Greenhouse evaluation of wheat cultivars’ performance in the coastal areas of Namibia

C. Gwanama ¹, L. Kanyomeka ^1,, O.D. Mwandemele ² and F. Mubiana ³

1. University of Namibia, Department of Crop Science, Ogongo Campus, Private Bag 5520, Oshakati, Namibia. Email: cgwanama@unam.na

2. University of Namibia, Private Bag 13301, Windhoek, Namibia

3. Sam Nujoma Marine and Coastal Resources Research Centre, University of Namibia, Henties Bay, Namibia.

Abstract

The coastal areas of Namibia have a desert climate and in most places either the soil or potential irrigation water when available, or both, are saline, making crop farming based on glycophytic species impracticable. Production ventures of other food, such as mushrooms are promising in these areas. However, substrates for the cultivation of mushrooms are mainly of plant material. The requirement for plant stover which could be used in alternative agriculture systems has necessitated a search for crop species that could be grown in these arid coastal environments. This study evaluated four wheat cultivars from CIMMYT with enhanced salinity tolerance for possible introduction to the area. All four cultivars performed similarly, showing more tolerance for plant growth than the local control at all developmental stages. The plant height differences were not consistent among the cultivars during the growth season, showing differences in growth rate, rather than potential total stover production.  Grain yield was also higher in the cultivars from CIMMYT, although only one cultivar was statistically different from the control. Plant height did not correlate with yield; therefore it was unrelated to the assimilate partitioning of the varieties, so that it could at best only be used as an indicator of general plant health. The four wheat varieties from CIMMYT have the potential for production in the coastal areas of the Namib Desert.

Key words:

Cultivar, Electrical conductivity, Salinity, Tolerance, Wheat, Yield

Introduction

Namibia is an arid country with half of its land area covered by two deserts. The remainder of the country receives less than 600 mm of precipitation, making crop-based agriculture difficult. Apart from prevalent livestock farming, alternative farming systems are being encouraged. Among these, mushroom farming has shown great potential and receives good promotion and reasonable adoption. The mushroom production technology requires mainly the presence of dry plant matter containing lignin, cellulose or hemicelluloses. The requirement of water in mushroom production is much less than for ordinary crop production. Due to their perishable nature, however, mushroom production is most profitable in close proximity to the markets. This requires production in urban and peri-urban areas.

Due to unfavourable economic circumstances, the majority of the local population cannot afford the prices of cultivated mushrooms. One sector of the economy which is renowned to be a large consumer of cultivated mushrooms in Southern Africa is the hospitality industry. Namibia has a booming tourism industry, especially at the Atlantic Coast. A number of farmers along the Namibian coast have adopted mushroom farming. Sadly, the cost of production is still prohibitive, because the substrate has to be hauled from further inland, as the 200 km-wide strip of land extending along the coast is desert. Options have been considered for farmers to grow the substrate. This is hampered by prevailing high soil salinity and the salinity of the meagre supply of water that is available for irrigation. Availability of salt tolerant crops and cultivars could mitigate this problem.

Soil and irrigation water salinity is one of the most serious stresses in agriculture and affects about one billion hectares of land under crop production globally. It is a natural phenomenon in areas where evapotranspiration is greater than precipitation (Rains and Goyal, 2003). Saline environments present three hazards to crop growth, namely, water stress arising from negative water potential, ion toxicity associated with either Na⁺ or Cl^-, and nutrient imbalance from excess of Na, Cl and B, which results in inadequate potassium, nitrate or phosphate uptake (Gorham et al., 1985). Excessive saline conditions reduce the plant’s transpiration and growth rates. Plants must spend more energy to acquire water, thus diverting energy from the growth processes. Additionally, salinity affects crop quality and is manifested in reduced product size, as well as change in colour and appearance (Rhoades et al., 1992).

