Applications of dry chain technology to maintain high seed viability in tropical climates

Measure the moisture status of the seeds

First of all, it is important to know the SMC prior to and after drying, but obtaining a precise measurement of SMC is a destructive and relatively slow process by means of a standard oven test, which involves weighing a seed sample, drying in an oven for up to 17 h (depending upon the species), and reweighing to determine the weight loss (ISTA, 2022). Moisture meters can be used for non-destructive measurements of SMC, but these are generally calibrated only for a limited number of crops (Hay et al., 2023). Alternatively, eRH can be measured within a few minutes and non-destructively using a water activity meter or a relative humidity meter (see e.g., Ndinya et al., 2017; MSBP, 2022; Hay et al., 2023). However, such equipment is not readily available in rural communities or to individual farmers. Thus, the use of relative humidity indicator cards is recommended to provide a quick estimate of eRH, which can be readily converted to SMC if needed (Bradford, Dahal & Bello, 2016; Thompson et al., 2017). These humidity indicator strips change color in response to RH and can quickly indicate the RH inside a container containing a seed sample. All that is required to estimate SMC is to insert an indicator strip with the seed sample inside hermetic container (e.g., a clean, dry plastic water bottle), allow it to equilibrate (generally within 30 min), and compare the color to the chart provided with the RH indicator strip/card. For larger quantities of seeds, representative sampling (such as by using a trier to sample large bins) is important (Bradford, Dahal & Bello, 2016; ISTA, 2022). As mentioned, eRH measurements can be converted to MC estimates, and online tools are available to facilitate these conversions for many crop and plant species (see e.g.,  https://ser-sid.org/viability/moisture-equilibrium and http://www.dryingbeads.org/tools).

Introduction to zeolite drying beads

Briefly, drying beads are composed of zeolite crystals that have sites that can tightly bind individual water molecules. Once the sites are occupied, the water can be removed by heating the beads to over 250 °C for approximately 2 h, or until all beads have reached this temperature. The beads are cooled in water-proof containers to prevent reabsorption of water from the atmosphere and then stored hermetically until used. Inside of a closed container, seeds will equilibrate with the RH of the air at the ambient temperature. When beads are present, they will absorb the water from the air, reducing the RH in the container to near zero. The seeds will then lose water to this low RH atmosphere through the vapor phase until the beads reach their capacity or the forces binding water in the seeds become equal to those in the beads. Even an excess quantity of fully activated beads generally will not reduce SMC below about 2–3%, at which point the binding energy of some seed components equals that of the beads. If a specific final MC is desired, the weight of beads to store with a given quantity of seeds at a known MC can be calculated from the amount of water in the seeds and the bead water-holding capacity (Bradford et al., 2018). A drying beads calculator is available to calculate the correct weight of beads to use considering seed quantity, initial SMC (or eRH), ambient temperature and oil content for 69 different crop species in order to achieve a given final MC when the beads are saturated (see: http://www.dryingbeads.org/tools). For larger-scale applications, such as with commercial seeds, a pragmatic approach is to first incubate seeds with 40% of their weight of beads (0.4 ratio). After one or two days, the seed eRH can be easily measured (see also the chapters below on the application of the dry chain in Costa Rica and Nepal); if it is at the desired level, the beads can be removed, and the seeds packaged for storage. If the SMC (or eRH) is still too high, the process can be repeated a second time, which is generally sufficient unless the initial SMC is very high.

Effects of the dry chain on seed moisture content, germination percentage and germination timing

In all the studies reviewed, drying beads quickly lowered the MC of the seeds, without damaging them or affecting germination, except for the results of Trail et al. (2022). In this study, zeolite beads (stored with the seeds) dried okra, sorghum and velvet beans to ultradry conditions (<5% MC), detrimentally affecting seed viability in okra and velvet beans but not sorghum. The authors of this study observed that the ratio of beads to seeds was probably too high, considering the initial low moisture contents of seed samples. This highlights that, when using drying beads as a seed desiccant, it is important to utilize the correct ratio of beads to seeds, based on initial moisture content and specific seed characteristics (e.g., oil content) to prevent over-drying of sensitive species. Moreover, such as for long-term germplasm storage, where lower moisture levels are desirable, the seeds should be incubated at ambient (or higher) RH for several days after removal from storage to hydrate the seeds via the vapor phase prior to imbibition in liquid water. This will prevent imbibitional damage that can occur with some species (e.g., legumes) when they are rapidly imbibed from very low MC (Bewley et al., 2013). This rehydration procedure was not employed in the study by Trail et al. (2022), who noted that the damage observed in germination tests of okra and velvet bean seeds appeared to be associated with imbibitional damage.

