Carolyne Dürr

A review of tillage effects on crop residue management, seedbed conditions and seedling establishment

There is considerable discussion about the influence of soil management techniques on soil erosion, water use and conservation, and more recently carbon dioxide sequestration and waste disposal. The soil–atmosphere interface, particularly the seed bed layer is of particular concern to agronomists and soil scientists because it is the focus of the physical processes affecting crop establishment and biological activity. This paper evaluates the current knowledge (1) in modeling seedling emergence and residue decomposition, (2) seedbed structure and its resulting physical conditions, and (3) tillage operations affect on seedbed structure and residue distribution.

Cumulative effects of cropping systems on the structure of the tilled layer in northern France

Abstract A field experiment was initiated in 1989 in northern France to evaluate the cumulative effects of cropping systems on the structure of the tilled layer of a loamy soil. Three cropping systems involving different crop rotations and cultivations (early or late sowing, early or late harvesting) were compared. The soil structure was evaluated after each winter wheat sowing by a morphological analysis of the ploughed layer. We determined the proportion of highly compacted zones, i.e. the zones with a massive structure and no visible macropores in the soil profile. These zones, defined as Δ zones, result from severe anthropogenic compaction. The creation of Δ zones depended largely on the soil moisture at the time of field operations and the characteristics of the machinery used. Maximum compaction occurred during harvesting in wet conditions because of high axle loads. In contrast, little compaction was produced by seedbed preparation, which involved lower axle loads and wide tyres. Consequently, changes in soil structure depended to a large extent on the cropping system. However, the proportion of Δ areas was not stable, but fluctuated greatly from one year to the next. Δ zones could quickly disappear from the ploughed layer. We, therefore, detected no irreversible effects on the structure of the ploughed layer, even for the cropping system that produced the highest annual risk of soil compaction. The combined effects of tillage and climate led to fewer compacted zones in the surface layer. Because this layer was mixed with deeper layers during soil inversion at the next ploughing, this contributed to fewer Δ zones in the whole layer. The loss of Δ zones over the whole cultivated layer could not be explained by this effect alone and the reduction in soil compaction was probably due to the combined effects of loosening by mouldboard ploughing and climatic and soil fauna activities in the ploughed layer below the seedbed. The initial soil structure had a major effect on seedbed fragmentation. When preparing seedbeds in the autumn, the proportion of remaining clods depended greatly on the initial state of the ploughed layer even when using a power rotary harrow. In spring, the number of remaining clods was still dependent on the initial compaction, but they were fewer and no differences were observed between rotary and combination harrows.

Seed mortality in the soil is related to seed coat thickness

Models that quantify the effects of cropping systems on weed dynamics are useful tools for testing innovative cropping systems. In these models, seed mortality in the soil is a key parameter to account for the cumulated effect of cropping systems over time via the soil seed-bank. Since seed mortality is difficult to measure, our objective was to develop a method to estimate it from easily accessible information. Seeds of 13 weed species were buried 30 cm deep in fields and were recovered regularly for 2 years to measure their viability. Seed mass, dimensions, shape, and protein and lipid contents as well as coat thickness were measured. To estimate seed mortality of species not included in the study, we searched for relationships between mortality rates and seed traits. Seed viability mainly decreased during the second year of burial, with mortality rates ranging from 0.01 to 0.63 seeds·seeds 21 ·year 21 , depending on the species. Seed mortality decreased with increasing seed coat thickness. No correlation was found with other measured traits or with seed persistence data in the literature. These results were confirmed when the effects of phylogenetic relatedness with phylogenetically independent contrasts were included. The thickness of the seed coat, which varied between 17 and 231mm over the range of species studied, can protect the seed from external attacks in the soil and slow down seed decay. This trait can be easily measured via X-ray images and could be used to estimate the seed mortality rate for a wider range of species.

QTL analysis of seed germination and pre-emergence growth at extreme temperatures in Medicago truncatula

Enhancing the knowledge on the genetic basis of germination and heterotrophic growth at extreme temperatures is of major importance for improving crop establishment. A quantitative trait loci (QTL) analysis was carried out at sub- and supra-optimal temperatures at these early stages in the model Legume Medicago truncatula. On the basis of an ecophysiological model framework, two populations of recombinant inbred lines were chosen for the contrasting behaviours of parental lines: LR5 at sub-optimal temperatures (5 or 10°C) and LR4 at a supra-optimal temperature (20°C). Seed masses were measured in all lines. For LR5, germination rates and hypocotyl growth were measured by hand, whereas for LR4, imbibition and germination rates as well as early embryonic axis growth were measured using an automated image capture and analysis device. QTLs were found for all traits. The phenotyping framework we defined for measuring variables, distinguished stages and enabled identification of distinct QTLs for seed mass (chromosomes 1, 5, 7 and 8), imbibition (chromosome 4), germination (chromosomes 3, 5, 7 and 8) and heterotrophic growth (chromosomes 1, 2, 3 and 8). The three QTL identified for hypocotyl length at sub-optimal temperature explained the largest part of the phenotypic variation (60% together). One digenic interaction was found for hypocotyl width at sub-optimal temperature and the loci involved were linked to additive QTLs for hypocotyl elongation at low temperature. Together with working on a model plant, this approach facilitated the identification of genes specific to each stage that could provide reliable markers for assisting selection and improving crop establishment. With this aim in view, an initial set of putative candidate genes was identified in the light of the role of abscissic acid/gibberellin balance in regulating germination at high temperatures (e.g. ABI4, ABI5), the molecular cascade in response to cold stress (e.g. CBF1, ICE1) and hypotheses on changes in cell elongation (e.g. GASA1, AtEXPA11) with changes in temperatures based on studies at the whole plant scale.

