Buckwheat: Potential Stress-Tolerant Crop for Mid-Hills of Eastern Himalaya under Changing Climate

Abstract

Under changing climate, identification and diversification of cropping systems having higher stress resilience and adaptability for fragile mountain ecosystems of Eastern Himalayan Region (EHR) are paramount. Lesser known and underutilized crop like buckwheat (BW) with year-round cultivation potential and having higher stress tolerance to prevailing stresses (low pH, low moisture) could be a crop of choice for abating malnutrition among hill inhabitants. Proper time of sowing of the crop is between mid-September and mid-December seemingly essential for better grain yield to the tune of 15.0–18.0 q ha−1, and the crop is found suitable to be grown all through the year for higher green biomass (12.6–38.4 q ha−1). Enhanced exudation of low-molecular-weight organic acids (LMWOA) like oxalic acid by buckwheat increased the solubilization of fixed forms of free phosphorus (P) to the extent of 35.0 to 50.0 micro gram per plant in ideal acid soil of the region (P) in acid soil. In addition, relatively increased resilience to moisture stress with improved stress physiological attributes adds more potentiality for enhancing cropping intensity of hill slopes of EHR. Few genotypes namely IC377275 (18.97q ha−1), IC26591 (17.1 qt ha−1), IC14890 (16.32q ha−1), and Himapriya (15.27q ha−1) are emerging as high-yielding types for productive cultivation in acid soils. Studies on the combined effects of acid soil and moisture stress would aid in novel crop improvement of buckwheat in EHR.

1. Introduction

Eastern Himalayan Region (EHR) of India being one of the hot spots of biodiversity comprises ≈56% of the area under low altitude and 33% mid-altitude, and rest under high altitude is distinctly characterized by diverse edapho-climatic constraints and distress-ridden geography. Farming in this region of India is primarily rainfed and uniquely represented by complex diverse risk (CDR) prone type [1]. Presently, the cropping intensity is only 120% indicating that about 80% of that area remains vacant during the rabi season due to severe water scarcity as most of the rainwater received during the rainy season is lost as runoff through sloppy land. The region receives an average annual rainfall of 220–250 cm out of which large part (70–80%) is received during rainy season. The major lands of the region being sloppy terrain has been suffering from various degrees of degradation [1]. Wide spread soil erosion, soil acidity, nutrient mining, eroding biodiversity, acute moisture stress during winter season are some of the major concerns threatening the food and livelihood security of the region [1, 2].

Buckwheat (BW) is one of the lesser-known alternative gluten-free pseudo-cereal crops, which is having immense potential to act as life support for tribal inhabitants with multipurpose utility in the Himalayas (Figure 1). Its short duration (80–100 days) with early maturity makes it suitable for cultivation under marginal and degraded lands of mountain ecosystem on sustainable basis, which holds a great promise for increasing production in the hilly regions of India through its inherent potential for sustainable yield [2, 3].

Figure 1.

Buckwheat (Fagopyrum esculentum) cultivation at mid-hill slopes of EHR.

Buckwheat (Fagopyrum esculentum) is an herbaceous crop with round and hollow knotted stem, generally green but sometimes tinged with red. The leaves are heart-shaped with reticulate venation, and inflorescence is a compound raceme that produces laterally flowered cymose clusters. The flowers are usually dimorphic with two forms of flowers, one with long style and short stamens and the other with short style and long stamen and their color varies from white or light green to pink or red. Buckwheat is necessarily cross-pollinated. The seeds have dark brown, tough rind, enclosing the kernel of seed. It is three-sided to form triangular because of which the name of the crop is known as “buckwheat” (Figure 2). The seed of buckwheat (achene) is a fruit and not a grain. Therefore, botanically, buckwheat is not a member of the family Gramineae or Poaceae, even though it is looking similar to the grains of cereals [4]. The root system is dense, fibrous with a deep taproot. Most of its roots are concentrated in the top 10 inches of the soil. The crop is reported to help in soil binding and check erosion during the rainy season. Buckwheat is used for both grain and greens. The tender shoots are used as a leafy vegetable, and the flowers and green leaves are also used for extraction of rutin which is used in medicines. The presence of a high content of the metabolite rutin in their foliage has substantial medical importance for increased capillary fragility for hypertension, leading to hemorrhage, purpurea, and bleeding from kidney. The crop is also used as green fodder. The pericarp of the buckwheat seed is used as suitable stuffing material for pillows [5], packaging material, as base material for heating pad and as raw material for mattresses. The buckwheat is used in a number of culinary preparations, alcoholic drinks, etc. Buckwheat flowers are also an excellent source of nectar for honey. Because of its fast-growing ability, it can escape weed competition (Figure 3). It is reported to be used as staple food among the Monpas and Sherdukpen tribes of Tawang district of Arunachal Pradesh since rice cannot be grown (because of agro-physiological constraints) in this region due to high altitude.