Other than the described over-drying risk of sensitive species (Trail et al., 2022), the germination of seeds dried with beads was always higher than the controls (i.e., seeds conserved in open storage and/or undried seeds in hermetic containers) after different periods of storage (Table S1). For example, Bakhtavar et al. (2019) demonstrated that maize seeds dried with beads and conserved in hermetic plastic bags for only 4 months had a faster mean germination time (MGT) when compared with seeds with a higher pre-storage MC and/or conserved in porous containers. A slower MGT is often observed with seed ageing (Demir et al., 2011; Bradford & Bello, 2022). Yahaya, Sinniah & Misran (2022) also showed that seeds dried with beads had faster germination when compared to seeds dried using traditional drying systems or oven drying after six months of storage. Seeds conserved following the dry chain performed better than controls also in terms of seedling vigor (percentage of seedling emergence, seedling length, dry biomass or seedling vigor index) when compared with controls after different storage periods (e.g., Guzzon et al., 2020, six months of storage; Kamran et al., 2020, five months of storage; Yamalle, Khade & Singh, 2020, twelve months of storage; Hilli & Vyakaranahal, 2019, nine months of storage). Plants derived from seeds of two cotton genotypes dried with beads and hermetically stored produced plants with more bolls and sympodial branches, that bloomed earlier, subsequently producing higher yields when compared to sun-dried seed lots (Kamran et al., 2020, five months of storage). Moreover, drying beads enabled faster drying while maintaining high seed viability when compared with traditional methods such as sun drying (Nassari et al., 2014; Kamran et al., 2020; Yahaya, Sinniah & Misran, 2022) or drying with cow dung ash (Sultana et al., 2021).

Effects of the dry chain on physiological indicators of seed ageing

Seeds dried with desiccants and stored hermetically showed reduced seed deterioration, as indicated by physiological indicators of seed ageing. For example, seeds dried with beads and conserved in hermetic containers had lower levels of malondialdehyde (MDA) when compared with undried seeds and seeds stored in porous containers (Bakhtavar et al., 2019, four months of storage; Bakhtavar & Afzal, 2020a, eighteen months of storage; 2020b, four months of storage). MDA is one of the by-products of lipid peroxidation and has been used to quantify seed deterioration due to oxidation (Gianella et al., 2022). Tahir et al. (2023) highlighted that rice seed quality in terms of germination and antioxidant defense mechanisms was preserved when dried to 10% MC with desiccant beads and stored hermetically for six months.

Bakhtavar & Afzal (2020a, eighteen months of storage) found the lowest levels of reducing sugars in quinoa seeds using the dry chain, when compared with poorly dried seeds and seeds stored in porous containers. Reducing sugars, which can interfere with the normal functioning of membranes, are formed during ageing as a result of sugar hydrolysis under high SMC due to the formation of reactive oxygen species (ROS). Therefore, high levels of reducing sugars, together with lipid peroxidation, are among the markers of the biochemical deterioration associated with seed ageing (Murthy & Sun, 2000). Kamran et al. (2020, five months of storage) found that cotton seeds dried with zeolite beads and stored hermetically did not show a significant increment in free fatty acids (FFA). Increases in FFA, due to hydrolyzation of lipids, is another hallmark for seed and grain deterioration (Wang et al., 2020) and was particularly evident in the experiments of Kamran et al. (2020) in seeds stored in porous containers.

