is Not Flowering Time in Spring Barley

Understanding the stages of floral development is one of the major goals of crop breeding to produce new varieties that are better adapted to environmental cues with improved yield. The phase transition is a quantitative trait that is predominantly genetically controlled with a complex genetic network integrating endogenous and environmental factors. In recent years, it has become evident that the heading date in small grain cereal crops is one important stage that has been extensively studied and highly associated with environmental adaptation and yield. Heading date has a complex genetic architecture that makes it a target trait in barley (Hordeum vulgare L.) breeding programs. Fine adjustment of heading date is important for understanding other developmental traits such as leaf area, plant height, tillering, and grain number (Li et al., 2006; Alqudah et al., 2016). In addition, it is also considered as a decisive stage for improving yield and yield components (Esparza Martínez and Foster, 1998; Li et al., 2006; Cuesta-Marcos et al., 2009; Pasam et al., 2012). The timing of heading in barley has a substantial impact on range-wide eco-geographical adaptation and improving the yield, which is clearly shown in accessions from North-Western Europe and North America (with reduced response to long days), that are late in heading (Turner et al., 2005). This adaptation habit allows the barley plants to extend their vegetative phase that in turn increases biomass accumulation and grain yields (Turner et al., 2005). However, there appears to be confusion among barley researchers when the barley inflorescence (i.e., spike) shows the heading [Zadoks, Z50–Z59 (spike out of the flag leaf sheath), Figure 1B] and awn tipping appearance [(Z49), Figure 1A]. Such confusion can ultimately lead to inaccurate phenotypic results and interpretation especially in the context of identified stage-specific QTL/transcriptomes, or underlying genes.

Impacts of drought on pollination of Trigonella moabitica (Fabaceae) via bee visitations

This experiment was conducted to evaluate the effect of drought stress on the attractivity of Trigonella flowers to worker honey bees, seed set and yield of T. moabitica under semi-arid Mediterranean conditions during the flowering stage. At this stage plants were exposed to one of three water regimes: either well-watered, moderately-watered, and drought stress. Results showed that flowers of well-watered plants attracted more bee flower visitors compared to plants grown under drought stress conditions. Plants grown under drought stress conditions were shorter, had fewer numbers of inflorescences, and fewer flowers per plant compared to well and moderately-watered plants. Furthermore, drought-stressed plants produced lower quantities of viable pollen grains, and lower pod and seed set.