Figure 2.

Whole grains of buckwheat having unprecedented nutritional value.

Figure 3.

Buckwheat (F. esculentum (L.) Moench) crop with full canopy foliage before its flowering at mid altitudes of Meghalaya.

1.1 Types of Buckwheat

Fagopyrum Moench is an annual or perennial herb, belonging to the family Polygonaceae. Fagopyrum is derived from the Latin word fagus (beech) and the Greek word pyrus (wheat) as the achene resembles beechnut and is used like wheat. The common name buckwheat refers to two cultivated species of Fagopyrum namely Common buckwheat (F. esculentum Moench) and Tartary buckwheat (Fagopyrum tataricum Gaertn). One of the most important distinguishing characters between F. tartaricum and F. esculentum is in their achene morphology. Achenes of F. tartaricum are grooved with angles rounded below and sharply acute above, while achenes of F. esculentum are not grooved with sharply acute angles (Table 1). The growth characteristics of Common and Tartary also differ in their physiological responses to cold and drought. The epigenetic regulation due to DNA methylation in Tartary buckwheat confers resistance to the effects of cold weather [6]. Tartary buckwheat is also more resistant to drought than Common buckwheat. Due to their ability to adapt to different climatic variables and water-stress regimes, cold temperature, and nutrient-deficient acid soil of EHR, the buckwheat is considered potential crop for cultivation at higher altitude. The examples for common varieties for Tartary buckwheat are Himgiri, Sangla B-1, whereas for common buckwheat varieties are Himapriya VLVgal-7 and PRB-1.

Type of cultivar	Plant Height (cm)	Leaf type	Inflorescence	Achene type	Flowering time
F. esculentum Moench	30–90 cm	Leaf is simple, and leaf blade is triangular, having papillate veins on both surfaces. Leaf base is cordate or nearly truncate.	Racemose/corymbose and axillary/terminal white or pink flowers with heterostylous.	Dark brown in color, ovoid shaped, and sharply trigonous.	September to March
Fagopyrum tataricum (L.) Gaertn	30–70 cm	Petioles has long blade, leaf blade is broadly triangular, both surfaces papillate along veins, and leaf base is cordate or truncate.	Racemose and axillary/terminal in position. Each inflorescence consists of white or greenish flowers.	Blackish-brown in color, narrowly ovoid in shape, trigonous with grooved surfaces with acute angles rounded below middle.	May to October

Table 1.

Distinct characteristics of different types of buckwheat.

1.2 Origin and distribution

The center of origin of buckwheat is the Himalayan region, which stretches across western China to northern India. It was first cultivated in inland Southeast Asia. From there, it has spread to Central Asia and Tibet, and then to the Middle East and Europe [7] and southwest to North China, then further to the Korean Peninsula and to Japan. In Japan, buckwheat was one of the most important foods since around 800 AD. Buckwheat is believed to have been introduced to Europe around 1200 to 1300 AD particularly in Ukraine, Germany, and Slovenia, which later spread to Belgium, France, Italy, and Britain. According to FAO, in 2017, buckwheat was cultivated in 25 countries with the total cultivated acreage of 3,940,526 ha with the total production was 2,056,585 t. Russia and China have the highest area under buckwheat cultivation [8]. In India, buckwheat is cultivated at high altitude regions of Jammu and Kashmir viz. Sonamarg, Baktour, Kupwara, Machil, Dawar, Nilnag, Gogjipathar Sind Valley, Ladakh and Zanskar, Kargil and Drass sectors, and Gurez Valley and in other states of India namely Himachal Pradesh, Uttarakhand, Sikkim, Arunachal Pradesh, West Bengal, Meghalaya, Assam, Manipur, and Nagaland. In southern parts of India, buckwheat is also cultivated in parts of Nilgiris and Palani hills.

