Effects of cold storage of germinated vegetable seeds prior to fluid drilling on emergence and yield of field crops

Germinated seeds of five vegetable crops were stored at 1 °C in aerated water or humid air for periods of up to 15 days and then fluid drilled in the field. Viability of germinated seed was maintained in carrot and parsnip for a minimum of 15 days, salad onion for 9 days, lettuce for 6 days and cabbage for 3 days. When salad onion, lettuce and cabbage seeds were stored for longer than these times there was a reduction in the number of seedlings emerging. In species where storing seeds for up to 15 days caused a reduction in emergence an adjustment to the seed rate related to the length of storage can be made to achieve the desired population and still retain the advantages of fluid drilling pre-germinated seeds. This is possible because with one exception (an increase in the variation of salad onion stem diameters) the performance of plants from all stored germinated seeds after emergence was similar to plants from freshly germinated seeds.     

Embryo Water Status and Development of the Recalcitrant Species Quercus robur L: Determination of Water Relations Parameters by Pressure-Volume Analysis

The water status and water content of embryos of Quercus roburhave been determined during development. Using psychrometers,changes in osmotic potential at full imbibition and the proportionof water at zero reciprocal water potential, R₀, were estimatedfrom curves of embryo relative water content and reciprocalwater potential. The specific moisture content of embryos decreasedduring development, with a concomitant increase in the osmoticpotential. However, R₀ also increased such that, during thelast four weeks of development, there was little net changein the content of osmoticum per dry weight. Evidence is presentedthat starch stored in the cotyledon may be the major site ofmatrix-bound water, R₀. Isolated embryonic axes had a slightly higher osmotic potentialthan the whole embryo but the R₀ was much lower. The differencesbetween whole embryo and axis water relations parameters emphasizethe need to take account of R₀ when discussing seed water statusand survival. The results are considered in relation to currentmodels of seed water status and recalcitrant behaviour. Key words: Quercus robur L., embryo water relations, development, starch     

Seed Development in the Recalcitrant Species Quercus robur L: Water Status and Endogenous Abscisic Acid Levels

Water status and endogenous ABA levels were measured duringthe development of the embryonic axis and cotyledons in fruitsof Quercus robur L. As dry matter accumulated in the embryoduring development, both moisture content and osmotic potential(     

Embryo Water Status and Survival in the Recalcitrant Species Quercus robur L: Evidence for a Critical Moisture Content

The responses of Q. robur L. fruits, seeds and embryonic axesto desiccation are characterized and discussed in relation tocurrent knowledge of recalcitrant seed behaviour. A relationshipbetween viability and seed moisture content is described. Thisrelationship was unaffected by rate of drying, year of harvestor presence of the pericarp. Desiccation sensitivity did notincrease with storage. Excised embryonic axes survived to lower moisture contents thanintact seeds. However, in the intact seed, loss of viabilityappeared to be determined by a critical moisture content inthe cotyledons. Consequently, the level of desiccation tolerancewithin the axis attached to cotyledons was not determined byaxis drying rate. A link is drawn between the difference in the desiccation toleranceof embryonic axes and of cotyledons, and estimates of theirdifferent levels of matrix-bound water. The results presentedare consistent with a critical moisture content for survivalwhich is determined by the loss of all free cellular water.This hypothesis takes account of the differential desiccationsensitivity of seed tissues and differences in desiccation tolerancebetween species.     