Most crop and grass species are glycophytes, meaning they grow best in salt unaffected areas. Only a few plants are halophytes — plants that thrive in salt affected areas (Grattan and Oster, 2003).  Plant salinity tolerance involves many mechanisms at both the cell and plant level. At the cell level the plant may use compartmentation, which means salt may be taken up and compartmentalised in the vacuole to prevent ion toxicity by protecting enzyme or ribosomal activity. The plants that use compartmentation concentrate the salts in the vacuoles, while keeping the cytoplasm at normal osmotic concentrations. This is usually the case with halophytes. Other plant species use ion selectivity, i.e., plant cells use plasmalemma-located ion pumps (such as sodium or proton pumps) to keep unwanted ions outside, while simultaneously producing organic osmoregulators to control internal tugor pressure (Lerner, 1985). Sometimes the ion exclusion mechanism is not restricted to the cell level. Ions may be absorbed into the roots by mass flow, but subsequently excluded from tissues of other organs through the path through which they are translocated. The two mechanisms have a common feature, or are alternative procedures of the same principle. This can be summed up as being ‘the exclusivity of the symplasm and the promiscuity of the apoplasm’ (Gorham et al., 1985). The halophytes, however, are very unproductive plants in terms of total dry matter accumulation. As tolerance involves the diversion of energy from growth and productive processes, it must be associated with good productivity for it to be desirable to the plant breeder, the agronomist and the farmer.

There is a threshold salinity for each crop at which yield becomes affected. Halophytes have high thresholds, while glycophytes have low threshold salinity levels. Among the agricultural crops, the most tolerant species are barley, cotton and sugarbeet. The more economically important staple cereals — wheat and maize — have only slight saline tolerance. Rice (Oryza sativus), the staple of two thirds of the world’s population, is among those with the lowest salinity tolerance (Rhoades et al., 1992). Nonetheless, there is within species variation for salinity tolerance in many species. For example, these authors have one variety of rice developed in Japan which is reported to have good salinity tolerance. Wheat (Triticum sativum L) has been hybridised with wheatgrass, resulting in offspring with elevated tolerance levels (Rains and Goyal, 2003). Within each species and variety, tolerance varies at different developmental stages. The most sensitive crop stage is emergence. Maize, despite being a generally intolerant crop, has about the best salinity tolerance at emergence.

Salinity, whether of the soil or irrigation water, is defined as the total dissolved concentration of major inorganic ions. Its objective measure is the amount of substance per unit volume as mmol/l or mg/l. However, for practical reasons the electrical conductivity (EC) of the saturated extract of the soil or the EC of the water is commonly used as a quick approximation. The EC is measured in dS/m (or dmho/m).  At 25°C, 1 dS/m approximates 700 mg/l total dissolved salts (Rhoades et al., 2003). Soils with EC greater than 4 dS/m are classified as saline soils (Grattan and Oster, 2003). On the basis of EC, water is classified as follows: <0.7 dS/m, non-saline water, suitable for drinking and irrigation; 0.7 – 2 slightly saline and suitable only for irrigation; 2 – 10 moderately saline, usually primary drainage water; 10 -- 25 highly saline, secondary drainage water; 25 -- 45 very highly saline, usually groundwater with high water table; and > 45 is brine, which is seawater (Rhoades et al., 2003).

A mathematical model for the yield reduction caused by saline soil has been proposed by Maas and Hoffman (1977). This model is expressed as:

RY (%) = 100 - B(ECe - A)

Where RY is the relative yield, B = slope of salinity curve, A = threshold salinity and ECe = average electrical conductivity of saturated root zone extract. The relationship shows that above the threshold salinity, the yield falls with increasing EC. Glycophytes have a low value of A and high value of B. The converse is true for halophytes. Values for both A and B are crop specific and are available in various sources in the literature.

Production of crops in saline soils depends on the use of good quality irrigation water. This idealistic scenario rarely exists, and a compromise usually has to be reached accepting water of lower quality. Irrigation with saline water is possible, provided certain measures are strictly implemented (Rhoades et al., 2003). The most important of these is to add an adequate leaching fraction to the eva-potranspiration demand in the calculation of irrigation water depth. Other strategies include blending with good water, sequential use and cyclic use of saline water (Grattan and Oster, 2003). “Sequential use” refers to using waters of differing salinities (as available) in a predetermined sequence to take into account the sensitivities of the crop at various developmental stages. For example, a crop may be irrigated with non saline water during the emergence stage and switched to saline water in later stages. “Cyclic use” means using waters of reduced salinity only at the more sensitive stages of crop production (Rhoades et al., 1992).

