Chile Seed Germination as Affected by Temperature and Salinity¹

New Mexico Chile Task Force Report 2
Robert Flynn, Richard Phillips, April Ulery, Richard Kochevar, Linda Liess and Magdalena Villa, State Seed Analyst, New Mexico Department of Agriculture
College of Agricultural, Consumer and Environmental Sciences, New Mexico State University

Authors: Respectively, Associate Professor and Extension Agronomy and Soils Specialist, Cooperative Extension Service, Project Manager, Cooperative Extension Servic, Assistant Professor, Soils, Department of Agronomy and Horticulture, State Seed Analyst, New Mexico Department of Agriculture, Research Specialist, Extension Plant Sciences, Student, Department of Agronomy and Horticulture.

Research Summary

For economic reasons, chile is planted early, which often exposes the seed to adverse conditions that affect stand establishment. High levels of soil salinity, common in many areas of the Southwest, also influence stand establishment, since chile is particularly sensitive to salt levels. This laboratory study evaluated how temperature and salinity affect seed germination.  Four chile seed types (fresh green, fresh jalapeño, dry red, and fresh red) were subjected to incubation temperatures of 10°C (50°F), 15°C (59°F), and 20°C (68°F) with varying levels of saline water during germination. The saline levels were 1, 3, 5, 7, and 9 deciSiemens per meter (dS/m). All chile types also were tested using standard germination protocols, with the salt content of the wetting solution remaining constant. Germination was evaluated 6, 9, 12, 15, 18, and 21 days after placement in the incubators. Seeds incubated at 10°C (50°F) did not germinate. At an incubation temperature of 15oC (59°F), less-than-optimal germination occurred during the 21-day test. After only 12 days, all chile types tested at 20oC (68°F) came to within 90% of maximum germination.  Salinity levels at or above 7 dS/m reduced germination by as much as 19% under 20°C (68°F) conditions and by as much as 27% under 15°C (59°F) conditions. Extrapolating these results to field conditions suggests that chile should be planted into a seedbed with salinity less than 3 dS/m and an average temperature of 20°C (68°F) for 12 days. Although salinity may have a relatively small impact on final germination, it can still have a major impact on stand establishment and, ultimately, on yield. Furthermore, there was a 13% difference in germination among varieties under standard conditions. That variation should be investigated further to improve consistent germination for all chile seed.

Background

Uniform stand establishment for chile pepper production is essential to maintaining profitable yields. A 1999 survey of commercial chile fields in southern New Mexico indicated that as many as 64% of the farms started the season with poorly stocked chile stands (Phillips, unpublished data, 2000). Poor stands are a result of many factors, including seed quality; environment; water and soil quality; site preparation; plant uniformity; irrigation practices; and other management practices. This study focused on two critical environmental factors: temperature and salinity. By the very nature of the commercial chile industry in New Mexico, these two environmental factors always will offset the uniformity of chile stand establishment. Learning to manage them is a crucial factor in producing profitable yields.

Processor perspective

The balance between processing capacity and the length of the chile growing season dictates the timing of planting and harvesting. Processors need to spread the harvest out to match their processing capacity. To maximize their use of capital investment in equipment, processors contract the flow of raw materials over as long a period as possible. Growers’ decisions about planting dates are controlled by two factors: the processors’ need to balance capacity and product quality, and supply and demand.  Fresh green chile, jalapeños, and fresh red chile have a very short storage life before processing, so processors must move the raw material quickly. Red chile is a more stable product, but it, too, loses quality with time. It is important that processing plants begin processing early and continue throughout a maximum harvest window.

Supply and demand affect the time of planting, since growers realize that raw materials are in short supply during the processing plant start-up phase.  Historically, if the farmer can deliver chile early, the demand is unlimited. This is in contrast to midseason circumstances, when the processing plants are full and growers are forced to slow down delivery of raw material. These two factors -- processing capacity and the growers’ willingness to gamble on early delivery -- result in many acres of chile being planted into a growing environment with less-than-optimal biophysical requirements.

Environmental perspective

Soil temperature. While spring weather in the southwestern United States practically begs farmers to start planting, it is soil, not air, temperature that governs seed germination. Some sources give minimum temperatures for germination. These are the lowest temperatures at which germination will occur for specific crops. The length of time for germination will be much longer at less-than-optimal temperatures. Planting at lower temperatures also results in greatly reduced germination.