Seed quality can also be estimated through conductivity tests that measure electrolytes leaking from seeds. Changes in the organization of cell membranes occur during seed dehydration to preserve them in the dry state (ISTA, 1995). During subsequent imbibition, higher vigor seeds reorganize seed cellular membranes and repair cellular damage quicker and to a greater extent than lower vigor seeds, which corresponds to lower measurement of electrolyte leakage (ISTA, 2022; Marin et al., 2018). Yamalle, Khade & Singh (2020, twelve months), Bakhtavar & Afzal (2020a, eighteen months) and Sultana et al. (2021, nine months of storage) showed that seeds of onion, quinoa and mung beans conserved with the dry chain showed the lowest values of seed leachates upon imbibition when compared with undried seeds and/or seeds conserved in porous containers. High α-amylase activity also indicates vigorous seeds with low deterioration (Marques et al., 2014). Bakhtavar & Afzal (2020a, eighteen months of storage) showed that quinoa seeds stored in hermetic plastic bags exhibited maximum α-amylase activity, thus demonstrating the effectiveness of maintaining low SMC in hermetic bags to preserve seed quality. Moreover, maize and wheat seeds dried with zeolite beads and conserved at low MC in hermetic plastic bags showed the highest starch and protein contents, when compared with control treatments (Bakhtavar et al., 2019, four months of storage; 2020b, four months of storage).

Effects of the dry chain on insect and fungal infestations

Seeds dried to low MC and then stored in hermetic containers also were less susceptible to insect and fungal infestations as well as to mycotoxin contamination (Afzal et al., 2017). Maize seeds dried with zeolite beads and stored in hermetic containers showed reduced insect and fungal infestations when compared to undried seeds in rural communities of the Guatemalan highlands (Guzzon et al., 2020, 6 months of storage). Sultana et al. (2021, nine months of storage) confirmed that the dry chain reduced the frequency of seed borne pathogens in mung beans stored in hermetic containers. Seed conservation with the dry chain approach also greatly reduced infestation by the lesser borer in maize (Bakhtavar et al., 2019, four months of storage), bruchid beetles in mung bean and groundnut (Kunusoth et al., 2012, seventeen months of storage; Sultana et al., 2021, nine months of storage; Singh & Mishra, 2022, six months of storage) as well as rice weevil in wheat (Nelwadker et al., 2022, twelve months of storage). Similarly, the dry chain reduced seed weight loss due to insect damage as well as oviposition and insect respiratory activity in maize and mung bean (Bakhtavar et al., 2019, four months; Sultana et al., 2021, nine months of storage). Singh & Mishra (2022, six months of storage) observed that groundnut pods conserved with zeolite beads reduced weight loss due to insect damage as well as fecundity of bruchids while minimizing losses in seed germination.

The high moisture content of stored grains can also cause increases in mycotoxin occurrence (e.g., fumonisins and aflatoxins) that are responsible for severe health effects in consumers, such as liver necrosis and tumors, depressed immune esophageal cancer, stunting and neural tube defects (Bryła et al., 2013; Wu, Groopman & Pestka, 2014). Bakhtavar et al. (2019, four months of storage; 2020b, four months of storage) determined that drying maize and wheat seeds with zeolite beads and storing them in hermetic plastic bags completely prevented aflatoxin accumulation during storage. Taken as a whole, these results demonstrate that the application of the dry chain can potentially prevent postharvest losses that account for 40% of food losses in developing economies (Claes et al., 2021). In particular, aflatoxin contamination poses a serious health threat to humans and livestock as well as a significant economic burden, causing an estimated annual loss of 25% or more of the world’s food supply (WHO, 2018; Dahal et al., 2023).

Comparison between zeolite drying beads and other desiccants

Some experiments dealt with the comparison between zeolite drying beads and other desiccants, mainly silica gel, which is one of the most used reactivatable desiccants for seeds, due to its low cost and high availability. Hay et al. (2012) demonstrated that zeolite beads showed a higher affinity for water over silica gel, particularly at low MC (<9–10%), being able to dry rice seeds to lower moisture content than silica gel. In fact, silica gel has a higher water-holding capacity than beads at high RH, but its ability to absorb water declines linearly with RH. Thus, at low SMC, where eRH is low, the capacity of silica gel to absorb water is reduced compared to beads. In contrast, drying beads will absorb a specific amount of water across a wide range of RH, and remain absorbent down to very low RH. Thus, their absorptive capacity does not change as the seeds dry, making it easy to calculate the amount of water that they will absorb (http://www.dryingbeads.org/tools). In addition, Kunusoth et al. (2012) highlighted that zeolite beads can be more reusable than silica gel, as they regain their full potential capacity after reactivation, whereas silica gel will gradually lose its capacity with repeated reactivation. Nivethitha et al. (2020) showed that drying beads, at the same bead:seed ratio, can dry okra seeds to lower moisture contents and faster than silica gel. Similar results were obtained by Nassari et al. (2014) in drying tomato seeds with zeolite beads and silica gel. Sultana et al. (2021) confirmed that drying beads, as well as other desiccants such as silica gel, sodium aluminum silicate and activated alumina, can be effective desiccants for mung bean seed storage activities.

