1. Introduction
Climate change toward higher temperatures, greater aridity, and more frequent erratic climate events is posing a major threat to agricultural sustainability [
1]. The development of new crops adapted to arid conditions is among the promising approaches to meet the projected demand for food production under expanding aridity. The introduction of a crop into a new region requires comprehensive studies of the crop’s responses to environmental factors. The adoption of a crop from a traditional rain-fed system in an intensive, mechanized, and irrigated cropping system is particularly challenging, requiring examination and adjustment of management practices, from field preparation, sowing, irrigation, mineral nutrition, and plant protection, to harvesting techniques.
Tef [
Eragrostis tef (Zucc.) Trotter] is a cereal crop that is cultivated mainly in Ethiopia and the Horn of Africa [
2]. Tef has a thin culm, long narrow leaves, and a thousand-kernel weight of 200–400 mg, making it the smallest-grained cereal [
3]. Recently, tef has been gaining popularity in many Western countries due to its dual potential—as a gluten-free “super food” grain [
3], and as high-quality forage [
4,
5]. In Israel, the demand for tef grain is further increased by the Ethiopian descent citizens who continue to consume it for their traditional cuisine [
6].
Tef has a C4 photosynthetic pathway, which allows for efficient utilization of high solar radiation. It is highly tolerant to various stressful environments, such as marginal soils, water-logging, salinity, and drought [
7,
8,
9]. In addition to tef’s remarkable resilience, its ability to produce grain and/or fodder in a relatively short growing season (60–90 days) [
6] enables drought escape and water savings. Thus, tef offers great advantages for arid region cultivation and holds promise for expansion of its production to currently uncultivated lands.
One of the basic details in mechanical farming is sowing depth. As a general rule of thumb, seeds should not be sown deeper than five times their width [
10]. Tef’s tiny seed size, 0.4–0.7 mm width and 1–1.4 mm length, hence calls for a maximum sowing depth of 2–3 mm. Tef is traditionally sown in Ethiopia by manual seed broadcasting and lightly covering with soil to prevent drying out [
11,
12], probably due to limited access to agricultural machinery. In addition, local farmers believe that the seeds are so small that they will not emerge from greater depths [
13]. However, under hot and dry conditions, deeper sowing is recommended to guarantee seedling establishment [
14]. In Oregon, USA, the recommended sowing depth for tef is between 3 mm and 15 mm [
5]. Sowing tef at 0 mm or at greater than 15 mm depth had negative effects on plant height and seedling emergence [
13,
14,
15], with no seeds emerging from a sowing depth of 50 mm or greater [
13].
Based on our previous study [
16] and published data [
4], tef is sown in Israel during the spring (March–April), at the onset of the hot and dry summer. Rainfall in Israel in March, and even more so in April, is very limited and scattered, and therefore irrigation is essential to securing proper germination and crop establishment. Shallow sowing would require up to two irrigation applications per day to avoid seed desiccation, which can sum to 150 mm during its 10–15 days of establishment, about half of the seasonal water requirement of the crop. Therefore, deeper sowing might be key to saving water during the crop’s establishment. Optimal sowing depth should provide a sufficiently thick soil layer to maintain the humidity required for germination, but not be so thick as to confer excessive mechanical resistance to the seedling’s emergence. Knowledge of the effects of sowing depth is also necessary for the development of advanced agricultural practices, including—but not limited to—the choice of sowing machinery.
Plant lodging, defined as the permanent displacement of the stem from the vertical, which is caused by environmental conditions as well as morphological plant traits [
17], is the most significant yield-reducing problem in tef, accounting for up to 35% of yield losses [
18,
19]. While certain studies relate tef lodging to stem characteristics (stem lodging) [
20], others [
18], including our recently published paper [
21], point to shoot–root junction traits (root lodging) as major contributors to tef lodging. In wheat, deeper sowing has been shown to induce a longer, deeper root crown, thus improving plant anchorage and reducing root lodging [
17] (and references therein). Hence, reduced lodging could be an additional benefit of deeper sowing in tef. The objective of this study was, therefore, to determine the effects of sowing depth on tef plant emergence, development, lodging, and productivity.
2. Materials and Methods
2.1. Plant Material
Six tef genotypes representing the range of thousand-seed weights were selected for the current study from the diversity panel assembled in our laboratory [
6]. These genotypes consisted of three seed-size groups: small, medium, and large, with each size group including a brown-seeded and a white-seeded genotype (
Table 1).
2.2. Test-Tube Experiment
The effects of sowing depth on seedling emergence and establishment were studied in a test-tube experiment for all six genotypes. Seeds were sown in 50 mL test tubes filled with brown-red sandy loam soil composed of 76% sand, 8% silt, and 16% clay (
Figure 1).
A hole was drilled at the bottom of each tube to allow drainage and covered with a thin layer of fabric to avoid soil leakage. A factorial experimental design was employed with the six genotypes, each sown at four depths: 0 cm (control) with 1–2 mm soil coverage, 1 cm, 2 cm, and 3 cm. Test tubes were filled with dry soil to the designated level, watered, and 20 seeds from a single genotype were scattered on the soil surface of each tube. To ensure an accurate sowing depth, preweighed soil was added to match a soil layer of 1, 2, and 3 cm in the 50 mL test tube, while the 0 cm treatment seeds were gently mixed into the top 1–2 mm of soil. After sowing, tubes were wrapped with aluminum foil to avoid root exposure to light. A total of 24 tubes—one tube for each of the six genotypes and four sowing depths—were sown in each of five growing cycles, referred to as experimental blocks.