Some crops require fluctuating temperatures on a daily or seasonal basis. In other crops, germination is affected by the interrelation of temperature and light. For example, celery requires temperatures below 50°F to germinate, if it is held in the dark at a constant temperature. In diffused light, however, it will germinate at 70°F; and with a 10-degree day/night fluctuation, it will germinate at an 85°F day temperature.  A review of the chile seed germination literature (Bosland and Votava, 2000) indicates that day/night temperature fluctuations of 30°/15° C (86°/59°F) at 16 hours of light and 8 hours of darkness is biologically the best temperature and lighting range for chile germination.

Salinity. Soil salinity, if not properly managed, can become a limiting factor for chile stand establishment. Excess salinity within the plant root zone has a deleterious effect on plant growth. Tanksley and Phillips (1985) reported a decline in seed germination for the red chile variety, ‘NuMex R Naky’, at 7.8 and 9.4 dS/m. Growth suppression usually is initiated at some threshold value of salinity that varies with crop tolerance and external environmental factors. Plants generally are tolerant during germination but become more sensitive during the emergence and early seedling growth stages (Rhoades et al., 1992). It is imperative to keep salinity low in the seedbed during germination and stand establishment. It also is important to realize that salt tolerance data cannot provide accurate, quantitative crop yield losses from salinity for every situation, since actual responses to salinity vary with other conditions, including agronomic factors, soil and water management, climate, crop variety, and growth stage.

Crop choice perspective. Crop variety can have a profound effect on stand establishment under different environmental conditions. Since chile is an important rotational crop in the Southwest, a brief overview of chile types could help explain differences that arise when considering what varieties to plant under less-than-optimal conditions.

The place of origin for all commercial chile peppers grown in the Southwest is Latin America (DeWitt and Bosland, 1996). While production has been adapted to temperate climates by growing chile as an annual plant, it originated in a frost-free climate and grew as a perennial (Bosland et al., 1996). There are four major chile types grown in New Mexico. They include fresh green, jalapeño, dry red (paprika), and red cayenne. How each of these commercial chile types responds to soil temperature and salinity conditions is not well understood. Therefore, a replicated study conducted under controlled environmental conditions was performed to observe the interaction of these two critical environmental factors.

Hypothesis

The hypothesis of this study was that low temperatures coupled with high salinity would negatively affect chile seed germination.

Materials and Methods

The New Mexico Department of Agriculture’s Seed Testing Laboratory, approved by the Association of Official Seed Analysts (AOSA), conducted the germination tests in conjunction with research associates of New Mexico State University’s Chile Task Force.

Twenty treatments, consisting of five salinity levels for each of four chile types with four replications, were arranged in a completely randomized design within each temperature regime (10°C, 15°C, and 20°C). Time was considered a split plot with multiple observation times (6, 9, 12, 15, 18, and 21 days after placement). Salinity levels were 1, 3, 5, 7, and 9 dS/m. There were four types of chile seed used: ‘AZ-20’ (a fresh green chile); ‘Sonora’ (a dry, red paprika type); ‘TAM’ (a jalapeño type for fresh market); and ‘Large Red Thick’ (a cayenne type). The experimental unit was 100 seeds placed on standard AOSA-approved seed germination blotter paper. Seeds were randomly chosen for each treatment without regard to any physical characteristics or seed viability. A total of 6,400 seeds were tested.

The AOSA standard germination test for pepper (Capsicum sp.) was used as a control or standard for comparison to the imposed incubation temperatures. The standard seed test was run at 20°C (68°F) in 16 hours of darkness followed by 30°C (86°F) for 8 hours of light and used distilled water (electrical conductivity < 0.1 dS/m) to saturate the blotter and moisten the seeds. Saline solutions were prepared from a mixture of sodium and calcium chloride reagents in a 1:1 equivalent ratio to attain electrical conductivities of 1, 3, 5, 7, and 9 dS/m. The amount of each chloride salt in the test solutions was determined graphically from U.S. Salinity Laboratory Handbook 60 (USDA, 1954). These solutions were used to saturate the germination blotters and moisten the seeds during testing.

The protocol included random selection of 100 seeds per temperature per replicate per variety of seed; and saturating the blotter paper with distilled water or saline solution for each combination of salinity, replication, and variety within each temperature. Seeds were placed evenly across the test surface and each blotter was then rolled and folded in such a way as to keep the seeds enclosed. The rolled blotter papers were placed in plastic bags labeled with seed type, salinity level, test temperature, and replication number. Each bag was then placed in the appropriate temperature-controlled germination chamber.

Six observations were made during the 21-day period to evaluate seed germination. The criteria used to evaluate seed germination were a primary root at least 5 mm long in the root hair zone, elongating hypocotyls above the root hair zone, and no lesions or decay from mold.