Concerns on the use of drying beads

Despite the overall positive results obtained by the zeolite drying beads, some concerns arose from their use. In particular, Hay et al. (2012) and Hay & Timple (2013) reported that beads did not seem to work to full capacity in a bead-seed system, and therefore calculating the quantity of beads to use to reach the target moisture content was not always straightforward. While these observations deserve further research, it is also important to highlight that, if the drying beads are not fully reactivated before use (by heating to >250 °C, for two hours), their absorptive capacity could be less than expected. It is therefore good practice to check the maximum capacity of the beads prior to use if their storage conditions since reactivation are not known. This can easily be done by weighing a small quantity of beads, placing them in a sealed container over water (as on a screen supported above the water), and weighing again after incubating at least several hours, or until weight is constant (Ndinya et al., 2017). The weight gain relative to initial weight indicates the current bead adsorptive capacity, which should be compared to that of recently reactivated beads.

Seed drying protocols and applications

In the publications analyzed (Table S1), different drying treatments were used for various species at a range of initial MCs. Several drying protocols, including different beads-to-seed ratios and storage conditions, were used in the different experiments, with some authors (e.g., Kunusoth et al., 2012; Guzzon et al., 2020) suggesting that to maximize the drying potential of zeolite beads, these should be replaced or reactivated during the drying process. It emerged from our review that, in some experiments, zeolite beads were stored together with seeds (see e.g., Ndinya et al., 2017; Sultana et al., 2021; Nelwadker et al., 2022; Trail et al., 2022), or removed before storage in hermetic containers (Arjun & Pratima, 2014; Guzzon et al., 2020; Bakhtavar et al., 2019). It is not necessary to store the desiccant together with the seeds as long as the seeds, once dried to low MC suitable for conservation, are then packaged in hermetic containers to maintain the MC achieved (Bradford et al., 2018). This means that after the initial drying, beads can be removed, reactivated and reused. This reduces the investment required for the acquisition of sufficient zeolite beads. In addition, it is important to note that bead drying can be combined with other drying systems, particularly air and sun drying. Ambient conditions, whether in sun or shade, are universally used to dry seeds and are effective in removing moisture to equilibrium with the ambient RH. However, as seed eRH cannot be lowered below the ambient RH, this often may not be sufficient to reduce SMC to levels for safe storage. However, the remaining moisture can easily be removed using drying beads to lower SMC to levels safe for sealed storage and extended longevity (see, e.g., the section below on the application of the dry chain in Costa Rica). This reduces the quantities of beads (or reactivation cycles) that would be required to process a given quantity of seeds. An initial stage of air or sun drying is recommended, particularly for seeds that are wet at harvest, such as tomatoes or melons, so that no surface water remains on the seeds if they are to come in direct contact with the beads. When beads absorb liquid water quickly, they emit heat, which can be damaging if beads are mixed directly with wet seeds.

The reusability of the beads is particularly important since it was highlighted that zeolite drying beads are currently logistically and economically inaccessible to individual farmers (Kunusoth et al., 2012; Guzzon et al., 2020; Musebe et al., 2020). It has been proposed that, to increase the access to drying beads, “drying centers” could be organized in farming communities in tropical areas (Dadlani, Mathur & Gupta, 2016; Bradford et al., 2018; Guzzon et al., 2020). These centers should be organized within infrastructures that are already exist and are being used by local farmers, such as community seedbanks or agricultural cooperatives. Drying kits, including beads, hermetic containers, ovens to reactivate the beads and instruments to evaluate MC and/or eRH (such as DryCards, reusable cards with a strip of relative humidity indicator paper embedded in them; Thompson et al., 2017) can be provided to interested institutions to spread the use of this technology. In this context, it would be important to learn from the experience of the “blue drums”, a drying system developed by the Millennium Seed Bank of the Royal Botanic Gardens, Kew (UK), consisting of a sealable, hermetic plastic drum containing silica gel, placed within a central cone made of metallic net from which bags of seeds are hung for drying (Sutcliffe & Adams, 2014). These drums are already being used as a low-tech drying method in seed banks in 47 different countries and territories across the globe (Martens, 2018). In this review of the current available literature, most of the studies on the dry chain were realized in seed laboratories in controlled conditions. More research on strategies for applying this technology in rural communities of low- and middle-income countries in tropical areas, as well as studies on the socio-economic implications of the use of dry chain technology are needed. We provide some examples of different strategies to implement the dry chain in rural communities in the following sections of this paper.