This study was part of the search for crops that can grow in Namibia’s saline coastal soils, using locally available water for irrigation, which is usually also saline. This initial experiment was a greenhouse trial, but the encouraging results have necessitated a field trial which is in progress.

Materials and Methods

Four wheat cultivars (= cultivated varieties), which were reported to be salinity tolerant by the suppliers, were obtained from CIMMYT, Mexico. These were ‘PWB 34’, ‘Karchia 65’, ‘Sorawaki’ and ‘Chinese Spring.’ A non-tolerant variety grown locally in South Africa and Namibia, called ‘Kariga’, was added as a control.

Two soils (top 20 cm) obtained from the vicinity of Henties Bay town were used for evaluation. One of these was obtained from the University of Namibia’s SANUMARC Centre, and the other from a community cooperative training centre outside Henties Bay, called Tulongeni. [Another soil from a place designated Solitude was used but subsequently discontinued, because its salinity was so high (equal to sea water) that there was zero emergence]. The Namibia coastal area receives less than 50 mm of rain, which falls between January and March. Soil collections were carried out in June 2009. A control soil from Ogongo in Northern Namibia was included. Ogongo receives a mean annual rainfall of 450 mm, distributed between December and March. During the previous season, however, 850 mm had been received. Since the study was conducted more than two months after the rain season, the top 20 cm of the soil was practically air-dry. Textural descriptions of the soils are given in Table 1.

The experiment was arranged in a randomised, complete block design with three replications, using the soil x source of irrigation water as the blocking factor, for ease of irrigation management. Treatments with the two coastal soils were irrigated with water obtained from each respective site. However, the treatments on Ogongo soil were irrigated by tap water from the Henties Bay Municipality. All the irrigation water was slightly saline. The experiment thus did not have an absolute control with non saline conditions. Electrical conductivities of the irrigation water are given in Table 2.

Planting took place on 4 June, 2008 in the SANUMARC greenhouse. Four seeds per pot, representing an experimental unit, were planted in a 20-cm diameter pot at a depth of 1 cm. A balanced liquid fertilizer for floricultural use was applied at planting, and repeated two weeks after emergence and six weeks after emergence. Plants were irrigated every three days. The greenhouse temperature was maintained between 22 and 28 °C. Average plant height (cm) was measured at six, eight, ten, twelve and fourteen weeks after emergence (wae). The number of tillers per plant was taken at one month after emergence, two months after emergence and at 100 % anthesis. At harvest, grain weight per pot was measured. Data on the number of tillers were transformed before analysis using the Ö(n+1) transformation to stabilise the variances. The log(n) transformation was used for the grain yield. The rest of the data were analyzed without transformation.

Table 1. Textural properties and electrical conductivity of soils used in the wheat cultivar performance trial

Table 2. Electrical conductivity of irrigation water used in the wheat cultivar performance trial

Results and Discussion

The minimum EC for a soil to be deemed a saline soil is 4 dS/m (Grattan and Oster, 2003). Although the two soils from the Henties Bay area had far greater EC values than that from Ogongo, all the soils used could not be classified as saline. Two factors were responsible for this. The first was that the groundwater tables for all these areas were not shallow (not within 2m). The second was that all the soils were extremely sandy (Table 1), therefore possessing very little matric suction. The combined effect of these factors was that there was very little soil solution moving as capillary rise and adding inorganic ions to the surface horizons.  The water used for irrigation in the experiment fell in the range EC 0.8 to 1.8 dS/m (Table 2) and is classified as slightly saline and suitable only for irrigation (Rhoades et al., 2003).