Emergence of seedlings of sugar beet (Beta vulgaris L.) as affected by the size, roughness and position of aggregates in the seedbed

Laboratory and field experiments were carried out to quantify the effects of the size and roughness of aggregates placed in the seedling path of sugar beet, in order to help in decision making for soil tillage and sowing operations. Graded aggregates (10, 15, 20, 30, 40, 50, 60 and 70 mm longest axis) were either laid on the soil surface or included in the soil over the seeds. The percent emergence decreased exponentially with aggregate size over 10 mm when the aggregates were included in the seedbed. The result was the same with aggregates laid on the soil surface, but for aggregates over 30 mm (mass>10 g). Aggregates on the soil surface could be lifted by the seedlings until their weight exceeded the seedling emergence force. Larger aggregates or aggregates in the soil layer could not be moved. Seedlings which did not emerge remained blocked in small cavities in the aggregate surface. No seedlings were blocked under smooth aggregates or glass beads. The experimental results fits with a model giving the probability to meet a hole according the distance covered and the diameter and density of holes. The results obtained under controlled conditions were similar to those obtained in field experiments for a wide range of aggregate sizes. These results will be incorporated into a computerised seedbed generator to simulate the effects of seedbed structure on seedling emergence.

Emergence and early growth of an epigeal seedling (Daucus carota L.): influence of soil temperature, sowing depth, soil crusting and seed weight

Abstract This study describes and analyses how seed placement and seed weight influence the response of emergence and early growth of carrot seedlings to changes in seedbed conditions (temperature and surface structure). The first experiment was carried out in a glasshouse with different sowing conditions (1, 3 and 5 cm sowing depths, 2 and 5 mm wet or dry crusts) and two seed weight ranges. The final emergence percentages (40–94%) and times from 50% germination to 50% emergence (55–105°Cd) varied widely. Seed weights had a marked influence only in the most extreme conditions. Seedling growth was then analysed in growth chambers at two temperatures (10 and 20°C), for the same two seed weight ranges and different times of growth in the dark. Hypocotyl elongation rates and growth forces decreased when this time increased. Heavy seeds had longer final hypocotyl lengths and greater growth forces, which explained their better emergence from deep sowing and with surface obstacles. Growth after emergence both in the glasshouse and growth chamber was influenced by seed weight and time from germination to emergence: the seedling weight at emergence depended only on initial seed weight; the seedling relative growth rate was not influenced by the initial seed weight, but decreased with increase in time before emergence. This was due to a decrease in cotyledon photosynthetic efficiency. Poor seedbed structure and seed placement control affect not only emergence but also early growth. These results provide basic information for modelling the emergence and early growth of dicotyledon epigeal seedlings.

Effects of seed production and storage conditions on blackgrass (Alopecurus myosuroides) germination and shoot elongation

Abstract Blackgrass is a common winter annual grass weed in autumn-sown crop rotations in Atlantic European countries. Control with a minimum amount of herbicides in integrated cropping systems would be facilitated by modeling the effect of cropping systems on its demography. To develop the submodel relating weed seed bank to emerged seedlings, laboratory experiments were conducted to analyze and quantify seed germination and shoot elongation. These processes were studied as a function of environmental conditions during seed production (nitrogen availability, water deficit, plant density, and crop) and of seed characteristics (seed weight, harvest date, storage length, and dry-stored/soil-buried). Nonlinear equations relating germination and shoot elongation to time calculated as cumulated degree-days were fitted to the observed germination and shoot elongation data. These were used to estimate parameters for germination proportion and rate, as well as final shoot length and elongation rate. Recently harvested seeds germinated best and fastest when they were collected in spring crops compared with winter crops. Germination proportion and rate increased with seed storage length, especially for seeds collected in winter crops. Midgermination time decreased with seed weight and water deficit during seed production; it increased with nitrogen amounts available to the mother plants. Maximum shoot length increased with seed weight and plant density during seed production. It decreased with nitrogen availability to mother plants and with storage length, irrespective of whether the seeds were dry-stored or buried in soil. Elongation rate was highest for early- and fast-germinating seeds. Time to midelongation increased with maximum shoot length. These germination and preemergence growth models can now be combined with other submodels to develop a blackgrass emergence model. Nomenclature: Blackgrass, Alopecurus myosuroides Huds. ALOMY.

