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B io Factsheet www.curriculum-press.co.uk Number 258 Biology of cereals 1: Sorghum, a C4 plant These adaptations are reflected in the water use efficiency of sorghum (Table 2) This Factsheet: • Summarises the importance of cereals worldwide • Describes and explains the adaptations of sorghum, a C4 plant • Reviews the very many types of exam questions that are set on sorghum in particular and cereals in general Table 2. Water use efficiency of sorghum Cereal Cereal crops such as sorghum are a major source of nutrients all over the world. Cereals grow well in many different environments, their grains are easy to store, contain a wide variety of nutrients and are often high in particularly important ones such as vitamin B. They also have high energy and fibre content. Water use efficiency/ kg water used per kg dry mass produced Sorghum 300 Wheat 350 Maize 550 Typical exam Question Scientists investigated the effect of the water-holding capacity (WHC) of soil on the yield of sorghum and maize. Seeds of both plants were planted in plots containing high WHC soil and in plots containing low WHC soil. The yield of sorghum and maize from all the plots was measured at the end of the investigation. The graph shows the results. Sorghum 6000 Tracey Slotta @ USDA-NRCS PLANTS Database 5000 Sorghum is a cereal that is well adapted to survive in hot, dry conditions (Table.1). yield/kg per hectare Table.1 Adaptations of sorghum Feature Explanation Extensive, deep root system Maximises water absorption Curled, small leaves Reduces water loss by evaporation/transpiration 3000 2000 1000 Reduces surface area over which transpiration can occur and traps layer of moist air 0 Reduces water loss by evaporation/transpiration Increased sclerenchyma and silica Reduces wilting Tolerant of high temperatures and heat shock proteins (HSPs) activate at very high temperatures Can grow in tropical conditions HSPs help prevent damage to proteins wheat LWC wheat HWC Markscheme Reduced stomatal numbers and stomata in pits sorghum HWC (a) No. of seeds sown; Planting pattern/density; Planting depth; Watering regime; Aspect; Wind/shelter; Soil pH; (b) (i) yield for sorghum greater than yield for wheat; WHC had little effect on sorghum yield; WHC had much greater effect on wheat yield; (ii) sorghum tolerant of dry soils/ adapted to dry conditions; wheat not well adapted to dry conditions; yield for wheat is better in HWC soil / little difference in yield for sorghum; Traps layer of moist air, reducing the diffusion gradient sorghum LWC (a) Suggest three factors that would have to be controlled during this investigation to ensure that the results were valid (3). (b) (i) Describe the results shown (2) (ii) Suggest an explanation for the results (2) Waxy leaves composed of esters Wax doesn’t melt and shiny and fatty acids with a melting surface reflects heat point much higher than nontropical plants Leaves have rows of motor cells along the midrib of the upper surface of the leaf, enabling the leaves to roll up. 4000 1 Bio Factsheet 258. Biology of cereals 1: Sorghum, a C4 plant www.curriculum-press.co.uk Photorespiration Photorespiration isn’t on many specifications but that doesn’t stop the Examiners from setting questions on it – they often don’t mention the term but give you a diagram of it that tests your ability to apply biological principles that are on the spec to a topic that isn’t. In other words, they are testing your ability to think on your feet. Nice people these Examiners! In the light-independent stage of photosynthesis, the enzyme ribulose bisphosphate carboxylase (Rubisco) catalyses the combination of carbon dioxide with ribulose bisphosphate (RuBP). The 6C compound formed immediately breaks down into two 3C compounds ( GP) which are then converted into triose phosphate ( (TP) using ATP and NADPH from the light dependent stage (Fig. 1) Fig. 1 The Calvin cycle (C3 pathway) CO2 Light Dependent Stage occurs in thylakoids CO2 + RuBP Rubisco catalyst carbon fixation 6C Unstable & breaks down 2GP (Glycerate-3-phosphate) Regeneration of RuBP ATP (from Light dependent stage) NADPH (from Light dependent stage) ADP NADP 2TP (Triose phosphate) Light Independent Stage (Celvin Cycle) occurs in stroma Synthesis of polysaccharides, amino acids, lipids and nucleic acids However, Rubisco can also bind with oxygen. In hot, dry conditions plants close their stomata to prevent excess water loss. But this also stops CO2 entering the leaf. When CO2 levels inside the leaf fall to low levels Rubisco combines with O2 instead of CO2. This process is called photorespiration. It is called photorespiration because: • • • It occurs in light It requires oxygen, like aerobic respiration It produces CO2 and water However, it does not produce ATP and it reduces carbon dioxide assimilation because some of the intermediates in the Calvin cycle are degraded. Photorespiration therefore reduces crop yields. • GP is the first stable product of the light-independent stage. It is a three- carbon compound and for this reason the Calvin cycle is also called the C3 pathway. • Plants such as sorghum are C4 plants. They have evolved an additional way of initially fixing the carbon dioxide and preventing photorespiration. 