1.3 Nutritional value

The buckwheat seeds are rich in protein, amino acids, and minerals (≈11–12%). The crop possesses higher content of lysine, tryptophan, arginine that are not present in cereals, and it is enriched with polyphenols, sterols, vitamins, and flavonoids (Table 2). It is reported that buckwheat sprouts could be used as a fresh vegetable in salads and be used for various other purposes including natural vegetable juice material. The biological value of buckwheat seed proteins is very high (93), as compared to pork (84), soybean meal (68), and wheat (63). The slowly digested or non-digested starch of buckwheat groats is important to diabetics, as it helps to flatten the glycemic response curve. A slow release of glucose from starch could prolong endurance during physical activities, and the duration of satiety is prolonged as well. The economic importance of buckwheat rests mainly with the high nutritive value of their grains (Table 2). The buckwheat has a long flowering period and thus served as the source of nectar for honey.

Grain or byproduct	Moisture (%)	Protein (%)	Fat (%)	Fiber (%)
Whole grain	10.0	11.2	2.4	10.7
Flour	12.1	7.8	1.5	0.7
Groats	10.6	11.2	2.4	0.6
Hulls	8.0	4.5	0.9	47.6

Table 2.

Nutrient composition of buckwheat grain.

2. Agroclimatic suitability of Buckwheat for hill slopes of EHR

Buckwheat can be grown on a wide range of soil types. But it is best suited to light- and medium-textured soils, such as sandy loam, loam, and silt loam of EHR. Moreover, buckwheat is acid soil-tolerant crop. Generally, the moisture content of the soil in winter season is less due to water scarcity, which can adversely affect the productivity of any crop in this geographically significant bio-diverse region. But buckwheat is a plant which can grow under such adverse moisture stress condition due to its extensive root networking system (Figures 4 and 5). During the moisture-stress conditions, the root of buckwheat go dipper and explore for water vis a vis essential nutrients.

Figure 4.

Buckwheat cultivation at mid-hills of Meghalaya following standardized agronomic practices [2].

Figure 5.

Root architecture, stomatal structure, and energy-dispersive x-ray analysis (EDAX) of high-yielding buckwheat under water stress and non-water stress conditions [2].

Buckwheat is frequently used as catch or emergency crop because it grows rapidly and matures early. For the cultivation of buckwheat, winter months are preferred. It was recorded that when the crop was cultivated in October/November, it gave a potential yield of 9.7 and 11.0 q/ha, respectively (Table 3). As the crop showed potential yield during this period irrespective of adverse moisture stress condition, buckwheat was found to be a promising crop during this period as most of agricultural land in this region becomes fallow due to water scarcity and has low moisture content in the soil. In addition, a fast-growing cover crop such as buckwheat is most useful for reduced chemical or nonchemical weed suppression. Buckwheat will shade and smother weeds or outcompete them for soil moisture and nutrients. Moreover, both living buckwheat plants and buckwheat residues have an allelo-pathic effect on weed germination. Research shows that the allelo-pathic effects of buckwheat can last about 30–60 days. Buckwheat is used not only for human but also for livestock and poultry. The crop is also suitable as green manure. On incorporation, the buckwheat biomass rapidly decomposed in soil and adds N, P and organic matter to the soil. Incorporating buckwheat into the soil improves soil health by enhancing the soil structure of the topsoil, making it more friable and increasing the water infiltration rate.

Sowing time	Plant height (cm)	Duration (day)	Grain yield (q/ha)	Straw yield (q/ha)
October	87.8	142	9.73	10.2
November	54.6	163	11.0	10.2

Table 3.

Performance of buckwheat at hill slopes of Meghalaya.

Buckwheat can be grown on soils poor in nutrients and sandy soils, and on stony fields. It is a low-input crop not sensitive to disease and pest, and there is normally no need for application of insecticide, fungicide, herbicides, and does not require any irrigation for the successful cultivation. The farmers of EHR are basically small and marginal. Therefore, farmers of this region can easily adopt the crop because of low-input cost and get maximum benefit within a small period of time. In EHR, 3923.6 sq km areas is under shifting cultivation. As buckwheat requires less input, abandoned areas due of shifting cultivation can be utilized effectively to grow the crop in large scale in this region. The prevailing agro-ecological condition of EHR region is suitable to producing buckwheat, and therefore, the crop is a potential crop for the farmers of this region. Therefore, buckwheat is becoming a potential contingency crop with substantial stress tolerance. Generally, in the upland conditions of EHR, the moisture content of the soil is more likely to reach its critical level of 7.0–10.0% compared to lowland condition (9.0–23.0%). Cool and moist climate favor the growth of buckwheat. Heavy soil with excessive moisture is not suitable for buckwheat. Higher income from buckwheat cultivation makes the crop more remunerative and is found suitable for preventing soil erosion across hill slopes, restoration of soil fertility as well as better alternative source of green fodder under stressful environments of hill ecosystem. Appropriate technology transfer with needful policy interventions for adequate value addition and marketing might help in popularizing this crop in Eastern Himalayan conditions. Buckwheat could be robust non-cereal crop because it is more productive than other cereals with better nutrient content in the grain and green leaves under abiotic stress conditions of marginal hill environments [9].