Hermetic containers for seed storage

Several types of hermetic containers were used to store seeds in order to maintain the low moisture content achieved with desiccants, including plastic containers and drums (Afzal et al., 2019; Guzzon et al., 2020; Sultana et al., 2021), plastic bags (e.g., Super Bags, Bakhtavar et al., 2019), laminated aluminum foil packets (Hay et al., 2012; Yahaya, Sinniah & Misran, 2022) and glass jars (Trail et al., 2022). This highlights that diverse types of moisture-proof containers can be used to store dry seeds, and that this approach can utilize a variety of different locally available hermetic containers. While we have concentrated here on seeds, the same conditions apply for grains or other food products. Anokye-Bempah et al. (2023) demonstrated the potential of the dry chain (in terms of storage with beads within hermetic plastic bags) in maintaining the correct MC during storage and long-distance transportation of green coffee beans otherwise exposed to undesirable fluctuations in temperature and relative humidity, without affecting sensorial qualities. This shows the potential of employing the dry chain also for the postharvest storage of other foods and plant materials of great economic importance.

Future applications and research directions

Most of the experiments on the application of the dry chain technology were performed on seeds of food crops (cereals, pseudocereals, vegetables and legumes), due to their direct importance in food security and farmers’ livelihoods. In the future, it would be important to test this technology for the short- and medium-term seed conservation of wild species in local seed banks, restoration seed banks (sensu Merrit & Dixon, 2011) and in reforestation activities in tropical areas, considering that critical hotspots of plant biodiversity are located in tropical areas and that ecological restoration provides a significant opportunity for achieving global conservation goals in the context of climatic change (De Vitis et al., 2020). In this context, a relatively high percentage of plant species in humid regions have recalcitrant seeds that are intolerant of drying, and the species’ desiccation tolerance should be known before dry chain technology is implemented for seed collection, storage or restoration of such species (Wyse & Dickie, 2017).

Costa rica

Results

The Papaya Seed Production Group was among the first users of the dry chain technology in Costa Rica. Papaya seeds are produced in two regions of Costa Rica, one located in the Caribbean side of the country and the other in the Central Valley. The MC of the seeds available varied consistently between the two locations, with an average of 12.4% (±0.1 SD) for the Caribbean location and 9.0% (±0.03) for the Central Valley location. A second independent group of seed lots was tested, and the difference was even larger, ranging from 14.7% to 9.3%. This clearly indicated the necessity, especially for the location on the Caribbean side, to include a drying process in the seed production system. Ellis, Hong & Roberts (1991) reported that for papaya seeds, storage at a moisture content between 7.9% and 9.4% and 15 °C should maintain original seed germination capacity for a year. Following these results, a target of 8.5% seed moisture content was set for the papaya seeds. The Cromarty, Ellis & Roberts (1982) equation (with a linear R² = 0.96 with the SMC; Fig. 1A) was used to obtain the eRH at which the SMC would be ∼8.5% (using 30% seed oil content for ‘Pococí’ hybrid). In preliminary experiments it was observed that drying the papaya seeds to 3.02% did not affect the germination (Fig. 1B). Once dried to the desired eRH (∼50%), the seeds were packed in hermetic plastic bags and the seed germination was checked before and after drying with no changes (Fig. 1C).