2 Bio Factsheet 258. Biology of cereals 1: Sorghum, a C4 plant www.curriculum-press.co.uk Fig. 2 The C4 pathway CO2 1 (3C) Phosphoenol pyruvate Mesophyll cell 1. Within the leaf mesophyll cells, instead of CO2 being fixed by the enzyme Rubisco and combined with ribulose bisphosphate, it is fixed by the enzyme phosphoenol pyruvate carboxylase (PEPC) and combined with phosphoenol pyruvate (PEP) to form the fourcarbon compound oxaloacetate (hence, C4 pathway). NADPH AMP 2 NADP ATP 5 Pyruvate (3C) (4C) Malate 3. The malate then moves out of the mesophyll cells into the bundle sheath cells that are tightly packed around the veins of the leaves. 3 5. The pyruvate re-enters the mesophyll cells and recats with ATP, regenerating PEP. Bundle sheath cell (4C) Olalocetate 2. Oxaloacetate is reduced by NADPH into malate. Pyruvate (3C) 4 CO2 Calvin Cycle Polysaccharides, amino acids, lipids and nucleic acids 4. Inside the bundle sheath cells a decarboxylase enzyme converts malate into pyruvate and releases CO2. The levels of CO2 NADP inside the bundle sheath cells become extremely high: 10 – 60 NADPH times the CO 2 concentration found in mesophyll cells. This then enables the normal Calvin (C3) pathway to begin i.e.C4 plants exploit both a C4 and the normal C3 pathway. (4C) Malate Vein So what is the point of all this? Fig. 3 Transverse section through the leaf of a C4 plant PEPC has an extremely high affinity for carbon dioxide, binding it even when carbon dioxide levels are very low, and it has no affinity at all for oxygen. It also has a high optimum temperature – it works best in hot conditions (45oC), precisely when the plant may have closed its stomata, thus limiting the amount of CO2 that can enter. In fact the enzyme is so efficient that C4 plants can afford to keep their stomata closed (preventing water loss) much longer than C3 plants can. epidermis/epidermal cell The tightly-packed mesophyll cells surrounding the bundle sheath cells don’t let oxygen into them so Rubisco can’t bind to it when CO2 levels are low. Thus, C4 crops such as sorghum, maize and sugar cane are able to withstand higher temperatures and light intensities, lose less water via transpiration and hence provide greater yields than C3 crops bundle sheath (cell) mesophyll (cell) Extract from Chief Examiner’s Report:- Many candidates could not explain why the C4 pathway enables plants to grow well in hot conditions. Many candidates seemed to believe that the C4 pathway was a modified form of respiration. 3 Bio Factsheet 258. Biology of cereals 1: Sorghum, a C4 plant www.curriculum-press.co.uk Typical Exam Question (d) Species X has evolved an altered form of photosynthesis that helps it to concentrate carbon dioxide in some cells. The diagram shows the interconversion and transfer of some organic compounds, which contain either three carbon atoms (C3) or four carbon atoms (C4), in a leaf of species X. (a) The diagram shows sections through a typical leaf of three different cereal plants. Cereal B Cereal C Cereal A cuticle CO2 Cell A Cell B C3 compound stomata C4 compound Which cereal is most likely to be grown in hot, dry conditions? Explain your answer (2). (b) The diagram shows a section of the lower leaf epidermis of sorghum. Y 2 1 0 10 20 30 40 50 60 Light intensity/arbitary units 70 Markscheme 0 (a) 3 (b) 4 (c) X (d) (c) The graphs show the effects of light intensity and temperature on the rate of photosynthesis in two crop species of plant, X and Y. Rate of photosynthesis /arbitary units CO2 Leaf A; Thick cuticle; Few stomata; Sunken stomata; Reduce water loss; grows in hot, dry /arid conditions; reduces transpiration/water loss; ref to C4; Species X; Higher rate of photosynthesis at high light intensity/ light intensity is still limiting at high intensity; Higher rate of photosynthesis at high temperature / Y slows above 30oC / temperature is still limiting at high temperature; (i) C3 reaction rapidly fixes the CO2 / keeps CO2 concentration low; Maintains the concentration gradient; Allowing more CO2 to diffuse in; (ii) Decarboxylation of the C4 compound is rapid/ CO2 is released rapidly in B; Movement of the C3 compound from B to A is rapid and accelerates decarboxylation/ ref to feedback control; Ref to use of ATP energy to transfer C3 compound into cell A; (iii) High temperatures may result in high transpiration/ water loss; Stomatal closure reduces this; But also stops CO2 diffusion into the leaf; Decarboxylation in cell B maintains high CO2 concentration; Maintaining high rate of photosynthesis; Sorghum has relatively few stomata. Explain the significance of this (2) 5 C4 compound Use information in the diagram to explain: (i) how reactions in cell A result in a large amount of carbon dioxide to enter cell B (2) (ii) how very high concentrations of carbon dioxide are built up in cell B (2) (iii) the significance of the fact that the stomata of species X are only open for approximately 65% of the time that those of species Y are open (3). 