In one of the field experiments carried out at mid-hills of Meghalaya during 2014–2015, it was found that the highest mean yield of buckwheat was recorded for the month of sowing in October (9.83 q ha⁻¹) followed by sowing in November (9.45 q ha⁻¹) and December (9.09 q ha⁻¹) [10]. The lowest yield was recorded for sowing in the months of May (1.23 q ha⁻¹) and April (1.32 q ha⁻¹). As buckwheat needs cold and dry weather during maturity, sowing in October to December results in high yield as harvesting coincides with dry weather. High intensity rain in EHR during kharif has resulted in significantly lower yield of buckwheat as compared to yield of the crop sown in winter months. Among the months of sowing, significantly the highest HI was recorded in the month of November (34.0%) followed by December (33.5%) and October (32.6%). Meanwhile, the soil moisture measured was found to be highest (44.0%) during July and reached lowest during the months of January (14.5%) and February (15.1%) (Figure 6). Buckwheat production is influenced by various biotic and abiotic stress factors under fragile hilly ecosystems of Eastern Himalaya. In our study of higher grain production of the crop sown during the months of September to December, it was attributed to increased biomass production, favorable temperate climate, and relatively shorter days during rabbi season and also due to higher partitioning efficiency of the crop, even at the soil moisture status of 14.5–15.1% which is a common phenomenon during winter in the region (Figure 6) [10]. The standardized package of practice followed for cultivation of buckwheat at EHR is presented in Table 4 [11].

Figure 6.

Monthwise soil moisture regime and BW yield at mid-hills of Meghalaya.

Sl No	Type of practice	Description
1	Climate and soil requirements	Buckwheat favors cool and temperate climate and is best suited to light-to-medium textured, well-drained soils such as sandy loamy, loamy, and silt loamy. It shows a higher degree of tolerance to low-fertile acidic soils and low soil moisture conditions, but does not tolerate water logging. In case of water logged or high moisture in low land area, it can be successfully grown on raised beds. It will produce a crop on newly cleared land, drained marshland, or on other marginal land.
2	Field preparation	As the buckwheat has very fine root system, fine tilth is necessary to get an optimum plant stand. Buckwheat can grow well after one plowing or field could be prepared by one deep plowing followed by two harrowing/tilling and planking, which results in good germination and uniform stand of the crop. Being a cover crop, it does not require extensive land preparation and can grow well on poorly tilled soil.
3	Sowing and seed rate	The growing season varied with altitudes and rainfall pattern in EHR of India. The crop can be sown in mid-altitude after harvesting of kharif crops (rice or maize) particularly during the beginning of winter months like September to December. Healthy and disease-free quality seed @ seed rate of 25–30 kg/ha is sufficient for growing buckwheat as a grain crop. Seed rate of about 50 kg/ha is required when buckwheat is grown as a cover crop/fodder crop/vegetable crop. Sowing should be done in rows of 30–35 cm apart and 10–15 cm from plant to plant spacing, depending upon varieties. In buckwheat, higher seed rate is generally used to promote faster canopy development and higher population for better weed control. The seeds should be placed in 3–5 cm depths. Thinning may start at 15–20 days after sowing to maintain optimum population.
4	Nutrient management	Buckwheat generally grown by farmers in residual fertility after harvesting maize or rice in upland areas without adding other nutrients input. However, for getting a good yield, recommended dose of nutrients, that is, 40 kg N, 40 kg P2O5, and 25 kg K per hectare should be applied through organic manure. FYM @ 5 ton along with 1 ton VC, 150 kg oilcake, and 150 kg rock phosphate is recommended for growing buckwheat in one-hectare area. Application of bio-fertilizers such as Azospirillum spp. and Azotobacter spp. also resulted in improving buckwheat productivity and soil fertility.
5	Water management	Generally buckwheat is grown in hill slopes as a rainfed crop. If there is severe water stress, irrigation should be given at critical stages of crop growth in buckwheat like pre-flowering and fruit formation stage.
6	Intercultural operations and weeding	Although buckwheat plants are very good competitors for weeds and generally their fast-growing capacity make them a smother crop. Under such conditions, one weeding and hoeing at 20–25 days after sowing is helpful for raising a good crop.
7	Pests and diseases	Buckwheat is not attacked heavily by diseases or pests. However, some diseases like leaf spot (Septoria polygonicola), root and stem rot (Phytophthora jagopyri), powdery mildew (Erysiphe polygoni), rust (Puccinia jagopyri), etc., were observed. Selection of seeds from disease-free plants is a pre-requisite to prevent the diseases. Seed treatment using Trichoderma viride at the rate of 4 gm/kg of seeds and soil application of Trichoderma viride@ 2.5 kg mixed with 50 kg sand or well-rotten FYM is recommended to control the diseases.; Major pests of buckwheat includes bruchids (Acanthe celidsobtectus), grain moth (Cephitinea sp.), cut worm (Cirphis spp.), and storage beetles (Mycetophagus spp.). Bird damage, particularly by doves, has been observed in this crop. Prevention is the best strategy to avoid insect problems in storage of buckwheat grains. Proper drying of grains is necessary to reduce infestation. In case of heavy infestation, neem oil (1500 ppm) @ 3 ml/liter water can be sprayed for effective management of pests.
8	Harvesting	To avoid shattering, the crop should be harvested when 75% of the grains turn brown or dull gray in color. Being of indeterminate growth habits, the plant shows irregular time of maturity. If the harvesting is delayed, shattering will start which may cause huge loss. After harvest, the crop should be allowed to dry for 2–3 days and then threshed. The seeds must be well-dried and kept at about 14% or less moisture for the safe storage of buckwheat grains.