As previously mentioned, in the three community seed banks involved in this project (see Fig. 2), the most commonly grown crops are maize and beans. One key need was to provide the communities with an affordable method to know whether their seeds were dry enough for storage. For this, an inexpensive dry-circle RH indicator card was designed and produced with instructions in local language (Figs. 2B and 2C). The main intentions in the design of the dry-circle card were to avoid using plastic and to maximize the reuse of the indicator to limit costs. At first, in all communities, all the dry-circles incubated with seeds exhibited pink color (Figs. 2B and 2D), indicating high eRH of the seeds, consistent with the high ambient RH in these locations (Table 1). Thus, seed drying beyond open-air drying was needed, so the communities were instructed on how to store zeolite beads with the seeds in a sealed container (beads equal to 40% of the total weight of the seeds, 0.4 ratio) for two days before storing the seeds in a hermetic container. An average of 9% and 11% SMC for maize and beans was detected after drying these samples. The training also included evaluating seed physiological quality employing a seed germination test. The germination tests were carried out two weeks after the storage in the hermetic containers, showing 91% germination in maize (average of five samples) and 78% in beans (average of 27 samples). However, in beans the values ranged from 62% to 94%, indicating a range of initial viabilities among the accessions.

Mexico

Introduction

The dry chain technology was introduced at the International Maize and Wheat Improvement Centre (CIMMYT, Texcoco, Mexico) in 2015 to support community seed banks in Guatemala and Mexico (see e.g., Guzzon et al., 2020). Moreover, the application of the dry chain was also tested within the CIMMYT Germplasm Bank itself (the largest germplasm collection of maize and wheat genetic resources in the world) as a potential opportunity to enhance seed processing and drying in the tropical research stations in Mexico where the regeneration of tropical maize material is carried out, especially in the research station of Agua Fría (Venustiano Carranza, Puebla State; hereafter referred as to AF) where a high occurrence of fungal and insect infestations as well as rapid losses of seed viability were observed. This research station has an old heat-based seed drier (6 DOOR; STONE CONVEYOR Co. Inc., Honeoye, NY, USA, 1967) that was no longer in use. In 2018 the heating system was removed and the drying cabinet underwent some renovations (painting and porous trays to allow air flow were added; see Fig. 3A). The aim of this project was to test the use of drying beads coupled with the air flow generated by the ventilator of the cabinet to enable fast drying of the seeds before they were transported to CIMMYT Genebank for the last phases of seed processing and packing (El Batán, Texcoco, Mexico State, hereafter referred to as GB), without adversely affecting seed longevity in storage.

Nepal

Materials and methods

The dry chain technology was evaluated on several crops, including onion, okra, bean and cucumber seeds (Figs. 4A, 4B, 4C and 4D, respectively). The experiments included comparison between seeds stored in cloth bags and in plastic hermetic containers with drying beads (beads equal to approximately 40% of the total weight of seeds, or 0.4 ratio) for up to 20 months under ambient temperature and RH conditions. Germination was tested at the Seed Quality Control Center of Nepal using standard paper towels seed testing methods at 25 °C.

Bangladesh

With support from the Horticulture Innovation Laboratory of the United States Agency for International Development (USAID), Rhino Research (Bangkok, Thailand) conducted projects aiming to create the foundation for spontaneous diffusion and large-scale adoption of drying technologies in Bangladeshi agriculture. The lack of efficient drying and storage systems in the hot, humid climate of Bangladesh poses a significant challenge to seed production and dissemination (Alam et al., 2018). Bangladeshi seed companies estimated that they lost 5–10% or more of their seeds from rapid ageing due to insufficient drying, a percentage that is worth tens of millions of dollars in horticultural seeds alone.

In contrast with many agricultural development projects that focus first at the individual farmer level, the concept of this project was to focus on the strongest links in the seed supply chain: the seed-producing companies. The approach was to encourage the major Bangladeshi seed production and agricultural processing companies to adopt this technology, enabling it to diffuse through commercial channels throughout the sector, including to smallholder farmers.

The project was initiated by signing Memoranda of Understanding (MOUs) with various Bangladeshi organizations, including Lal Teer Seed Ltd., Getco Agro Vision, Metal Seed Ltd. and Development Alternatives Incorporated (DAI). These companies were offered complete hands-on training of their key quality control employees on the use of zeolite bead-based seed drying methods. Seven week-long trainings enabled the trainees to implement these methods in their own facilities and follow up with questions and further training in the subsequent sessions. This enabled each company to adapt the basic technology to their own crop species, processes, and clients. A key adaptation by Lal Teer, for example, was to provide their seed growers with activated drying beads and hermetic containers, rather than require the growers to purchase them. The growers were instructed to place the seeds into the containers with the beads provided and send the sealed containers back to the company. The seeds then arrived at the company already dried and ready for further processing and storage, and the beads could be efficiently reactivated for reuse in company facilities. This system proved to be highly efficient, particularly in the high humidity conditions prevailing in Bangladesh that make air drying relatively ineffective. As a result, these companies have completely adopted the drying beads technology with good results.

