Hydroponic Saffron Cultivation and the Effects of Soil Salinity
This report explores the hydroponic cultivation of saffron (Crocus sativa L.). In particular, it explores how hydroponic cultivation affects various desirable qualities of the plant, including aroma, flavor, and longevity. Results are compared to conventional soil cultivation, with particular attention devoted to the effect of soil salinity upon formative aspects of the crop. Each of these twin concerns — hydroponic cultivation and sensitivity to environmental salinity stresses — is examined from the perspective of recent botanical investigations conducted by Milan Kordestani. The purpose, conduct, and findings of each of these concerns is notated per the standard taxonomy that offers an introduction, materials and methods, experimental results, and a detailed explanatory rationale.
Hydroponic Saffron Cultivation
Chapter 1. Introduction
Saffron is one of the world’s most expensive spice cultivars, and its husbandry is both difficult and time-consuming. Saffron is a triploid plant, and its violet flowers are incapable of producing seeds (Fig. 1). As a result, saffron, per se, is infertile, meaning propagation is strictly via daughter corms from the parent plant. In addition to the large number of pathogenic fungi and viruses that readily transfer between seasons when planting, harvesting of the red stigmas that are dried to create commercially salable saffron is extremely meticulous work. Consequently, only 6 kg of saffron can typically be derived from a hectare’s worth of plantings (Souret & Weathers 27). This is the case even when the electrolytes in the soil are titrated to provide the apparently optimal electrical conductivity (EC) of 1.0.
The delicacy of saffron makes it an attractive candidate for indoor growth. Although the plant has been grown commercially for centuries in the nations of Spain, Iran, and India — which feature a warm, dry climate and cheap labor concomitant with the extraordinary processing requirements of saffron harvesting and preparation for commercial sale — indoor and outdoor cultivation by hydroponic, or even aeroponic, means have been suggested as a means of combating the variable aridity and salinity of soils in these lands. The availability of saffron bulbs at the appropriate time of year makes the spice relatively straightforward to grow in greenhouse conditions. Yet, although the bulbs occupy little space, yields are typically disappointing. The commonly used figure of merit is that two pounds of fresh flowers yields less than one-half ounce of commercially viable saffron spice. Moreover, the average saffron corm which produces from one to three flowers, lowers per season, requiring harvesting of the bulb itself every one-to-two years so as to detach its new extremities for individual planting. Consequently, the yield of saffron per square foot is among the lowest afforded by any soil grown cultivar (Morgan).
Saffron culture without soil by the use of hydroponics has permitted the plant to thrive under conditions in which normal cultivation is either difficult or impossible. The use of hydroponics permits the growth environment and nutrition of the growing plants to be controlled most carefully, engendering better quality and higher yield of the resulting crop. Hydroponic cultivation is achieved by maintaining the roots of the plant in either a static nutrient solution that is continually oxygenated or a sporadically flowing nutrient solution (Fig. 2; Souret & Weathers 27).
Commonly, nutrient cultures for hydroponically growing saffron are not sterile insofar as they are not based upon distilled water. Nevertheless, the water should be autoclaved before use to reduce the number of microbial flora. Saffron grows best in water that is maintained at a pH between 6.0 and 6.25, that is, slightly acidic. Within the soilless hydroponic context, the individual saffron corms are planted in two-inch plastic mesh pots that are filled with an inert substrate (Souret & Weathers 28).
Chapter 2 Materials and Methods
Hydroponic cultivation requires a microfiber sponge that comprises four channels; a catchment pipe; a catchment tank; and four pipes that support inlet flow (Fig. 2). The pipes are commonly constructed of polyvinyl chloride (PVC) plastic that features equally spaced holes to mate with the aforementioned mesh pots. The catchment pipe is identical in form to the channels. The nutrient liquid is continually aerated and, stored in the catchment tank, is circulated through the channels and discharged through the catchment pipe. The entire system is placed outdoors to receive direct sunlight.
Chapter 2.1 Results
The biological success of the endeavor can be determined by examining the growth rates of the shoots and roots, the initial and final bulb weights, the sizes and weights of the stigmata, and the patterns of blooming. The commercial viability of the resultant saffron was gauged by grinding the stigmas and mixing them with ethyl alcohol, creating a solution that could facilely be subjected to thin-layer chromatographic analysis that separately measures levels of crocin, picrocrocin, and crocetin. Safranal, a fourth component of saffron stigmas, is difficult to measure by thin-layer chromatography because of its extreme volatility (Souret & Weathers 28).
Saffron plants cultivated hydroponically for six weeks produce corms that exhibit significantly less fresh weight increase as compared to plants grown in soil. However, the dry weight of hydroponically grown corms is significantly greater than that of soil-grown corms. In general, the biomass is eighty percent less in the hydroponically grown plants. Maximum root length is also significantly curtailed in the hydroponic scheme. Moreover, the roots are thicker and curlier, and a greater preponderance of contractile roots is observed. This may be because the roots of hydroponically cultured plants grow in direct contact with the plastic channel.