Vegetative growth

Cultivars showed good growth from emergence to maturity, except for scorching of the leaf tips for most of the growth period, but especially in the first three weeks after emergence. The average height of plants at 14 wae (just before anthesis) varied from 60 cm to 95 cm, which is normal maturity height for most wheat cultivars (Figure 1). Significant differences in plant height were observed throughout the period. The commercial variety ‘Kariga’ was always shorter than the other four varieties, though not significantly different from some of the rest. Observations of ‘Kariga’ in fields at Etunda in northern Namibia reveal that it has far greater vegetative growth than was realised in this experiment. This shows that salinity stress was significant in the experiment and confirms that the CIMMYT cultivars were salt tolerant. All CIMMYT cultivars, except ‘Chinese Spring,’ were significantly taller than the control at 6 wae. Until 12 wae, the varieties ‘Karchia 65’ and ‘Shorawaki’ were the tallest and fastest growing. However, this changed in the last weeks of growth, as ‘Chinese Spring’ was then the tallest. ‘PWB 34’ was ‘the average variety’ in terms of plant height at all stages.

The tolerance to salinity of the CIMMYT cultivars is estimated to be the same the same. The variation observed is likely to be genotypic differences for growth rate favouring the first two, while ‘Chinese Spring’ is a slow grower but has greater plant height at maturity. Indeed, all the test cultivars had entered into diminishing returns growth by 10 wae, while ‘Chinese Spring’ was still in the linear phase (Figure 1). This aspect of whether the differences in growth rate related largely to varietal salinity tolerance or to varietal phenological characteristics could not be clarified in the absence of completely non-saline irrigation water for a control. When the growth pattern of “Chinese Spring” is contrasted with the rest, it appears that the tolerance mechanisms are of specific duration, to the effect that tolerance in the four other varieties has started breaking down by the 10th week after emergence. This may imply the existence of different tolerance genes in “Chinese Spring” from the other three CIMMYT varieties. “Chinese Spring” is superior to the rest in total stover production. Nonetheless, its slow growth during the first several weeks makes it less competitive against weed competition.

The number of tillers per plant was only significantly different among the varieties at full anthesis (Table 3). The range of the data for individual pots was large, but variety standard errors were extremely high, pointing to the instability of this trait under saline environments. The highest number was for the variety ‘Shorawaki,’ followed by ‘Chinese Spring,’ and the least was for ‘Kariga’. Tillering ability depends both on the suitability of the environment and the specific bias of the variety for yield components.  The number of tillers is only important if it is a significant yield component. Association of the number of tillers per plant to the grain yield did not give significant correlation coefficients in this experiment. This was probably as a result of many tillers not producing grain, or varieties with lower numbers of tillers excelling in other yield components, such as spikelet number per tiller or spikelet size and 100 seed weight (data not available). Neither was plant height at any stage significantly correlated to grain yield. This means that the plant height was unrelated to the assimilate partitioning by the varieties and could only be used as an indicator of general crop health, and not yield potential.

Table 3. Summary of performance of the five wheat cultivars grown

Figure 1. Average plant height of five wheat cultivars grown on three saline soils at Henties Bay

Grain yield

Significant differences were found for the grain yield, with ‘PWB 43’ being the highest yielder, followed by ‘Chinese Spring,’ the two of them both yielding significantly higher than the control.  Taking the grain yield of ‘PWB 34’ as standard, the other cultivars had the following relative grain yields: ‘Chinese Spring’—93 %, ‘Shorawaki’—69 %, ‘Karchia 65’—65 % and ‘Kariga’—46 % (Table 3). Comparing the differences in plant height and those for grain yield, it is evident that if salinity tolerance were measured by survival only, the real amount of stress tolerance would be masked.

When the economic yield (grain yield) was taken as the tolerance criterion, it was seen that the differences between the susceptible control and the tolerant cultivars was vast. The reduction in yield is consistent with results of research by workers in Siberia, who found reductions of 1.9 Mg/ha (about 30 % yield reduction) in a non-tolerant wheat variety due to salinization of production fields (Bysovskaya and Malinina 2003).