The tubes were placed in a climate-controlled greenhouse at the Faculty of Agriculture, Food and Environment campus in Rehovot, Israel, under temperatures of 28/22 °C day/night, and were watered twice daily by microsprinklers. At the end of the growth cycle, after all plants reached the second leaf stage, plants were uprooted, and the soil was gently washed away to observe the root system.
The number of emerged seedlings and number of seedlings that reached the second leaf stage were recorded daily for each test tube. Time from sowing to emergence (TSE), time from sowing to fully expanded second leaf (TSL2), and time from emergence to second leaf (TEL2) were calculated as follows:
where
n is the number of seedlings emerged at each counting,
t is the time in hours from sowing to counting, and
N is the total number of seedlings emerged.
where
s is the number of seedlings with a fully unfolded second leaf at each counting,
t is the time in hours from sowing to counting, and
S is the total number of seedlings that reached the second leaf stage.
2.3. Pot Experiment
Three genotypes, RTC-119 (large seeds), RTC-400 (medium seeds), and RTC-19 (small seeds), were selected for the pot experiment (
Table 1). To enable accurate sowing depth, transparent cylindrical pots (8.2 cm diameter, 18 cm height), made from plastic soft-drink bottles, were used for this experiment. Holes were drilled at the bottom of each pot to allow drainage and covered with a thin layer of fabric to avoid soil leakage. A factorial experimental design was employed with three genotypes, each sown at four depths (0 cm (control), 1 cm, 2 cm, and 3 cm), and six replicates (a total of 72 pots). The type of soil used, number of seeds per experimental unit, sowing methodology, and scoring of seedling emergence and second leaf stage were as in the test-tube experiment. Once all seedlings reached the second leaf stage, all but one randomly selected seedling were removed from each bottle and the remaining plant continued to grow until harvest at flowering onset. Just before harvest, the lodging angle (between the main culm and the vertical) was measured. The plants were then uprooted, the soil was washed away, and number of tillers, crown diameter, number and diameter of crown roots, and shoot and root dry weight were assessed following the protocols in Ben-Zeev et al. [
21].
2.4. Field Experiments
Two field experiments were conducted in the summers of two consecutive years, 2019 and 2020, in two locations, the Hula Valley in Northern Israel (33°06′46.8” N, 35°35′03.7” E) and Revadim in Central Israel (31°46′04.8″ N, 34°49′02.8″ E), respectively. Soil type at the Hula experimental site was deep peat (52% sand, 43% silt, 5% clay, and ~10% organic matter) and at the Revadim site clay soil (clay 41%, sand 42%, silt 17%, and ~1% organic matter). A factorial block design was employed in both experiments with 2 genotypes (RTC-119 and 400) × 2 sowing depths × 12 replicates in Hula (2019), and 3 genotypes (RTC-19, 119 and 400) × 2 sowing depths × 5 replicates in Revadim (2020).
Soil preparation included shallow tilling and flattening with a heavy-duty leveler and crumble roller tool to obtain a smooth and even seedbed. Seeds were tested to confirm uniform germination rates and were treated with Vitavax® fungicide (thiram + carboxin, Gadot Argo, Israel) at a rate of 100 µL/10 g seeds. Experimental plots were mechanically sown using a Plotseed S seeder (Wintersteiger, Ried, Austria). Ten rows were sown in each 8 m × 1.6 m plot at two sowing depths (1 or 3 cm) using a sowing rate of 6 kg/ha (Hula) or 4 kg/ha (Revadim).
Seedling density was assessed 10 days after emergence. A metal frame with inner dimensions of 25 cm × 100 cm was placed across the seedling rows in two locations per plot and the number of seedlings within this frame area was counted.
Lodging was assessed visually by two independent surveyors twice a week, starting from six weeks after emergence (WAE), using a scoring method adapted from Caldicott and Nuttall [
22]. Lodging was scored on a 10-level severity scale (0 being a vertical nonlodging plant and 9 being a horizontal fully lodging plant), and lodging prevalence was determined (percentage of the entire plot area). Lodging index was calculated as the product of lodging severity × lodging score.
Plant height and panicle length assessments and harvest were performed at grain maturity, 12 WAE. Three plants, randomly selected from various parts of each plot, were measured from the soil surface to the bottom and top of the panicle and used to calculate the average culm length and panicle length, respectively. A metal frame of 25 cm × 100 cm was placed in the middle of the plot across six rows (excluding the two outer rows from each side) and all of the biomass bordered by the frame was manually harvested and collected into paper bags. Biomass samples were dried in a hot glasshouse (maximum temperature ~55 °C) for two weeks, weighed for total dry matter, and threshed using a LD350 thresher (Wintersteiger, Ried, Austria). Seed samples were cleaned of debris and weighed to determine grain yield (GY).
2.5. Statistical Analysis
Statistical analysis was conducted using JMP® Pro, Version 15, software (SAS Institute Inc., Cary, NC, USA, 1989–2019), and included two-way analysis of variance (ANOVA), Student t test, and Tukey HSD test (for factors showing a significant F ratio).
5. Conclusions
This study was motivated by the need to reduce the amount of irrigation water required for tef establishment under semiarid Mediterranean conditions. An optimal sowing depth should allow sufficient soil above the seed to maintain the required moisture for germination, while not inflicting penalties on seedling emergence or plant development. Despite the tiny size of tef seeds, which was thought to require a very shallow sowing depth, we showed that tef can successfully emerge from a depth of 1–2 cm without any apparent penalty. Tef even emerged from 3 cm depth, while showing reduced lodging, but this was coupled with a penalty on plant development and productivity. Therefore, we conclude that a 1–2 cm depth is optimal for sowing irrigated tef. Further research combining optimal sowing depth, sowing rate, and irrigation regimes with a wide genotypic range to evaluate plant development, lodging and productivity, could lead to advances in tef production in Mediterranean and other similar regions around the globe.