EFFECT OF SALINITY ON GROWTH AND LEAF YIELD
OF SPIDERPLANT (Cleome Gynandra L.)

GN Mwai¹, JC Onyango¹ and MOA Onyango²

ABSTRACT

This study was conducted to investigate the effect of salinity (NaCl) on growth and leaf yield of spiderplant. The experiment was laid out in a completely randomised design (CRD) with four replicates. Seeds of spiderplant (Cleome gynandra L.) were sown in soil-filled 20-litre plastic pots.  The pots were irrigated with tap water for up to four weeks after germination. The plants were then subjected to five levels of salinity by irrigating the pots daily with salt solutions of concentrations: 0 mol/kg (control), 0.07 mol/kg, 0.13 mol/kg, 0.20 mol/kg and 0.26 mol/kg. These solutions were made to exert osmotic potentials of 0 MPa (control), -0.3 MPa, -0.6 MPa, -0.9 MPa and -1.2 MPa, respectively, in the rooting medium. Data on leaf and general plant growth parameters were collected weekly, including fresh and oven-dried weight of the leaves, chlorophyll content, leaf number, leaf weight ratio, root/shoot ratio and days to 50% flowering.

Key Words: Salinity, Growth, Leaf yield, Spiderplant

FRENCH

EFFETS DE LA SALINITE SUR LA CROISSANCE ET LA PRODUCTION DES FEUILLES DU CHLOROPHYTUM CHEVELU ‘Vittatum' (Cleome gynandra L.)

NOTE DE SYNTHESE

Les semences du chlorophytum  (Cleome gynandra L.) sont plantées dans des pots en plastic d'une capacité de 20 litres remplis de sol. L'étude a pour objet d'examiner l'effet du sel sur la croissance et le rendement en feuilles du chlorophytum.  L'expérience est réalisée de manière complètement randomisée sur quatre échantillons semblables. Les pots sont arrosés avec de l'eau courante pendant quatre semaines après la germination.  Les plants sont ensuite soumis à cinq niveaux de salinité en arrosant les pots chaque jour avec des concentrations de solutions salées de: 0 mol/kg (échantillon témoin), 0,07 mol/kg, 0,13 mol/kg, 0,20 mol/kg et 0,26 mol/kg.  Ces solutions sont appliquées afin d'exercer des potentiels osmotiques dans le milieu d'enracinement, de respectivement 0 MPa (échantillon témoin), -0,3 MPa, -0,6 MPa, -0.9,MPa et–1,2 MPa. Des données sur les paramètres de croissance des feuilles et de la plante en générale sont recueillies chaque semaine, notamment sur le poids des feuilles fraîches et séchées au four, la teneur en chlorophylle, le nombre de feuilles, le ratio poids/feuilles, le rapport racine/pousse et le nombre de jours pour une floraison de 50%.

Les résultats indiquent que la salinité diminue considérablement la croissance générale de la plante, la teneur en chlorophylle de la feuille, la croissance et le rendement en feuilles et retarde la floraison. Ceci indique que le chlorophytum dispose d'une faible capacité à assurer la régulation de l'entrée, la translocation et la compartimentation du sel. Ainsi, cela permet l'absorption et la translocation de larges quantités de sel vers les pousses et les feuilles.  Ainsi, ce sel retardera ou freinera la division et la multiplication des cellules et réduira les taux photosynthétiques et accroîtra les taux de respiration. L'on observe une mortalité des racines qui est attribuée à l'effet indirect de la détérioration de la structure du sol due à la présence de concentrations élevées d'ions de sodium. Toutefois, les espèces peuvent survivre et continuer à pousser dans des conditions de stress modéré du sel.  Par exemple, le stress du sel de –0,3 MPa n'affecte pas la croissance de ces  plantes.  En outre, ces espèces de plantes peuvent survivre, pousser et se reproduire lorsqu'elles sont soumises à un stress de sel allant jusqu'à –0,9 MPa, bien que cela soit à des taux retardés, en raison de leur capacité d'ajustement osmotique. L'étude a conclu que même si le chlorophytum est sensible au sel, sa capacité observée pour l'ajustement osmotique constitue une caractéristique prometteuse, qui peut constituer, après des études plus approfondies, un critère de sélection et d'amélioration génétique des cultivars de chlorophytum.

