Nterpretation_of_Data_of_Stanberry_&_Lowrey_(1965);_Winter_Barley,_Nitrogen_&_Irrigation

Interpretation of data of Stanberry & Lowrey (1965); Winter Barley, Nitrogen & Irrigation Introduction: Plants are dependent on a range of essential nutrients, which are extracted from the soil by the roots. These are categorised as either macro or micro nutrients depending on the amounts required by the plant. One of the macro nutrients required for plant growth is nitrogen (Lack & Evans; 2001). Nitrogen is a component of amino acids and proteins, contributing to many structural and metabolic compounds in plants. It is a major component of chlorophyll which is required for photosynthesis, producing photoassimilates (Carbohydrates). The earth’s atmosphere is composed of 78% nitrogen (N2), however in its gaseous form nitrogen remains unavailable to plants, nitrogen is taken up by the plant roots as either nitrate (NO3-) or ammonium (NH4+). As a rule mineral nutrients taken up by the roots are transported to the shoots via the xylem and photoassimilates are transported to the roots via the phloem. In nitrogen deficient soils root growth is enhanced, in the Thornley model (1972) growth is dependent on the carbon supply from the shoots and the nitrogen supply from the roots. In principle using the Thornley model, an increase in the carbon concentrations should therefore lead to an increase in biomass partitioning towards the roots, whereas an increase in nitrogen concentrations would lead to an increase in biomass partitioning towards the shoots (Marschner, Kirkby & Cakman; 1996). Water is one of the most important factors limiting plant growth, (Lin, Bing-Cheng & Feng-Min; 2007), in an environment with limited water there is a trade-off between reproductive growth and investment in roots to increase the water supply (Chaves, Maroco & Pereira; 2003). If the water potential of a soil drops below -1.5 MPa it causes mesophytic plants to permanently wilt. This would be best avoided from an agricultural point of view as grain yield would be affected by the soil water potential, plants in infrequently irrigated soils may be prone to wilting. The typical root and shoot system is illustrated below, with a fairly evenly distributed root:shoot ratio. Figure 1: The typical root and shoot system for a plant under regular conditions in which the soil and nitrogen concentrations have not been altered. Notice the root:shoot ratio is roughly equal. CO2 fixed during photosynthesis is used in shoot/leaf growth and/or transported through the phloem to the roots, promoting root growth. H2O and nutrients (inc Nitrogen) are transported to the shoots/leaves via the xylem. Results: Nitrogen Fertiliser Added (kg ha-1) | 0 | 67 | 135 | 270 | | Yield of Grain (t ha-1) | Infrequent Irrigation | 0.87 | 3.05 | 2.95 | 3.57 | Frequent Irrigation | 0.82 | 3.49 | 4.54 | 4.81 | Table 1: Stanberry & Lowrey (1965) at Tucson, Arizona, grain yield of winter barley Hordeum vulgare receiving varying amounts of nitrogen fertiliser with infrequent and frequent irrigation. Discussion: Nitrogen is one of the nine macro nutrients essential for plant growth and reproduction (Lack & Evans; 2001). As illustrated in Stanberry & Lowrey’s results as the amount of nitrogen fertiliser (kg ha-1) in the soil is increased the yield of grain (t ha-1) is also increased. This is current with the Thornley model where an increase in nitrogen concentrations leads to an increase in biomass partitioning towards the shoots (see figure 2) and subsequently grain production (Marshcner, Kirkby & Cakman; 1996). As shown in Stanberry & Lowrey’s results with no nitrogen addition grain yield in an infrequently irrigated soil is 0.87 t ha-1, and in a frequently irrigated soil grain yield is 0.82 t ha-1. This shows the type of water irrigation system is almost “irrelevant” with respect to grain yield when the nitrogen levels in the soil are depleted or at least not elevated. When nitrogen fertiliser is applied to the soil only the top few cm (surface soil) receives this extra nitrogen, although a small amount of nitrogen is washed down through the soil profile due to water and gravity the surface soil retains the highest levels of nitrogen fertiliser. With the addition of 67 kg ha-1 of nitrogen fertiliser the grain yield rises dramatically, giving a 3.5 and 4.3 t ha-1 fold increase to infrequent and frequent irrigation respectively. The type of irrigation system is now limiting grain production, this is because although soil nitrogen concentrations are the same the water supply to the roots differs. Gases, mineral ions and organic molecules (solutes) all move into the tissues of plants in aqueous solutions, and are required to be dissolved in water before entry into the plant. In frequently irrigated soils the top few cm of the soil profile remains moist, allowing the root system to uptake nitrogen in its soluble form, this leads to increased biomass partitioning towards the shoots and hence a larger grain yield in frequently irrigated plants this is seen in table 1. In infrequent irrigation the plants are receiving larger quantities of water but less often, this means the top few cm of the soil profile (surface soil) are prone to “drying out”, with more water available further down the soil profile. This leads to less nitrogen availability, as the nitrogen is no longer in its soluble form required for absorption by the roots. Subsequently less nitrogen is transported to the leaves, which in turn means less leaf growth and grain production leading to more photoassimilates being transported (phloem) to the roots. The transportation of the photoassimilates to the roots increases root growth, this leads to plants under infrequent irrigation conditions to have larger root systems as the roots are required to locate sufficient water further down the soil profile this is illustrated in figure 3. With