Moreover, the government of Bangladesh has proactively adopted the concept of dry chain technology aimed at providing the necessary training, equipment and tools to the local seed producing communities. In support of this, the Bangladeshi Ministry of Agriculture (BAM) directed its public research organizations, such as Bangladesh Rice Research Institute (BRRI), Bangladesh Agriculture and Development Cooperation (BADC), and Department of Agricultural Extension (DAE), as well as academic institutions, to dry and store their high value seed using the dry chain. The Bangladeshi government has initiated a program to provide these basic supplies to rural communities and to individual farmers. They have successfully trained more than five hundred farmers through a funded project and equipped them with necessary dry chain tools to earn better livelihoods with improved seed quality. Apart from the government organizations, the Bangladesh Seed Association (BSA), representing the private seed sector, has emerged as an active support organization for all of the seed companies and seed-producing communities. BSA organizes various training workshops focusing on dry chain technology and continuously emphasizes its adoption through seminars, workshops, or webinars. Keeping in view the keen interest of all the key stakeholders of Bangladeshi seed sector, a new company named Rhino Bangladesh has recently been established by Rhino Research in collaboration with Lal Teer. The newly developed company has already started commercial production of Dry Chain commodities and is successfully meeting the needs of local and international seed market.

Pakistan

As previously mentioned above in the literature review section and in Table S1, researchers from University of Agriculture, Faisalabad (UAF) have widely tested the use of zeolite beads for seed drying and seed storage in diverse hermetic containers across different regions of Pakistan and with numerous crops. In addition to these research activities, UAF have also promoted the use of the dry chain technology among farmers at four locations (Athmuqam, Hattian Bala, Muzaffarabad and Rawlakot) of Azad Jammu and Kashmir, Pakistan. In these locations, UAF researchers demonstrated that germination and integrity of maize seed were maintained better in hermetic bags as compared to conventional bags at all locations (Khalid et al., 2024). The incidence of mold and insect infestations and aflatoxin contamination was also monitored in the stored seed. Maize seed preserved at low seed moisture contents (<12%) in Purdue Improved Crop Storage (PICS) bags (moisture- and oxygen-proof) showed higher seed quality, maintaining dryness during storage (Afzal et al., 2017). Educational and technical workshops were organized to share results and learnings from dry chain experiments with public and private sector stakeholders, including farmers.

Additionally, the dry chain technology was tested and adopted by vegetable seed companies in Pakistan that are currently utilizing desiccant beads for preservation of high value vegetable seeds. In particular, a study (also at UAF) on the use of zeolite drying beads was carried out on onion seeds, which is relatively short-lived in storage when compared to other crops (Walters, Wheeler & Grothenius, 2005). Onion seeds (100g for each experimental unit, 8% was the initial SMC) were stored in hermetic plastic containers after equilibrating at 6, 8, 10 and 12% MC. The MC of 6% was achieved by drying the seeds with desiccant beads, calculated by using the abovementioned drying beads calculator tool. The MCs of 10 and 12% were achieved by adding a calculated amount of water to the seeds, mixed thoroughly in an airtight container. The following equation was used to calculate the amount of water required to increase the seed moisture content of crop seeds up to desired moisture levels (initial and final SMC values are in percentage and on a fresh weight basis and initial seed weight is in grams).

Amount of water required (ml) = [((100 − Initial SMC)/(100 − Final SMC)) × Initial seed weight] − Initial seed weight.

In this experiment it was found that onion seed with 6% MC conserved in hermetic plastic container had generally superior seed quality when compared with seed stored in conventional cloth bags. Seeds stored in cloth bags at 10 and 12% MC showed less than 20% germination (tested in Petri dishes with filter paper at in an incubator at 25 °C for 10 days). Seed moisture content was maintained in hermetic plastic containers while MC fluctuated in cloth bags at ambient RH after six months storage (Fig. 5).

