Nezar H. Samarah

Size Distribution and Mineral Nutrients of Soybean Seeds in Response to Drought Stress

Abstract Accumulation of mineral nutrients in soybean seeds grown under drought stress may have a role in drought tolerance. Because drought stress reduces dry matter accumulation and seed size, drought stress may increase the concentration of mineral nutrients in soybean seeds and may decrease the content of nutrient per seed. The objective was to study the effect of drought on seed size distribution and whether different drought-stress patterns increase mineral nutrient concentration in soybean seeds similar in size. Two experiments were conducted, one in the greenhouse and one in the field. In the greenhouse experiment, three-drought stress treatments were imposed on soybean plants at beginning seed fill (R5) for 23–26 days: well-watered (WW), sudden severe stress (SS), and gradual stress (GS) imposed prior to severe stress. In the field experiment, irrigation and nonirrigation (rainfed) conditions were imposed on soybean plants at R5 for the entire seed filling period. Drought stress decreased seed size in the greenhouse experiment, but not in the field experiment. Within similar seed sizes, drought-stress treatments increased concentrations of phosphorus (P), potassium (K), calcium (Ca), molybdenum (Mo), manganese (Mn), copper (Cu), and zinc (Zn) in seeds above well-watered treatment, but this increase varied between the two experiments. This increase in nutrient concentrations in seeds from drought-stressed plants compared with well-watered plants for seeds similar in sizes indicates that the increase in nutrient concentration in seeds from drought-stressed plants is not necessarily due to a reduction in dry matter accumulation. Accumulation of minerals in seeds might be an important response in drought stress tolerance.

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.

Allelopathic effects of Pinus halepensis and Quercus coccifera on the germination of Mediterranean crop seeds

Agroforestry is a leading alternative for food security and forest conservation. A full understanding of positive and negative, i.e. allelopathic, interactions between crops and forest trees is necessary for producing crops and conserving forests especially within the threatened Mediterranean forest ecosystems. The present study explored the allelopathic effects of green and senescent leaf and soil extracts of two agroforestry trees—Pinus halepensis and Quercus coccifera—on the germination of wheat, barley, lentil, chickpea, and fababean as the major grain crops of Jordan. Results revealed that allelopathic effects reduced seed germination of these crops. Germination reduction reached a maximum of 75% in fababean treated with green extracts of Q. coccifera and differed among crops and extract sources, but not between tree species. Comparing between green and senescent leaf and soil extract, regarding their effect on germination percentage, it was noticed that these effects were similar in some crops and were different in others. Germination responses were generally different between cereals and legumes where cereals tend to be less affected by allelopathic influences than legumes, especially fababean. We suggest using cereals such as wheat and barley in agroforestry practices in the Mediterranean region of Jordan.

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.

Gene expression and activity of antioxidant enzymes in barley (Hordeum vulgare L.) under controlled severe drought

The objective of the present study was to determine the activity of antioxidant enzymes: superoxide dismutase (SOD), ascorbate peroxidase (APX), and catalase (CAT) and the expression of their genes in two barley genotypes under controlled severe drought. To fulfill this objective, 21-day-old barley plants of two genotypes: Rum and Yarmouk were exposed to controlled severe drought (25% field capacity) for 2, 9, and 16 days. The activity of SOD was significantly high in Rum genotype after 2 days of drought treatment. In Yarmouk genotype, the activity of APX was significantly high after 2 and 9 days of drought treatment. In Rum genotype, CAT2 was upregulated after 9 days of drought treatment and SOD and APX were upregulated after 16 days of drought treatment, whereas CAT2, SOD, and APX were upregulated in Yarmouk genotype after 2 days of drought treatment. The results indicate a unique pattern of activity and gene expression of the antioxidant enzymes in the two barley genotypes under controlled severe drought. Moreover, the data suggest that each genotype utilizes different molecular and biochemical responses under the same drought conditions.

Effect of maturity stage on germination and dormancy of fresh and air‐dried seeds of bitter vetch (Vicia ervilia L.)

Abstract Bitter vetch is an important legume crop cultivated for forage and seed yield in the Mediterranean, West Asia, and North Africa regions. Delaying seed harvest of bitter vetch may increase seed losses due to pod shattering; however, early harvest may influence seed dormancy and the ability of seed to germinate. Field experiments were conducted at two locations in Jordan to study the effect of maturity stage on germination and dormancy of fresh (before drying), air‐dried, and dry‐prechilled (at 5°C for 8 days) seeds of bitter vetch. Pods were harvested at three development stages: full‐seed pod (FS), yellow pod (YP), and brown pod (BP). Percentage of normal, abnormal, hard, and dead seeds at 14 days after planting in standard germination tests was recorded for the fresh, air‐dried, and dry‐prechilled seeds harvested at the three development stages. Germination counts of normal seedlings at 4, 7, 10, and 14 days after planting was used to calculate the germination rate index (GRI). For the fresh seeds, germination did not exceed 30% when seeds were harvested at the FS stage, and decreased to 0% at the YP stage. All non‐germinated fresh seeds harvested at the YP stage were dormant. Air‐drying and dry‐prechilling treatments improved seed germination at the YP and BP stages. Maximum germination was achieved in air‐dried and dry‐prechilled seeds harvested at the BP stage. The percentage of hard seeds was the highest for air‐dried seeds harvested at the FS stage. Percentage of hard and dormant seeds was decreased to the minimum when seeds were harvested at the BP stage and exposed to air‐drying and dry‐prechilling treatment before germination. Dry‐prechilled seeds had higher GRI than fresh and air‐dried seeds when seeds were harvested at the BP stage. In conclusion, maximum germination and minimum dormancy was achieved for air‐dried and dry‐prechilled seeds harvested at the BP stage. Dry‐prechilling treatment increased the speed of germination for seeds harvested at the BP stage. Presence of hard seeds reduced the germination of air‐dried seeds harvested at the YP stage. Bitter vetch seeds harvested at the YP stage may require scarification and a longer prechilling treatment to overcome seed dormancy observed at this stage.