the addition of 135 kg ha-1 of nitrogen fertiliser both irrigation regimes grain yield increases, due to frequent irrigation and hence increased availability of nitrogen to the plant roots the frequent irrigation system produces 1.5 times the amount of grain than a soil receiving infrequent irrigation (4.54/2.95 t ha-1). Eventually increasing the volume of nitrogen fertiliser does not show a continual exponential increase in the yield of grain, both types of irrigation system show this to be the case i.e. there is an “optimal” amount of Nitrogen beyond which there is progressively less response from the plants. As illustrated in Stanberry & Lowrey’s results, with the addition of 67 kg ha-1 of nitrogen fertiliser to infrequent and frequent irrigation systems grain yield increased by 2.18 and 2.67 t ha-1 respectively, however when the volume of nitrogen fertiliser was increased from 135 to 270 kg ha-1 grain yield only increased by 0.62 and 0.27 t ha-1. This is because nitrogen and water are no longer limiting factors affecting grain yield, another factor is limiting the yield, possibly another essential nutrient in too smaller concentrations and therefore affecting the growth and reproduction of Hordeum vulgare. Figure 2: The typical root and shoot system for a plant in frequently irrigated soil with high nitrogen concentrations. Notice the low root:shoot ratio, this type of root and shoot system is very prone to drought due to its shallow root system. Figure 3: The typical root and shoot system for a plant in infrequently irrigated soil with increased nitrogen, water is available further down the soil profile. Notice the high root:shoot ratio, this type of root and shoot system can withstand drought due to its extensive root system. In frequent irrigation systems plants tend to have low root:shoot ratios due to the sufficient supply of water in the surface soil and subsequently the availability nitrogen, this leads to higher partitioning of biomass towards the shoots according to Thornleys model and therefore higher yields of grain in plants with frequently irrigated soil. In infrequently irrigated soil plant roots tend to have a high root to shoot ratio this is because the surface soil is prone to “drying out” and therefore essential nutrients including nitrogen required by the plants are unavailable in the surface soil this leads to biomass partitioning towards the roots and subsequently larger and more numerous roots. Due to the low root to shoot ratio frequently irrigated plants are more prone to suffer from drought in the future as their roots aren’t as extensive and don’t extend as far down the soil profile as infrequently irrigated plants. Both types of irrigation regime increase in grain yield with the addition of nitrogen fertiliser from 67 – 270 kg ha-1, the grain yield for infrequent irrigation overall rose by 25% and the grain yield for frequent irrigation overall rose by 35%. These figures show a 10% difference in the recorded grain yields between the two different irrigation systems, showing water availability to affect grain yield in Hordeum vulgare. The availability of nitrogen is dependent upon water availability, as nitrogen has to be available in its soluble form for the roots of winter barley to utilize it. The overall increase in grain yield is significantly different, infrequent irrigation has an overall increase in grain yield of 2.70 t ha-1 and frequent irrigation has 3.99 t ha-1 this again illustrates that nitrogen uptake by the roots is dependent on water availability, with increased available nitrogen biomass partitioning towards the shoots occurs and consequently growth and reproduction of shoot material is increased including increased yield in grain for Hordeum vulgare. Abscisic acid (ABA) is a plant hormone known to mediate a number of important physiological processes in plants (Mundy & Chua; 1988). ABA is synthesized in the root system of plants under osmotic stress, and is then transported to a plants leaves. The transport of ABA to a plants stomata causes stomatal closure, restricting the production of photoassimilates and hence leaf expansion (growth) and grain yield. This therefore leads to an increase in photoassimilates (carbohydrates) partitioning towards the root system, stimulating root growth. The production of ABA by the roots provides an indication to the shoots in the variation of water availability through the soil profile (Zhang & Davies; 1989). The ABA-dependent increase in cytosolic Ca2+ in guard cells triggers stomatal closure due to cation and anion effluxes (Skriver & Mundy; 1990). In infrequently irrigated soils plants are undergoing increased ABA synthesis in their roots leading to biomass partitioning towards their roots rather than towards their shoots and therefore a decreased grain yield in plants that are in infrequently irrigated soils. This can been seen in Stanberry & Lowrey’s results where frequently irrigated winter barley produces a higher grain yield than that of winter barley in a infrequently irrigated soil due to the plants responses to osmotic stress. In summary if shoot growth is limited by Nitrogen and/or the supply of available water from the root system, then proportionally more photoassimilates are available for transportation by the phloem to the roots; the roots therefore have Nitrogen, water and photoassimilates all of which are required for growth consequently resulting in a higher root:shoot ratio. However if the shoot/leaves have an “adequate” supply of Nitrogen and water they can utilize the photoassimilates they produce for their own growth and grain production, at the expense of root growth, resulting in a lower root:shoot ratio. References: Chaves, M. M., Maroco, J. P. & Pereira, J. S. (2003). Understanding plant responses to drought – from genes to whole plant. 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