Mots-clés: Salinité; Croissance; Rendement en feuilles; chlorophytum.

INTRODUCTION

“Salt stress” is defined as the presence of excess amounts of salt in the soil, in concentrations sufficiently high to lower the chemical potential of soil water by between 0.05 and 0.1 MPa [1]. Soil salinization is the accumulation of soluble salts in the rhizosphere to concentrations high enough to affect plant growth and development [2, 3]. Halomorphic/ salinized soils are those whose properties are controlled by the presence of excess soluble salts (mainly chlorides, sulfates, carbonates and bicarbonates of sodium, potassium, calcium and magnesium) in the ion exchange complex [4]. When soils contain high amounts of soluble salts, they develop unfavorable characters which hamper cultivation operations and decrease plant growth, eventually becoming unproductive and unmanageable [3, 4].

The degree of soil salinization is an important determinant of whether a soil is agriculturally productive or not especially in irrigated lands, coastal areas, and arid and semi arid regions [5]. An understanding of the physiology of salt tolerance in agricultural plants is therefore essential for an effective approach to solving the problem of salinity in agriculture [6]. The degree to which a soil is salinized is a function of several factors including the period of time over which the salts have been deposited, frequency of deposition, salt content of ground water, chemical composition, and permeability of the underlying parent rock [2, 3].

The effects of salinity on plants have been classified into osmotic, nutritional and ion toxicity. The presence of high salt concentrations in the rooting medium has the primary effect of lowering the osmotic potential of soil water, thereby making soil water unavailable to the plant. As a result of reduced water uptake, a physiological drought develops in the plant, causing loss of turgor and cessation of growth [7]. If the stress is severe enough, actual death of plant tissues results as necrotic spots, marginal burns, falling of leaves, or death of the whole plant. Reduced leaf expansion, lower photosynthetic rates, raised CO₂ compensation points, reduced stomatal conductance, and altered patterns of respiration, metabolism and partitioning of assimilates are other physiological effects of salt stress. Ultimately, these effects are manifested as reduced biomass production and yield [8]. In addition to the above effects on plants, high soil salinity affects plant growth indirectly through the effects of the salts on the soil itself, usually marked by structural deterioration of the soil, referred to as deflocculation [9-11].

Most salinity effects have been observed to increase with increasing soil salinity [12].  The reduction in growth and yield of plants subjected to salinity is, therefore, quantitatively related to the concentration of salts in the growth medium [1]. The growth inhibition may or may not be reversible depending on such factors as the salt resistance of the species/variety, age and salt concentration [13].

African indigenous food crops constitute a complement of crops that are both nutritionally rich and well adapted to the region. However, these crops have in the past been ignored by policy makers, researchers and farmers as possible solutions to food insecurity problems in Africa. Considering the important role of leafy vegetables in the African diet and as sources of micronutrients, there is need to promote leafy African Indigenous Vegetables (AIVs) [16]. The spiderplant (Plate 1) is among the most widely consumed leafy AIVs in East Africa. Yet, little research has been carried out regarding its responses to environmental stresses, agronomic traits and selection/breeding. Following promotion of its growth and utilization, increased cultivation in low-rainfall areas in Uganda, Zambia, Zimbabwe and Cameroon are reported [17, 18].

The nutritional value of the spiderplant is reported to be comparable to that of other vegetables, being a good source of β-carotene and vitamin C, iron, calcium, magnesium and proteins [18]. It is also used as medicine, insecticide, acaricide, forage crop, crop protectant and anti-feedant, for oil extraction and as a source of yellow dye [19].