Table 4.

Standardized agronomic package of practices for cultivation of buckwheat at EHR.

3. Increased P efficiency of Buckwheat suits to acid soils of EHR

Phosphorus (P) is a major soil nutrient limiting crop production in acid soils of EHR because of its high P fixation and its very low diffusion coefficient reducing soil solution P concentration to less than plant absorption thresholds. When phosphate fertilizer is applied in soil, adsorption and precipitation processes potentially result in precipitating cation concentrations in soil solution, leading generally into sesqui-oxides, following the dissolution of clay minerals and the release of Al ions (Al³⁺) in acidic soils (pH<5.5), and hydroxyl-apatite in high pH and alkaline (pH>7.3) soils. About 99% of P absorbed by plants is primarily buffered phosphates into soil solution from adsorption sites of mineral and organic complexes. This means that almost all plant P uptake comes from the soluble soil Pi pool, suggesting that Po must first be converted to the Pi forms for plant uptake. In addition to white lupin, buckwheat (BW) has been classified as P uptake efficient [12]. Even when total soil P may be high, >80% still exists in forms that are unavailable to plants, with inorganic phosphorus (Pi) in most top soils between 25 and 75% and organic P (P_o) within the same range (Figure 6). Various mechanisms are involved in the efficient P uptake by plants such as white lupin (Lupinus albus L.) and buckwheat under P stress including changes in plant root morphology and plant physiology, whereby root exudates, for example, flavonoids by white lupin and low molecular weight organic acids (LMWOAs) by buckwheat are released [13] to hydrolyze a range of Pi and Po compounds in soil (Figure 7).

Figure 7.

Fate of added phosphorus in acid soil. P fixed either through adsorption, immobilization, and precipitation in low pH soils of EHR.

In the study conducted by Jasper and David [14], it was shown that Buckwheat (BW) could solubilize phosphorus (P) from fixed forms of soil P to subsequent crops. Calcium-bound P could solubilize more to enhance the free P (72% of inorganic pool) to the available fraction, and P uptake by BW (40 kg ha⁻¹) was remarkably higher than wheat (WHT) (16 kg ha⁻¹) from the inorganic pools. Following the cultivation, more P was added to available P pools after BW was compared to WHT, suggesting potential solubilization of P to subsequent crops. Buckwheat is often called a P scavenger because it can solubilize and take up soil P more efficiently than other plants. In its growing stage, the roots of buckwheat exude substances that help to solubilize P to the tune of 35.0-50.0 microgram per plant that may otherwise be unavailable to plants. The roots of buckwheat were also found to have a high storage capacity for inorganic P. As a result, when buckwheat plants are incorporated in the soil, they decay quickly, making phosphorus and other nutrients available to the succeeding crop [14].