Chemical Composition and Mineral Content of Common Vetch Seeds during Maturation

ABSTRACT Common vetch (Vicia sativa L.) is an important legume cultivated for feed grain and forage in the Mediterranean and Central Asia regions. Little information is available about the changes in seed mass, chemical composition, and mineral concentration in the developing seeds of common vetch. Therefore, the objective of this experiment was to study the changes in seed mass, moisture content, fat, fiber, proteins, carbohydrates, reducing sugars, and macro- and microelements in common vetch seeds sampled at four pod developmental stages described based on pod color. Common vetch pods were harvested at four pod developmental stages: 1) full-size seed (FS), 2) greenish-yellow pods (GY), 3) yellow pods (Y), and 4) brown pods (B). Common vetch seeds attained maximum mass accumulation when pod color turned to yellow (the Y stage), which was a good indication of seed physiological maturity in common vetch. Results showed that ash, fat, fiber, protein, and carbohydrate content increased between the FS and GY stage. There was no significant change in the accumulation of these components at the Y stage as compared with the GY stage. During the desiccation phase (the Y to B stage), there was an increase in carbohydrate and reducing sugars and a decrease in proteins. Seed content (μg seed−1) of macro- and microelements levels increased as the seed matured, reaching the maximum level by the GY stage [for phosphorus (P), sodium (Na), zinc (Zn), and manganese (Mn)], the Y stage [for copper (Cu)], and the B stage [for potassium (K) and iron (Fe)]. These results suggest that the majority of seed chemical composition was accumulated by the GY stage where farmers can harvest common vetch seed without reducing seed mass and nutrient loss. These data serve as a useful characterization of changes in chemical composition based on pod developmental stages for further studies.

Effect of air‐drying immature seeds in harvested pods on seed quality of common vetch (Vicia sativa L.)

Abstract Immature seeds of common vetch had low germination and were desiccation sensitive when seeds were extracted from pods before drying. Air‐drying immature seeds in harvested pods may improve their germination and vigour. The objective of this experiment was to study the effect of air‐drying immature seeds in harvested pods on seed germination and vigour of common vetch and to identify the vigour test that best discriminates seed vigour differences amongst seeds harvested at different maturity stages and drying treatments. Seeds of common vetch were harvested at five pod developmental stages: beginning of seed fill (BS), full‐size seed (FS), greenish‐yellow pod (GY), yellow pod (Y), and brown pod (B) and subjected to three drying treatments: seeds dried after extraction from pods (depodded), dried in pods detached from plants (podded detached) or attached to plants (podded attached). Seed germination and seed vigour, as estimated by the germination after three accelerated ageing (AA) treatments and by electrical conductivity of seed leachate, were evaluated for the harvested seeds. Seeds were subjected to three ageing treatments (100% RH and 39°C for 72 h, 39°C for 96 h, or 45°C for 48 h) to identify the AA treatment that best discriminates seed vigour differences amongst common vetch seed lots. Podded dried seeds detached from or attached to plants had higher standard germination, germination after three accelerated ageing treatments, and lower electrical conductivity of seed leachates than depodded dried seeds when seeds were harvested at the BS and FS stages of development. Although the podded dried immature seeds did not differ significantly in standard germination from the podded dried mature seeds, the podded dried immature seeds had lower vigour as estimated by the germination after 39°C for 96 h. Seed ageing at close to 100% relative humidity and 39°C for 96 h was the best condition to separate the differences in vigour among maturity stages and drying treatments. The 39°C for 96 h treatment can be a useful test to assess seed vigour of common vetch and needs further evaluation.

Physiological and Molecular Responses to Controlled Severe Drought in Two Barley (Hordeum Vulgare L.) Genotypes

Drought is a major abiotic stress that restricts growth and productivity of many crops. The objectives of this study were to assess the morphological and physiological responses of barley plants at the vegetative stage to controlled severe drought, and to test the expression of drought-responsive genes (HvHsdr4, HvNCED2, HvDHN1, HvDHN9, and HvCBF3) in a time course of drought treatment. Barley plants of two genotypes (Rum and Yarmouk) were exposed to controlled severe drought (25% field capacity) in a greenhouse experiment. The two genotypes showed a similar and significant reduction in net photosynthetic rate and plant height. At the molecular level, a differential gene expression of drought-responsive genes was shown between the two genotypes. The results indicate that drought-responsive genes can be useful marker genes to study the differential response of barley genotypes to drought stress conditions.