MATERIALS AND METHODS

Salt stress had pronounced effect on relative shoot growth rates (RSGR) (Table 1), with growth rates being progressively depressed as the level of salt stress increased. At the end of the experimental period, plants subjected to the highest stress had the highest RSGR values, indicating that growth was temporarily arrested rather than irreversibly inhibited, presumably resuming after osmotic adjustment had presumably taken place. Although root growth (Figure 3) was less responsive to salt stress compared to shoot growth, it was, nevertheless, significantly (p≤0.01) affected.

Leaf growth was significantly affected by salt stress (Figures 4 and 5). Statistically significant differences (p≤0.001) in leaf fresh weights were observed after only one week of salt treatment (Figure 4) in highly stressed plants (-0.9 MPa and -1.2 MPa). After the first week of subjecting the plants to salt stress, leaf fresh weight in the moderately stressed plants (-0.3 MPa and -0.6 MPa) seemed to recover from the osmotic shock, presumably after osmotic adjustment had taken place. The extensive burns in the leaf blades accompanying the decline in leaf fresh weight of -0.9 MPa treated plants could have been due to ion toxicity effects.

Leaf weight ratio (LWR) in all treatments declined progressively throughout the experimental period (Figure 5). From the fourth week of salinization, control plants had the lowest LWR, which increased with increasing salt stress. Thus, the effect of salt stress was to reduce the decline in LWR of stressed plants.

Leaf chlorophyll concentration decreased with increase in salt stress (Figure 6). There were no significant differences (p>0.05) between the control and moderately salinized plants (-0.3 MPa and -0.6 MPa). However, salt stress reduced chlorophyll content in -0.9 MPa plants by between 20-50%; and in -1.2 MPa plants by between 60-80%, and these differences were statistically significant (p≤0.001).

Results on days to 50% flowering show that salt stress significantly (p≤0.05) lengthened the period of the vegetative phase. Plants attained 50% flowering at 13, 15, 18, and 20 days respectively for the control, -0.3 MPa, -0.6 MPa and -0.9 MPa plants (Fig. 7). However -1.2 MPa treated plants died before attaining 50% flowering.

DISCUSSION

The presence of high salt concentrations in plant tissues increases the osmotic potential of tissues, leading to low plant water potential. Such osmotic stress leads to reduced cell expansion and cell division rates. Since it depends on turgor, cell expansion growth is very sensitive to water stress, being reduced at water potentials below -0.3 MPa due to decreased turgor pressure. Plants in this study were subjected to soil osmotic stress of up to -1.2 MPa, at which turgor is negligible [27].

Ion toxicity may also have a role in decreasing the rates of cell division and cell expansion; hence retarded plant height and reduced dry weight. Although ion parameters were not measured directly, their contribution to the observed response may be deduced. For example, accumulation of Na⁺ and Cl^- ions occurs in the stem tissues of sorghum and finger millet, leading to ion toxicity [21, 26]. In this study, the results support a contribution to reduced growth by ion toxicity effects, since extensive leaf damage in the form of marginal burns, necrotic patches and leaf death were observed within three to seven days of subjecting the plants to salt stress in the most stressed treatments (-0.9 MPa and -1.2 MPa), especially in the older leaves.