The absence of correlation of vegetative characters (plant height, number of tillers) with grain yield is highlighted by the fact that ‘PWB 34’ was the best grain yielder, while it was not the most prolific for vegetative growth. The tolerance mechanisms — either at cell or plant level — of these cultivars are unknown to the authors. It is not likely that they have the same mechanisms or that the cultivars have the same genetic background. ‘Karchia’ is a salt tolerant variety that has been around for some decades. This experiment employed a progeny of the original variety, but the hybridisations that were made to the original are unknown to us. If only intraspecific hybridisations or only crosses within the Triticeae tribe were employed, then it is likely that the mechanisms are of the salt exclusion type. The Triticeae tribe uses the ion exclusion mechanism which operates as various sub-mechanisms in the root, leaves and flowers (Gorham et al., 1985). In addition to ensuring a lower inorganic ion concentration in the cell than the surrounding, it is characterised by production of organic osmoregulators, such as sucrose and glycinebetaine, which maintain tugor pressure. The production of proline has been shown in these species not to be connected to salinity tolerance but to drought-tolerance. Salinity and drought are two phenomena which are usually confused, because they often occur together, the latter often acting as a cause for the former. Within the Poaceae, the Triticeae have the highest amount of salinity tolerance.

Significant differences in plant height were obtained in this study for the irrigation water/soil combinations at 6 wae, 12 wae and 14 wae (data not shown), but the trends from week to week were inconsistent, making comparison among soil/irrigation water treatments impossible. The treatment soil/irrigation water from Tulongeni had the best plant height results and therefore offers the best conditions for wheat stover production among the sites. This was also true for the number of tillers. The highest grain yields were obtained from the Ogongo soil/Municipality water combination. Correlations between the EC in the soil, or the EC in the irrigation water, with all the measured traits, were not significant (data not shown). This may have resulted from the fact that the range of the EC values in the irrigation water was too small to show differences in plant response. The low EC in the Ogongo soil must have risen quickly due to the irrigation water employed. Alternatively, the lack of differences in performance in the three soils may be due to the interaction of many ionic species in the soil, a factor which was impossible to control. Additionally, differences in the percent textural fractions of the soils employed would likely compound the results due to differences in toxic ion adsorption capacities and buffering (by the soil) thereof. Most studies which have evaluated the effect of salinity on plant performance have used simulated conditions with varying NaCl concentrations on single soils, rather than irrigation water with multi-ion derived salinity. The results of this study, however, are more applicable to farm conditions.

This study has confirmed tolerance of CIMMYT wheat varieties to salinity conditions at the Namibian coast. Where grain is the economic yield, the variety ‘PBW 34’ is the first choice, followed by ‘Chinese Spring’. For the Henties Bay area, there is an expressed interest in the farming community to find crops that give good, above ground, biomass. In that case the cultivar of choice is ‘Chinese Spring,’ followed by ‘Shorawaki,’ because of their superiority in plant height and number of tillers at maturity. The choice of wheat for this trial was mainly due to availability of tolerant cultivars. The suitability of this crop, by sheer coincidence, proves a great advantage to mushroom farming, since, whereas oyster mushrooms (Pleurotus spp) can grow on nearly any ligno-cellulosic material, button mushrooms (Agaricus spp) show greater selectivity. Wheat straw is the backbone of the button mushroom industry worldwide (Miles and Chang, 1977, Stamets and Chilton, 1983), and it is equally suitable for the production of oyster mushrooms.

Identification of suitable cultivars is only the first step in their production in saline environments. Of equal importance is the availability of irrigation water and its management. Pockets of slightly saline water are available in the Namibian coastal areas, coming from underground rivers that disappear as they enter the desert sands. Some of the water used in this experiment came from boreholes sunk in sand dunes within 200 m of the seashore. This water is less salty than the sea water and available in many places along the coast. It can be used successfully for irrigation, provided adequate leaching fractions are added to the crop evapotranspiration demands. The amount of irrigation water and the watering interval of three days in this experiment may be seen as giving water almost ad libitum. Further field studies are required to validate the salinity tolerance of these cultivars in the field, as well as to work out irrigation schedules that would be realistic on the farm.

Acknowledgements

We are greatly indebted to the University of Namibia for the funding of this experiment.  CIMMYT kindly donated the varieties for the trial. Further thanks go to Mr. F. Mwazi for management of the experiment and assistance on site.

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