The number of shoots and leaves per corm does not differ dramatically between hydroponic and conventional soil cultivation. However, bulbs grown in soil tend to produce leaves that are thirty percent longer than those grown hydroponically. The leaves also tended to emerge several days earlier than otherwise expected within the hydroponic setting. Yet, the percentage of bulbs that developed into commercially viable flowers was much less in the hydroponic setting than in conventional soil (Souret & Weathers 29).
A comparison of pigments and stigmas demonstrates that the plant tissues are both morphologically and biochemically similar to stigmas that are harvested from soil cultures and, furthermore, with prepared commercial saffron. In particular, crocin and crocetin concentrations are seen to be greater in hydroponically grown saffron bulbs. By way of contrast, picrocin concentration is significantly greater in plants cultured in soil. Safranal is present in very low concentration within hydroponically cultured bulbs, but this may be a result of its volatilization during the stigma drying process (Souret & Weathers 29).
Chapter 3. Discussion
It is seen that many factors influence the flowering of the bulbous plants. These include temperature, humidity, storage time, bulb size, and light intensity. For saffron in particular, it is noted that temperature; humidity during storage; and bulb size and weight are the most critical predictors of successful flowering. If the corms can be planted in the ground between their first and second seasons, flowering within the hydroponic setting will be markedly greater. Moist storage at cool temperatures is also seen to help.
The decrease in root length evidenced in plants that grow hydroponically is similar to a phenomenon seen in the hydroponic cultivation of beans. The root length decrease in saffron could be due to the amount of light that reached the corms (Fig. 3). Light has an inhibitory influence on root growth, and saffron should be kept in darker surroundings if possible. It is thought that hypoxia leads to directly to this suppression of root growth. The problem caused by decreased root length is its deleterious effect upon uptake of water and nutrients from the circulating liquid.
Hydroponically grown saffron also exhibits an unusual morphology of its roots. This may well be related to mechanical resistance resulting from the plastic tubing. Roots whose growth have been mechanically impeded are seen to be shorter and thicker than soil-grown roots and more frequently exhibit irregular or even bizarre shapes. This impedance doubtless results from the NFT channel in the hydroponic setting (Souret & Weathers 31).
The pattern of formation of contractile roots within hydroponically cultivated saffron correlates with previous research efforts (Halevy 97) that determined an inverse relationship between root length and planting depth in gladiolus flowers and further examined the effect of incident light intensity. A wide variety of bulbs and cormous geophytes exhibit underground movement, typically achieved by the formation of contractile roots, in order to reach the desired planting depth. It is clear that the formation of contractile roots in saffron plays a critical role in corm lowering.
It has also been demonstrated that the rate at which the saffron plant shoots emerge is inversely related to planting depth (Negbi 107). Leaf elongation is also seen directly to relate to planting depth. In the hydroponic system, where planting depth is obviously shallow as compared to soil planting depth, the shoots appear earlier. Nevertheless, no significant difference is evidenced in the number of shoots or leaves that are produced per bulb.
When commercially grown in soil culture, saffron corms are expected to achieve a flowering rate of twelve percent. By way of contrast, hydroponically grown corms only blossomed four percent of the time (Fig. 4). The precise mechanisms that underlie the decrease in flowering rate are unknown. However, one may assume that significant effects result from the radical environmental difference between soilless cultivation and soil culture. Nevertheless, the lesser quantity of stigmas obtained via hydroponic cultivation were found to exhibit a hard, brilliant color and to be of very select grade as far as commercial buyers are concerned (Souret & Weathers 32).
Assuming that the corms used for propagation are sufficiently large, results suggest that hydroponic cultivation in nutrient can produce high-quality saffron. Yet, further studies are required to address the issue of maximization of flowering so as to optimize saffron production within such carefully controllable environments.
Effects of Soil Salinity
Chapter 4. Introduction
Hydroponic cultivation can address the issue of water scarcity that plagues many regions where saffron is typically grown, such as Iran. However, salinity is another critically important factor involved in the flourishing of the saffron plant and its attendant commercial productivity. Salinity and water stresses combine to reduce water uptake through roots. Within irrigated soils, particularly within arid growth regions, the intensity of both salinity and water stress is variable. In arid conditions in particular, evapotranspiration depletes the water content of the soil and therefore reduces the matric potential and osmotic head of the soil. All of these factors serve directly to curtail water uptake by the roots.
The effect of salinity stress and the concomitant effect of potassium electrolytes on biomass accumulation in the roots and shoots of saffron were examined. I determined that salinity bore a significant effect on a wide array of growth characteristics. These included dry weight, leaf count, root density, and root volume. Moreover, it was noted that potassium content was able significantly to moderate the negative effects of NaCl. In particular, at various concentrations of NaCl, adding K to the root medium directly increased the number of roots per saffron plant. However, above certain excessive concentrations of NaCl, potassium titration bore no ameliorative effects. Overall, it was demonstrated that both relative water content and electrolyte leakage can be decreased by increasing the concentration of NaCl. I found that the best growth parameters and ratios of roots to shoots were found when the concentration of NaCl was 0.03M.