Plants have several mechanisms by which they deal with the presence of high salt concentrations in the growth medium. Most glycophytes avoid ion toxicity by excluding the salt ions at the root surface, or by excluding the toxic ions from metabolically active (salt-sensitive) sites such as the photosynthetic apparatus and enzymes. Exclusion may be at the whole-plant, organ, tissue or cellular levels. A plant may, for example, accumulate salts in the roots, hence avoiding translocation to the more sensitive shoots; it may also accumulate salts in leaf sheaths, in vascular tissues and bundle sheath tissue, or in older leaves, thereby protecting the photosynthetically important leaf blades and metabolically more active young leaves, respectively. In some plants, this compartmentalization of salts within the plant may have a sub-cellular component, whereby salt ions are accumulated in the vacuoles away from organelles. Plants that cannot regulate salt ion concentration in this way, and whose tissues are not innately salt-tolerant per se, encounter severe physiological dysfunction, leading to decreased growth rates and eventually, tissue and organ death [26]. Spiderplant, a glycophyte, seems to be a poor regulator of the entry of salt ions at both the root surface as well as translocation to the shoot, as indicated by the observed early and severe leaf damage symptoms exhibited at high salt concentrations. Passive uptake of salts with the transpiration stream would therefore have led to rapid accumulation of salts in the roots and shoots until the Na⁺ and Cl^- ions eventually reached levels at which tissue damage would result from ion toxicity or osmotic dehydration. Corresponding drops in leaf fresh weights (Figure 4) and chlorophyll content (Figure 6) may be viewed as supporting this argument.

Other than ion toxicity and osmotic effects, observed reduction in plant growth could be partially explained by indirect metabolic and physiological responses to salinity. Salt stress increases mesophyll and stomatal resistances, leading to reduced CO₂ assimilation and photosynthetic rates. This ultimately translates into smaller quantities of metabolites available for growth, hence reduced growth. Furthermore, salt stressed plants have higher rates of tissue respiration, which exerts its demand on the already low metabolite levels at the expense of growth [21]. Observed root damage could also have occurred due to soil deflocculation as a result of high Na⁺ concentration and subsequent high exchangeable sodium percentage (ESP). Visual observation of the potted soil at the end of the experimental period confirmed such deflocculation to have occurred in the highly salinized pots [4].

The conclusion here is that damage to shoot and root tissues as a result of direct ionic and osmotic effects, and root damage due to structural deterioration of the soil, contributed to the observed sharp drops in dry weight preceding the death of highly salt stressed plants in -0.9 MPa and -1.2 MPa treatments [3, 11].

Results that salt stress initially retarded relative shoot growth rate agree with observations made on sorghum [26]. Such depression of growth rate may be ascribed to the effects of salt stress on the physiological/metabolic processes responsible for plant growth. However, plants subjected to the highest salt stress later had the highest RSGR values, suggesting that growth was temporarily arrested rather than irreversibly inhibited, and resumed after osmotic adjustment had taken place. This agrees with findings on finger millet [1, 21].

Reduction in root growth could be attributed to reduced rates of cell division and cell extension, increased root death, and reduced root diameter [26]. Observations that roots are often more salt-resistant than shoots are similar to those reported for other species. The ultimate result of this is a higher root to shoot ratio (Figure 3) in salt stressed plants compared to unstressed ones. The current results are in agreement with this conclusion since after three weeks of salt treatment, the most stressed plants had the highest root to shoot ratios [6].

The primary effect of salt stress in many plant species is reduction of leaf growth rates, leaf initiation and emergence rates, and overall shoot development. Such reduction of leaf growth is reported in rice, beans and sorghum [25, 26, 28]. The observed decline in leaf growth may be attributed to either or both of two possible responses. The first is inhibition of initiation, emergence and growth of new leaves; and the second is leaf damage and acceleration of leaf senescence and death due to toxicity of Na⁺ and Cl^- ions [9].

The observed ability of moderately stressed plants (-0.3 MPa and -0.6 MPa) to initiate new leaves and retain older leaves for longer periods may be explained by their ability to recover from osmotic shock after osmotic adjustment had taken place. Extensive burns in the leaf blades accompanied the decline in leaf fresh weight in -0.9 MPa stressed plants, an effect attributed to ion toxicity effects. Other than retarded rates of leaf emergence and accelerated leaf senescence and death, reduced rates of cell division and cell extension could have contributed to reduction in leaf growth. The rapid response in leaf fresh weight could be attributed to a rapid message transduced via the xylem from the roots to the leaves when the roots were subjected to osmotic stress, which in turn reduced turgor and extension growth [25]. This effect may be mediated by phytohormones such as abscissic acid (ABA). Decrease in cell extension growth is also attributed to reduction in extensibility of expanding cell walls [26]. Ultimately, reduction in leaf growth results in reduced leaf area, leading to less photosynthesis and hence retarded plant growth as the supply of resources required to support growth become limited. Salt stress is also reported to reduce the activity of various enzymatic systems involved in plant metabolism hence affecting plant growth [1]. It has been observed that after osmotic adjustment has taken, salinity not only allows leaf growth to continue, but also to promotes the ability of plants to retain leaves for longer periods, which agrees with observations in this study that salt stressed plants had higher LWR. However salt stress is reported to reduce LWR in maize seedlings, which is contrary to the current results [29].

