Germination vigor is driven by the ability of the plant embryo, embedded within the seed, to resume its metabolic activity in a coordinated and sequential manner. Studies using “-omics” approaches support the finding that a main contributor of seed germination success is the quality of the messenger RNAs stored during embryo maturation on the mother plant. In addition, proteostasis and DNA integrity play a major role in the germination phenotype. Because of its pivotal role in cell metabolism and its close relationships with hormone signaling pathways regulating seed germination, the sulfur amino acid metabolism pathway represents a key biochemical determinant of the commitment of the seed to initiate its development toward germination. This review highlights that germination vigor depends on multiple biochemical and molecular variables. Their characterization is expected to deliver new markers of seed quality that can be used in breeding programs and/or in biotechnological approaches to improve crop yields.

https://www.uv.mx/personal/tcarmona/files/2010/08/Rajjou-et-al-2012.pdf

Seed Germination and Vigor

Productivity and management of rice–wheat cropping systems: issues and challenges

Rapid seed germination and stand establishment are critical factors to crop production under salt‐stress conditions. In many crop species, seed germination and early seedling growth are the most sensitive stages to salinity stress. Salinity may delay the onset, reduce the rate, and increase the dispersion of germination events, leading to reductions in plant growth and final crop yield. The adverse effects of salt‐stress can be alleviated by various measures, including seed priming (a.k.a. pre‐sowing seed treatment). The general purpose of seed priming is to partially hydrate the seed to a point where germination processes are begun but not completed. Most priming treatments involve imbibing seed with restricted amounts of water to allow sufficient hydration and advancement of metabolic processes but preventing germination or loss of desiccation tolerance. Treated seeds are usually redried before use, but they would exhibit rapid germination when re‐imbibed under normal or stress conditions. Various seed priming techniques have been developed, including hydropriming (soaking in water), halopriming (soaking in inorganic salt solutions), osmopriming (soaking in solutions of different organic osmotica), thermopriming (treatment of seed with low or high temperatures), solid matrix priming (treatment of seed with solid matrices), and biopriming (hydration using biological compounds). Each treatment has advantages and disadvantages and may have varying effects depending upon plant species, stage of plant development, concentration/dose of priming agent, and incubation period. In this article, we review, evaluate, and compare effects of various methods of seed priming in improving germination of different plant species under saline and non‐saline conditions. We also discuss the known metabolic and ultra‐structural changes that occur during seed priming and subsequent germination. To maximize the utility of various seed priming techniques, factors affecting their efficiency must be examined and potential benefits and drawbacks determined. For example, quality of the seed before treatment, concentration/dose of priming agent, time period for priming, and storage quality of the seed following priming treatment must be carefully determined. Furthermore, such assessments must be based on large‐scale experiments if seed priming is to be used for large‐scale field planting. A better understanding of the metabolic events that take place in the seed during priming and subsequent germination will improve the effective application of this technology. The incorporation of advanced molecular biology techniques in seed research is vital to the understanding and integration of multiple metabolic processes that can lead to enhanced seed germination, and consequently to improved stand establishment and crop yield under saline and non‐saline conditions.

Pre‐Sowing Seed Treatment—A Shotgun Approach to Improve Germination, Plant Growth, and Crop Yield Under Saline and Non‐Saline Conditions

Soil salinity is a major environmental constraint to crop productivity worldwide. The “biological” approach to this problem focuses on the management, exploitation, or development of plants able to thrive on salt‐affected soils. This chapter reviews strategies by which plants can be enabled to grow on saline soils. The first strategy is to prime seeds before planting by treating them with inorganic or organic chemicals and/or with high or low temperatures. The second strategy involves exogenous application of organic chemicals, such as glycine betaine, proline, or plant growth regulators, or inorganic chemicals to plants under salinity stress. Considerable improvements in growth and yield have been reported in a number of crops using these approaches. The third strategy is to employ selection and breeding. Major efforts have been made to develop salt‐tolerant lines or cultivars of crops using conventional plant breeding. However, the complexity of the tolerance mechanisms, lack of selection criteria, and variation in responses of plants at different developmental stages have resulted in only limited success. The emphases for developing salt‐tolerant lines/cultivars are now on marker‐assisted breeding and genetic transformation. The development of salt‐tolerant transgenic plants is still at an early stage but may become increasingly more effective as better knowledge of the complex mechanisms involved in plant salt tolerance is acquired. Furthermore, the rapid expansion in knowledge on genomics and proteomics will undoubtedly accelerate the transgenic and molecular breeding approaches However, to date, there are few conclusive reports indicating successful performance of transgenic cultivars under natural stressful environments.