The researchers from UAF also conducted several additional experiments to identify the most suitable hermetic containers for Pakistani farmers and seed producers. Several studies were conducted on seeds of vegetable crops, cereals, moringa (Moringa oleifera Lam.), oilseeds and pulses, testing multiple seed traits to evaluate seed quality (e.g., accelerated ageing, electrical conductivity of seed leachates, germination, insect infestation, grain damage and weight loss, MDA content, SMC, oil and protein contents, total soluble sugars, mycotoxin contamination). These studies clearly revealed that porous woven polypropylene and cloth bags are not recommended for seed and grain storage in the season of high RH and that the dry chain through hermetic storage of seeds at low moisture contents in different types of containers (PICS and Super bags, plastic drums and glass jars) helps to preserve seed quality throughout the supply chain (see e.g., Afzal et al., 2019; Afzal et al., 2020; Khalid et al., 2024).

To enhance the conservation of cereal seeds after harvest in spring season when weather is relatively dry (RH < 60%), hermetic bags were introduced to the farmers of South Punjab and Sindh with the support of NGOs (non-governmental organizations). This helps to protect stored grains from insects and fungi, and therefore mycotoxins, during the monsoon season when RH exceeds 90%. In the desert areas of Sindh, smallholder farmers are facing postharvest losses of cereal grains and pulses due to coastal rains that raise RH during the hot summer season; seed quality rapidly deteriorates in these extreme environmental conditions. In these areas, capacity-building initiatives were conducted to equip farmers with the knowledge and skills necessary for the proper implementation and maintenance of the dry chain technology. UAF collaborated with national and international NGOs, district governments, and farmer cooperatives for further technology dissemination in areas prone to and affected by natural disasters. In recent floods (2022), about 33 million people were affected and farmers had no wheat seed for the next growing season. The concept of dry chain technology was introduced to resource-poor farmers of flood-prone areas. The farmers received training on strategies to strengthen their own seed production as well as storage through use of hermetic systems. Low-cost hermetic drums of high-density polyethylene with a built-in digital hygrometer in the lid to measure eRH of stored seeds were introduced to the farmers and seed industry (Afzal et al., 2019). Farmers in flood-affected areas gained access to hermetic containers in order to safeguard their seeds from floods, excess moisture, pests, and diseases. As a result, the loss of viable seeds was reduced, ensuring a reliable seed supply for future planting seasons. These results highlight how the dry chain technology can have a significant positive impact on the livelihoods of smallholder farmers of humid regions of Pakistan.

South Pacific: Fiji and Tonga

Dry chain technology is being tested and used in the Pacific Island Countries and Territories (PICTs) since June 2022 as part of the work of the Seed Laboratory of CePaCT (Centre for Pacific Crops and Trees, Suva, Fiji), the largest genebank of the Pacific region, belonging to the Pacific Community (SPC). Drying beads, RH indicator cards, dataloggers and hermetic plastic boxes were acquired and are now being tested for drying of orthodox tree seeds at CePaCT as part of initiating long-term seed conservation activities at the Centre. After drying, seed lots are packed in heat-sealed aluminum pouches and conserved in freezers at −18 °C. Several experiments are ongoing at CePaCT, in collaboration with the Fiji National University (FNU), in order to: (1) test dry chain technology for the long-term storage of native and endemic Pacific tree seed species, as well as exotic species of economic importance; (2) compare seed drying with silica gel and drying beads; and (3) promote the use of this technology for small-scale seed drying for genebanks, seed enterprises and community seed banks in the PICTs.

As part of this activity, a one-week training event on seed conservation (12 participants) was carried out by CePaCT staff at MORDI Tonga Trust (Mainstreaming of Rural Development Innovation, Nuku’alofa, Tonga) in April 2023 and the dry chain was introduced to the Kingdom of Tonga (including drying beads, RH indicator cards and dataloggers). MORDI Tonga Trust currently conserves 94 accessions of 21 crops (including local Tongan varieties as well as commercial cultivars) in aluminum pouches and hermetic plastic buckets in its seed vault at 5 °C. These accessions are distributed to local farmers and growers across the Kingdom either as seeds or as plantlets grown in MORDI’s nurseries. The dry chain is being tested to enhance the drying of the seed accessions prior to long-term conservation in the seed vault, which at the moment is the only seed cold storage facility in the Kingdom.