Onion (Allium cepa L.) Seedling Emergence Patterns can be Explained by the Influence of Soil Temperature and Water Potential on Seed Germination

A theory is outlined that seedling emergence patterns in thefield are largely generated by the influence of soil water potentialon a critical event (i.e. the initiation of radicle growth)that determines the base water potential for germination. Abovethis base the mean rate and distribution of seedling emergenceare largely determined by temperature. This proposal was expressedas a parsimonious stochastic model to test whether patternsof seedling emergence recorded in field experiments could beexplained. The model applies current understanding of the influence ofwater potential and thermal time on germination under controlledconditions to the variable environment of horticultural seedbeds. Where possible, parameter values were taken from the literature,others were inferred from laboratory experiments on seed germinationand seedling emergence. The model accurately predicts the onsetof seedling emergence and the occurrence and timing of subsequentseedling flushes in a wide range of seed bed conditions. The study indicates that a co-ordinated use of existing seed-bedpreparation techniques to retain soil moisture for initial seedimbibition and limited irrigation, timed to coincide with thecritical period when radicle growth is initiated in the seedpopulation, may provide an approach to improving crop uniformitythat is simple and practical. Key words: Onion, germination, seedling emergence, thermal time, water potential     

The combined effects of a newly designed dibber drill, irrigation and seed covering treatments on lettuce and calabrese establishment

A comparison was made between seedling establishment following sowing with the Mk2 dibber drill and a conventional coulter drill at three sowing occasions with both lettuce and calabrese. Different seed covering materials used to cover dibber-drilled seed were also compared. With lettuce, the rate and uniformity of seedling emergence and the numbers of seedlings emerging were greater following sowing with the dibber drill than with the coulter drill. Seedling establishment was further improved with irrigation especially when seeds were sown with a small quantity of the hydrophillic polymer, Alcosorb, which avoided associated problems of increased soil impedance. Alcosorb was a more effective seed cover than a peat and vermiculite mix and better suited for mechanical metering. Results indicate that the dibber drill can be considered as a realistic and cheaper alternative to transplanting for establishing later-season lettuce crops. With calabrese, however, percentage seedling emergence on all dibber-drilled plots was lower than that on plots sown with the coulter drill. This result may have been due to seed-coat damage caused as seeds are plunged into the soil during sowing with the dibber drill.     

The effects of fluid drilling and seed covering medium on early carrot production under polyethylene mulch

Ungerminated seeds, fluid-drilled germinating seeds and fluid-drilled uniformly-germinated seeds were sown in furrows that were either uncovered or covered by soil and left unmulch ed or mulched with polyethylene sheet. The effects of these treatments on the emergence and growth of carrots for early harvest were examined. Leaving seeds uncovered or mulching them with polyethylene advanced seedling emergence and increased the percentage that emerged; leaving seeds uncovered also reduced the spread of emergence times and produced seedlings with larger cotyledons. Uniformly-germinated seeds emerged before germinating seeds and both emerged before ungerminated seeds. The best combination of treatments (i.e. mulched, uncovered uniformly-germinated seeds) reduced the mean number of days for emergence from the 52 required by traditionally sown seeds to 29. There was a more variable effect of seed treatments on the spread of emergence times and the percentage emergence. Earlier emergence generally led to roots reaching a marketable size earlier and more uniform emergence led to less variation in root weights at harvest.     

Quantitative genetic analysis of seed vigour and pre-emergence seedling growth traits in Brassica oleracea

The ability of seeds to germinate and establish seedlings in a predictable manner under a range of conditions has a direct contribution to the economic success of commercial crops, and should therefore be considered in crop improvement. We measured traits associated with seed vigour and pre-emergence seedling growth in a segregating population of 105 doubled haploid Brassica oleracea lines. The germination traits measured were: mean germination times for unstressed germination; germination under water stress or germination after a heat treatment; and conductivity of seed leachate. The seedling growth traits measured were: seed weight; seedling growth rate; and seedling size at the end of the exponential growth phase. There were some correlations, notably among germination traits, and between seed weight and pre-emergence seedling growth. Heritability of the various traits was typically in the 10–15% range, with heritability of conductivity and mean germination time under water stress 25 and 24% respectively. Collectively the results indicate that germination and pre-emergence seedling growth are under separate genetic control. Quantitative trait loci analyses were carried out on all measurements and revealed significant loci on linkage groups O1, O3, O6, O7 and O9. We suggest that genes at these loci are important in determining predictable seed germination and seedling establishment in practice.