Integrated management of damping-off diseases. A review

Damping-off is a disease that leads to the decay of germinating seeds and young seedlings, which represents for farmers one of the most important yield constraints both in nurseries and fields. As for other biotic stresses, conventional fungicides are widely used to manage this disease, with two major consequences. On the one hand, fungicide overuse threatens the human health and causes ecological concerns. On the other hand, this practice has led to the emergence of pesticide-resistant microorganisms in the environment. Thus, there are increasing concerns to develop sustainable and durable damping-off management strategies that are less reliant on conventional pesticides. Achieving such a goal requires a better knowledge of pathogen biology and disease epidemiology in order to facilitate the decision-making process. It also demands using all available non-chemical tools that can be adapted to regional and specific production situations. However, this still is not the case and major knowledge gaps must be filled. Here, we review up to 300 articles of the damping-off literature in order to highlight major knowledge gaps and identify future research priorities. The major findings are (i) damping-off is an emerging disease worldwide, which affects all agricultural and forestry crops, both in nurseries and fields; (ii) over a dozen of soil-borne fungi and fungus-like organisms are a cause of damping-off but only a few of them are frequently associated with the disease; (iii) damping-off may affect from 5 to 80% of the seedlings, thereby inducing heavy economic consequences for farmers; (iv) a lot of research efforts have been made in recent years to develop biocontrol solutions for damping-off and there are interesting future perspectives; and (v) damping-off management requires an integrated pest management (IPM) approach combining both preventive and curative tactics and strategies. Given the complex nature of damping-off and the numerous factors involved in its occurrence, we recommend further research on critical niches of complexity, such as seeds, seedbed, associated microbes and their interfaces, using novel and robust experimental and modeling approaches based on five research priorities described in this paper.

Heat stress during seed filling interferes with sulfur restriction on grain composition and seed germination in oilseed rape (Brassica napus L.)

In coming decades, increasing temperatures are expected to impact crop yield and seed quality. To develop low input systems, the effects of temperature and sulfur (S) nutrition in oilseed rape, a high S demanding crop, need to be jointly considered. In this study, we investigated the effects of temperatures [High Temperature (HT), 33°C/day, 19°C/night vs. Control Temperature (Ctrl T), 20°C/day, 15°C/day] and S supply [High S (HS), 500 μm SO2−4 vs. Low S (LS), 8.7 μM SO2−4] during seed filling on (i) yield components [seed number, seed dry weight (SDW) and seed yield], (ii) grain composition [nitrogen (N) and S contents] and quality [fatty acid (FA) composition and seed storage protein (SSP) accumulation] and (iii) germination characteristics (pre-harvest sprouting, germination rates and abnormal seedlings). Abscisic acid (ABA), soluble sugar contents and seed conductivity were also measured. HT and LS decreased the number of seeds per plant. SDW was less affected due to compensatory effects since the number of seeds decreased under stress conditions. While LS had negative effects on seed composition by reducing the FA contents and increasing the ratio S-poor SSPs (12S globulins)/S-rich SSPs (2S albumins) ratio, HT had positive effects by increasing S and FA contents and decreasing the C18:2/C18:3 ratio and the 12S/2S protein ratio. Seeds produced under HT showed high pre-harvest sprouting rates along with decreased ABA contents and high rates of abnormal seedlings. HT and LS restriction significantly accelerated germination times. High conductivity, which indicates poor seed storage capacity, was higher in HT seeds. Consistently, the lower ratio of (raffinose + stachyose)/sucrose in HT seeds indicated low seed storage capacity. We demonstrated the effects of HT and LS on grain and on germination characteristics. These results suggest that hormonal changes might control several seed characteristics simultaneously.

Are penetrometer measurements useful in predicting emergence of sugar beet (Beta vulgaris L.) seedlings through a crust

Soil crusting strongly affects seedling emergence. Laboratory experiments were carried out to analyse the emergence of sugar beet (Beta vulgarisL.) seedlings from beneath a crust, and to determine whether mechanical models using penetrometer measurements could predict the final emergence rates. Wet and dry crusts were created under a rainfall simulator. An ascending micropenetrometer was used to measure the crust strength in conditions as close as possible to those encountered by seedlings. The mode of the emergence was observed for both micropenetrometer probe and seedlings. While 94% of the emerged seedlings penetrated the wet crusts, only 6% broke it. The probe penetrated the wet crust in only 46% of cases, and broke it in the other 54%. In contrast, 39% of the seedlings emerged directly through a crack in the dry crust and 55% emerged by breaking (5% lifted a detached fragment and 1% penetrated the crust). In the same conditions, the probe emerged directly through a crack in only 4% of cases; 67% by breaking and 29% by lifting a crust fragment. The force recorded by the penetrometer was 0.05 – 0.80 N, and it varied with the mode of emergence. The seedling growth force distribution was 0 – 0.30 N (mean = 0.09 N). Mechanical models comparing the force exerted by the micropenetrometer probe to the seedling force distribution according to the mode of the emergence were used to predict final emergence rates. Penetrometer measurements appeared to overestimate the mechanical resistance encountered by the seedlings, leading to an underestimation of emergence rates under all emergence conditions. This overestimation of crust strength was attributed to the rigidity of the probe as opposed to the flexibility of hypocotyls.