The ability of buckwheat (F esculentum) roots to acquire P was characterized by morphological features and chemical changes in the rhizosphere (Figures 8 and 9). Jasper and David [14] have shown that higher shoot growth was achieved with a P supply between 5 and 100 μmol/liter. Root biomass and root length with increased root hairs was found at low P levels. Root exudates of low-P plants have relatively lower pH values than exudates of high-P plants, and thereby they increased the solubility of FePO₄ and MnO₂ to a greater extent. Enhanced hydrolysis of glucose-6-phosphate by exudates from low-P plants was due to an increased “soluble” acid phosphatase activity (Figure 9) which was commonly enhanced with P deficiency. In the rhizosphere soil of buckwheat, some depletion of organic P forms was also observed (Figure 9).

Figure 8.

Effects of root exudate components on nutrient availability and uptake by plants and rhizosphere microbes. OA = organic acids; AA = amino acids including phytosiderophores, Phe = phenolic compounds.

Figure 9.

P uptake by intact roots in acid soil. The ability of the roots to excrete organic acid is associated with several physiological mechanisms under low pH soils of Meghalaya.

The mechanisms conferring P efficiency have to be associated with either the acquisition of P nutrient from the environment or the movement and distribution within the plant or the utilization in metabolism. Efficient acquisition of P under insufficient supply like acid soils of EHR may be due to a higher ability of the buckwheat crop to explore the soil by a more extensive root system (with rapid development, higher root-to-shoot ratio, finer and longer roots and root hairs), or a greater ability to absorb P from a dilute solution (i.e., an efficient uptake mechanism) [2, 3, 10]. Exudation of reducing, chelating, and/or acidifying substances by the roots, resulting in solubilization of soil P, to the extent of 35.0-50.0 microgram per plant may also be of great importance.

Further as buckwheat is known for P solubilization, Aliyeh et al. (2018) have conducted extensive field studies to explore its potential in intercropping production systems. Intercropping of buckwheat can enhance the productivity of cropping systems through increased soil nutrient availability and thereby increasing plant nutrient use efficiency. One 2-year field study with different intercropping ratios of fenugreek and buckwheat and fertilizer types on nitrogen (N) and phosphorus (P) concentrations was undertaken at the research farm of Shahrekord University, Iran. The experiment comprised sole cropping of fenugreek (F), buckwheat (B), and three intercropping ratios fenugreek and buckwheat of 1:2, 1:1, and 2:1 under three fertilizer types: chemical fertilizer (CF), integrated fertilizer (IF), and broiler litter (BL). The results revealed that intercropping substantially increased total aboveground dry matter, total seed yield, N and P tissue content as well as their uptake. Efficiency of applied N use and applied N recovery in the intercropped plots was also higher as compared to the sole cropping. The intercropping ratio of F:B (2:1) emanated as the most suitable for improving nutrient use efficiency. The IF and BL have shown remarkable benefits in terms of increased total seed yield, tissue N and P contents, and their uptake in sole and intercrops. The intercropping of fenugreek–buckwheat in the ratio of 2:1 with the application of integrated fertilizer and broiled litter were emerged as promising for improving productivity under semiarid growing conditions of Iran or similar ecosystems elsewhere [15].