Some Prospective Strategies for Improving Crop Salt Tolerance

Rice is one of the most important food crops in the world, and staple for more than half of the global population. Looming water crisis, water-intensive nature of rice cultivation and escalating labour costs drive the search for alternative management methods to increase water productivity in rice cultivation. Direct seeded rice (DSR) has received much attention because of its low-input demand. It involves sowing pre-germinated seed into a puddled soil surface (wet seeding), standing water (water seeding) or dry seeding into a prepared seedbed (dry seeding). In Europe, Australia and the United States, DSR is highly mechanised. The development of early-maturing varieties and improved nutrient management techniques along with increased availability of chemical weed control methods has encouraged many farmers in the Philippines, Malaysia, Thailand and India to switch from transplanted to DSR culture. This shift should substantially reduce crop water requirements, soil organic-matter turnover, nutrient relations, carbon sequestering, weed biota and greenhouse-gas emissions. Still, weed infestation can cause large yield losses in DSR. In addition, recent incidences of blast disease, crop lodging, impaired kernel quality and stagnant yields across the years are major challenges in this regard. In this review, we discuss the experiences, potential advantages and problems associated with DSR, and suggest likely future patterns of changes in rice cultivation.

Rice direct seeding: Experiences, challenges and opportunities

Poor crop establishment was identified as a major constraint on rainfed crop production by farmers in the tribal villages of Rajasthan, Gujarat and Madhya Pradesh served by the Krishak Bharati Cooperative (KRIBHCO) Indo-British Rainfed Farming Project (KRIBP). On-farm seed priming with water was chosen as a low cost, low risk intervention appropriate to the farmers' needs. In vitro screening of the effects of priming on the germination of seeds of local and improved varieties of maize, upland rice and chickpea provided ‘safe limits’ – the maximum length of time for which farmers should prime seeds and which, if exceeded, could lead to seed or seedling damage. Recommended safe limits were 24 h for maize and rice and 10 h for chickpea, with only minor varietal differences. These recommendations were then tested in on-station trials in Dahod, Gujarat. Farmer-managed trials were conducted for chickpea in three villages in the rabi (post-monsoon) season in 1995–96; for maize and upland rice in eight villages in the kharif (monsoon) season in 1996; and for maize and chickpea in 15 villages in the 1996--97 rabi season. Farmers modified these recommendations to ‘overnight’ for all three crops. Evaluation of the technology by farmers involved focus group discussions, matrix ranking exercises and two workshops. Direct benefits in all three crops included faster emergence, better stands and a lower incidence of re-sowing, more vigorous plants, better drought tolerance, earlier flowering, earlier harvest and higher grain yield. Indirect benefits reported were earlier sowing of rabi crops because of the shorter duration of the preceding kharif crop, earlier harvesting of rabi crops that allowed earlier migration from the area, with better chance of obtaining off-season work, and increased willingness to use fertilizers because of reduced risk of crop failure. In matrix ranking exercises in four villages in the kharif 1996, 95% of farmers indicated that, even after only one exposure to the technology, they would prime seed in the following season. Similar exercises in four villages in rabi 1996–97 revealed that 100% of collaborating farmers intended to continue seed priming. From 21 villages, 246 farmers attended two workshops to share their experiences of seed priming and resolved to continue with the technology.

P. A. Khan

Papers

On-farm seed priming in semi-arid agriculture: development and evaluation in maize, rice and chickpea in india using participatory methods