Findings that leaf chlorophyll concentration decreased with increasing salt stress are reported for sorghum, finger millet and wheat [9, 21, 26]. The photosynthetic capacity of a plant is determined by several factors including photosynthetic pigment composition (chlorophyll content), CO₂ fixation capacity, light intensity and the activity of various enzymes. Furthermore, the efficiency of light-capture to drive photosynthesis is directly correlated to chlorophyll concentration in leaves [26]. Therefore, observed decline in leaf chlorophyll concentration would contribute to reduction in photosynthesis, and ultimately plant growth.

Plant development results from cell differentiation, and is outwardly perceived as the morphological formation of various tissues and organs such as roots, shoots, leaves, flowers and fruits [21]. Reported responses of plant development to salt stress include delaying of flower emergence, enhancement of fruit ripening, and acceleration of leaf senescence [1]. Results show that salt stress significantly lengthened the vegetative phas, and agree with those reported for other species [20]. It has been reported that ion toxicity delays flowering in rice, and that salt stress does not affect cell differentiation in finger millet [21, 30]. Similar inferences may be made in the case of spiderplant. For instance, it was observed that the initiation of new leaves seemed to improve in the moderately stressed plants after osmotic adjustment had taken place (Figure 4), suggesting that salt stress retarded the growth of already initiated leaves rather than inhibiting the differentiation process that leads to leaf initiation. The same inference may be made for flower emergence. In agreement with this view are reports that salt stress had no effect on development and growth potential in beans, peaches and rice [1].

In leafy vegetable crops such as spiderplant, the length of the vegetative phase is important for the productivity of the crop and the longer the vegetative period, the higher the expected leaf yield [19].  Observations that salt stress delayed flower emergence, lengthened the vegetative phase, and produced higher leaf weight ratios in salinized plants seem to be beneficial. However, this would only hold true if higher leaf yields were obtained from the salt stressed plants. This is hardly the case since salt stress also caused leaf damage, accelerated the rate of leaf senescence and generally reduced leaf growth, which may have far outweighed any benefits gained by a longer vegetative phase and a higher leaf weight ratio. Thus, leaf yield of unstressed plants growing over a shorter vegetative period may well be higher compared to leaf yield of salt stressed plants growing over a longer vegetative period. However, these responses of spiderplant to salt stress may form a basis for the selection and breeding for higher leaf yields in salt-affected soils.

CONCLUSION

Spiderplant, being a glycophyte, is expectedly salt sensitive. Hence both growth and leaf yield of this species are depressed by salt stress. However, the results of this study also show that the species can survive and continue growth under conditions of moderate salt stress. A salt stress exerting an osmotic stress of -0.3 MPa does not affect the growth of this species; and it can survive, grow and reproduce when exposed to a salt stress of up to -0.9 MPa, albeit at retarded rates. This ability, attributed to the species' capacity for osmotic adjustment, is viewed as a trait that through further investigation may form a criterion in the selection and breeding of salt-resistant cultivars of spiderplant [18]. As yet, very limited selection for desirable agronomic traits in spiderplant has been done. There is, therefore, need to incorporate screening for salt resistance in such selection.

Plate 1: Flowering spiderplant shoot at Maseno University Botanic Garden

Table 1
Effect of salt stress on relative shoot growth rate (RSGR) in Cleome gynandra( L.)

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