4. Buckwheat is potential moisture stress tolerant crop of EHR

Buckwheat is commonly cultivated under marginal and degraded lands of EHR with poor nutrients, lesser moisture regimes, and low-input agriculture [2]. Since moderate-to-severe moisture stress prevails during post-kharif season in the region, buckwheat cultivation is subjected to varied levels of moisture stress and invariable soil acidity that impacts at root and shoot level [2, 3]. Since crop growth largely depends on the ability of roots to acquire essential water and nutrients from the rhizosphere, the impediments at root system by toxic elements (Al and Fe) hamper the ability of the roots to support highly metabolizing shoot system especially under low soil moisture stress condition [16]. Under short-term water stress, plants increase their water use efficiency (WUE) by reducing stomatal aperture and thereby reducing transpiration rate; however, under conditions of prolonged water deficit, plants frequently produce leaves with reduced stomatal conductance resulting from altered stomatal density (SD) and size [17]. Under continued moisture stress, crop adaptability manifest to the extent of change in root growth, stomatal structure, and cuticle synthesis as major means of stomatal and nonstomatal barrier for reduced water loss through plant leaves [18]. Possible changes in root and shoot growth alterations, stomatal characteristics, and leaf surface features would be remarkably varied under lowest moisture stress levels of 10–15% that could be reached during winter months of Meghalaya. Since the root system is slowly explored as the key source for moisture stress tolerance and adaptation [19] with an array of optimum root traits or phenes, plant ability to alter the root system would increase the stress tolerance substantially. Buckwheat has unique advantage for modifying root in accordance with prevailing soil water status under soil moisture constraints as evident in our study indicating the versatile crop adaptability in terms of altering root plasticity [2, 10].

Differential and incremental increase in the leaf pigments especially protective pigments having antioxidant capacity such as chlorophyll b and carotenoids that are found as a suitable rescue system for the crop to thrive and impart stress adaptability under moisture stress conditions for balanced light absorption and reduced photo-inhibition at higher altitudes of Eastern Himalaya [20]. Chlorophyll a to b ratio decreased under moisture stress (3.07) compared to 5.5 under controlled conditions, substantiating the higher accumulation of secondary pigment chlorophyll b by buckwheat. Increased chlorophyll b and carotenoid content by buckwheat could be useful in two major photoprotection systems, viz., (i) as antioxidant which can scavenge free radicals generated by excess solar radiation energy and (ii) through NPQ enhancement to emit excess solar radiation energy [3, 10]. Regulation of photosynthetic apparatus by inducing necessary changes in leaf pigments under fluctuating light conditions under hilly locations of Eastern Himalaya is important for onset of appropriate physiological mechanism and adaptation in stress-resilient crops like buckwheat [10, 18].

Stomatal index has reduced significantly both in abaxial and adaxial surfaces to the extent of 72.6 and 55.1% under moisture stress conditions compared to control [3]. The high-resolution SEM images show that under moisture stress, the stomatal size significantly decreased or closed partially in abaxial surface, whereas under controlled conditions stomatal aperture remains open to a greater extent (Figure 5). Gas exchange is regulated by controlling the stomatal aperture and density on the epidermis [21]. Stomatal size and stomatal density were considerably influenced by plant species and abiotic environmental perturbations such as changes in atmospheric CO₂ concentration, light intensity, temperature, soil water, and nutritional status [22]. Besides stomatal control, increased cuticle synthesis and accumulation are major means of non-stomatal regulation or barrier for free water loss through plant leaves that is a prime adaptive mechanism, which is able to amply prevent water loss under moisture stress conditions [23].

4.1 Genetic variability for physio-morphological and yield traits in Buckwheat under acid soils of Meghalaya

Another field experiment conducted during 2021–2022 with 44no of buckwheat germplasm under native acid soils of hill slopes in Meghalaya revealed significant genetic variability among buckwheat genotypes for various physio-morphological and yield traits. The levels of leaf carotenoids were found higher in high yielders to the tune of 32.6 and 27.2% compared to moderate and low yielders, respectively. High yielders have shown reduced chl a/b ratio compared to moderate and low yielders possibly owing to their increased chl b synthesis, which is known to act as both stress pigment and antioxidant [2, 3]. Total root length (TRL) and root surface area (RSA) had the range of 18.7–123.3 cm and 5.69–48.0 cm² per plant. High yielders have reduced TRL than moderate and low yielders to the tune of 30.9 and 28.2% implying that high yielders possibly harbor other rescuing physiological mechanisms under acidic soil. Shoot length and number of leaves had mean values of 32.7 cm and 21.8 and within the range of 12.5–50.3 cm and 11 to 38.7, respectively. High yielders have shown increased shoot length and more number of leaves to the tune of 30.8 and 32.7% and 52.2 and 76.3% as compared to moderate and low yielders. Seed yield in high yielders was found higher to the tune of 163 and 688%, respectively. High yielders viz., IC377275 (18.97 qt/ha), IC26591 (17.1 qt/ha), Shimla (16.4 qt/ha), IC14890 (16.32 qt/ha), IC37288 (15.5 qt/ha), and Himapriya (15.27 qt/ha) were emerged as promising buckwheat genotypes for cultivation in acid soils of Meghalaya (Unpublished data).