Chapter 5 Materials and Methods
Eighteen jars were prepared for the cultivation of saffron corms and were titrated to salinities ranging from zero to 0.12M. The texture of the chosen soils was improved by sufficient intermixture of animal manure, and the base of each pot was covered with netting in order to afford improved drainage. At the end of the flowering stage, the plants were harvested and the fresh weights of flowers in general and stigmas in specific were individually measured.
Chapter 5.1 Results
I found that the maximum number of days taken to germinate was not significantly affected by the salinity. However, the number of days taken to flower directly decreased as the salinity level of the soil increased. With the most highly saline solutions, flowering was up to fifty percent faster. They also determined that the number of plants per pot was unrelated to the level of salinity (Fig. 5). However, the fresh weights of the saffron flowers — defined as the weight of stigma, stamens, and corolla collectively — did vary significantly with salinity. The least saline solution demonstrated nearly an eighty percent increase in gross flower weight over the most saline. In terms of dry flower weight, those solutions with intermediate salinity — ranging from 0.05 to 0.09 mm — demonstrated dry weight increases of more than eighty percent with respect to the control sample. All told, salinity was negatively correlated with fresh weight of the flowers and fresh weight of their commercially important stigmas.
Chapter 6. Discussion
Since saffron can only propagate via corms, any improvement in the quantity and quality of crops must result from concomitant changes to the corms. It is critical, then, to understand the roles imposed by various environmental stresses, including nutritional factors, and their effects upon the enzymatic and metabolic properties of the corm (Keyhani et al.). It was clear that the highest and very lowest salinity levels resulted in delays in the germination of corms. It is known that chloride is an essential nutrient for plant growth, and its suppression deleteriously influenced the longitudinal growth of the roots.
Under normal field conditions, chloride deficiency is a rare occurrence. When chloride is highly concentrated in the root zone, its effects include competition with nitrate in uptake; balance of regulation of cations and anions; and the specific metabolism of organic acids (Khold & Eslam-Zade 495). Since chloride is an active osmotic ingredient in the vacuoles of plant cells, the increase of external osmotic pressure causes a disorder in osmotic regulation within the plant cells and high levels of both sodium and chloride accrue.
The number of days for emergence was similar to the number of days required for germination. This means that an increase in salinity level decreased the number of days required for emergence. This is explained by the fact that salt, which exists in water in ionic form, is healthy for plants up to a certain amount. It is during the absorption and transpiration of water that the plants take up these ions. Chloride aids with overall metabolism, while sodium is required to regenerate phosphoenol pyruvate in such CAM and C4 plants as saffron.
I also observed that increasing salinity tended to decrease the resultant height of the plants except at the 0.02 mm salinity titration. In fact, plants grown in 0.02 mm soil were significantly taller than their fellows. Although soil salinity reduced both the heights of the plants and the diameters of the safflower stems, it accelerated both flowering and maturation. It appears that acceleration of flowering served to alleviate environmental stress upon the plants (Torbaghan et al. 426). Osmotic problems that occur under salinity stress were not as expected, however. Genetic drought resistance mechanisms have arisen over the past century within saffron plants in Iran. Three distinct mechanisms of resistance include drought escape, drought avoidance, and drought tolerance. The drought avoidance particularly seen in saffron explains the ability of the plant to complete its life-cycle before the water deficit becomes too pronounced. The mechanisms include fast phonological development; flexibility of growth, often irrespective of predicted seasonality; and transition of pre-flowering photosynthesis.
The fresh weight of the flower was seen to increase with increasing soil salinity up to intolerably high concentrations in the range of 0.1 mm and greater. The most important parameter — that of stigma yield — was also evidenced with increasing soil salinity, although this trend reversed when the concentration exceeded 0.05 mm. In general, stigma yield was seen to be far more sensitive to salinity stress than was simple flower yield. It is seen, in general, that saffron is highly resistant to salinity, albeit bulb production decreases with rising salinity.
Differing salinity levels were seen to have variable effects upon different organs and systems within the saffron plant. The growth rate of both fresh and dried mass of all parts of the plant decreases dramatically with increasing salinity. Moreover, the most sharply affected part of the plant by increasing salinity are the roots. Increasing the salinity beyond 0.1 mm tended to decrease the relative water content of the resulting flowers. Moreover, both the rate of flowering and the size of the resultant flowers decreased dramatically when salinity exceeded 0.1 mm. While the roots exhibited the most sensitivity, the corm and the leaf were perceptibly less sensitive to these environmentally imposed salinity stresses
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