5. Possible response of Buckwheat for combined acid soil and moisture stress (drought stress) conditions of EHR

Abiotic stresses commonly prevailing in fragile ecosystems of EHR such as drought, heat, and soil acidity could potentially cause extensive losses to hill agricultural productivity and thereby impact food security. Under changing climate, resource-poor farmers are exposed to a range of challenges such as variability in weather patterns, soil acidity that manifested low soil fertility with nutrient depletion which might act solely or in combination to increasingly limit potential crop yield. As a result, crop plants are needed to be tolerant of both drought and acid soil stress as Asia has the second largest area of acid soils in the world with distinct interactions with seasonal drought across many countries [24]. Under such conditions, crop growth largely depends on the ability of roots to explore the soil for absorption of water and nutrients. To enhance cropping intensity and crop productivity development of the crops with resilient root system like buckwheat would remarkably enhance crop yields under low-moisture of acid soil conditions of hill slopes of EHR [25, 26].

Even though the use of lime, phosphate fertilizers, organic matter, and irrigation are highly productive in acid soils, as evident elsewhere, liming is not a realistic alternative economically in EHR because of geographical hill and logistic terrains, unfavorable climate, and due to the high cost for low-input resource-poor farmers. Extensive utilization of external inputs might cause undesirable side effects under fragile ecosystems practicing sustainable organic farming that would substantially threaten the environment. Moreover, liming can raise soil pH transiently and overcome toxicity problems only in the surface soil leaving the subsoil usually unaffected and deeper incorporation of lime is technically difficult and expensive. Therefore, holistic understanding of plant response to individual and combined stress factors are important in mountain ecosystems as they offer window of opportunities for novel crop improvement for enhanced crop adaptability and food security. As Al toxicity is the most important factor limiting plant growth on acid mineral soils, plant roots undergo rapid inhibition and damage that limits uptake of nutrients and water.

Root elongation is reduced under water stress with two distinct differences between acid soil Al stress and drought stress: (i) Under drought stress, shoot growth is affected more than root growth, whereas short- and medium-term acid soil Al strongly reduce root growth without affecting shoot growth [27]; (ii) Al toxicity reduces cell elongation along the entire elongation zone [28], whereas under water-deficit stress only basal and central elongation zones growth is inhibited, but maintained toward the distal and apical elongation zones. The maintenance of root elongation in the root apex is accomplished by some physiological mechanisms such as osmotic adjustment, alteration in cell-wall extension, and deposition of abscisic acid (ABA) [29]. The different response of root elongation under acid soil Al and drought stress in climate-resilient crop like buckwheat appears to give ecological advantage because of its versatile fibrous roots [2, 3]. Reduced inhibition of root growth by Al in the Al-toxic subsoil allows buckwheat to forage more nutrient-rich surface soil efficiently for nutrients and water, while the relative maintenance of root growth under drought allows the roots to grow into the subsoil for improved foraging for deficient nutrients like phosphorus [2, 30]. However, under field conditions, how declining soil moisture increased mechanical impedance to be overcome by resilient crops like buckwheat having potential to release organic acids need to be studied.

6. Conclusions

Specific stress-tolerant traits harbored by buckwheat substantially emulate it as suitable crop for higher grain productivity during stressful conditions of winter months (mid-September to October) in EHR but found better for producing higher green biomass all through the year. Root morphological traits, root exudation, stomatal attributes, and photo-protective pigments were significantly enhanced and emanated as efficient stress rescue mechanisms in buckwheat to overcome inimical moisture stress and soil acidity conditions of hill slopes in EHR under changing climate. In view of its increased resilience and sustained productivity by identified high-yielding genotypes viz. IC377275 (18.97q ha⁻¹), IC26591 (17.1 qt ha⁻¹), IC14890 (16.32q ha⁻¹), and Himapriya (15.27q ha⁻¹) under stressful and marginal hill environments, Buckwheat was found to be potential stress-resilient and remunerative crop for increased cropping intensity and food security with prevailing low-input agriculture in EHR.

Acknowledgments

Authors thankfully acknowledge the director, Indian Council Agricultural Research (ICAR) for NEH region, Umiam, Meghalaya, for rendering constant guidance and support to carry out this research work on buckwheat. We thank our technical assistant and supporting staff Ms. Claribel Christy Iawim and Akash Kharumlong for their tireless field and laboratory work under the project.