0.1 mm 6 AT P C4 compound C3 compound 6 X 5 4 Rate of photosynthesis /arbitary units 3 Extract from Chief Examiner’s Report (d) This part of the question was simply about diffusion but many candidates appeared to “give up” apparently because they were intimidated by the unfamiliar diagram. Most candidates did not even mention diffusion, nor the maintenance of concentration gradients; candidates need more experience of applying simple biological principles to unfamiliar material. 2 Y 1 0 0 10 20 30 Temperature /0C 40 One of the crops is well adapted to growing in tropical conditions. Suggest, with reasons whether this is species X or Y (2) 4 Bio Factsheet 258. Biology of cereals 1: Sorghum, a C4 plant www.curriculum-press.co.uk Scientists are collaborating worldwide to try to understand how sorghum can tolerate the high temperatures that would kill many other important crop plants. One suggestion is that their heat tolerance is due to a substance known as GB. (a) Explain how this leaf anatomy helps the plant achieve high rates of carbon fixation at high temperatures (6). (b) Germinating seeds use the enzyme alpha amylase to break down starch. Scientists investigated the effect of temperature on alpha amylase in seeds of sorghum and rice that were germinating. The graph shows the results. Scientists investigated this by growing two varieties of sorghum, one containing high levels of GB and one containing low levels of GB, in both distilled water and in sodium chloride solution (sorghum is often grown in irrigated water that contains sodium chloride). They measured the rate of photosynthesis of all the plants. The table shows the results. Variety of sorghum 4 3 Rate of photosynthesis / arbitrary units Distilled water NaCl solution High GB 31 18 enzyme activity /arbitrary 2 units Low GB 33 8 1 sorghum What does this data tell us? • The scientists wanted to find out whether GB levels affected photosynthesis, hence yield • So they selected varieties with high and low GB levels and then grew them in distilled water (no NaCl ) and in water that was similar to the irrigation water (with NaCl) • Simple Biology tells us that, normally, plant roots growing in salty water will have water pulled out of them by osmosis. Worse, in hot conditions, irrigation water will evaporate, making the remaining soil even saltier • In the NaCl solution the variety with high GB achieved more than twice the rate of photosynthesis of the variety containing low GB • This suggests that the GB is lowering the water potential of the sorghum roots, stopping water being withdrawn osmotically, enabling the plant to absorb water, survive and grow 0 10 rice 20 30 40 50 temperature / 0C 60 70 80 (i) Describe the results (3) (ii) What do these result suggest about the tertiary structure of alpha amylase in sorghum and rice? (3) (b) (i) in both, enzyme activity increases and then falls; alpha amylase in sorghum has higher activity at all temperatures; alpha amylase in sorghum has higher optimum temperature; alpha amylase in sorghum has higher maximum activity; (ii) tertiary structure/active site of alpha amylase of sorghum more stable; able to form more enzyme-substrate complexes; high temperatures affect H bonds more than other types of bond; so alpha amylase in sorghum may have less H bonds; 2. (a) tightly packed mesophyll cells; prevent O2 cannot reaching bundle sheath cells; light independent stage/Calvin cycle occur in bundle sheath cells; malate moves from mesophyll cell to bundle sheath cell; decarboxylation of malate maintains high CO2 concentration in bundle sheath cells; PEP carboxylase has high optimum temperature / greater affinity for / no affinity for O2; PEP carboxylase not denatured; photorespiration is avoided; photorespiration prevents carbon dioxide entering the Calvin cycle; from which biomass is produced; photorespiration also destroys compounds in the Calvin cycle Heat shock proteins Despite sorghum being able to tolerate and take advantage of high temperatures, yields of sorghum can be severely reduced if it is exposed to very high temperatures during germination or at the young seedling stage. When sorghum seedlings are exposed to temperatures above 37°C, they produce heat shock proteins ( HSPs ). These intracellular proteins help protect enzymes against denaturation and help prevent damage to proteins as they are being synthesised. However, very young sorghum seedlings are unable to activate sufficient HSPs and this is why they are particularly vulnerable. 1. Thick waxy cuticle; Sunken stomata; Reduces water loss; Adult/embryo plants able to tolerate high temperatures; C4 pathway; Allows photosynthesis when stomata closed/more efficient at high temperatures; Dense/wide/deep root system; Allows water to be collected from large area; Rolling of leaves; Reduces area from which water can evaporate; Practice Questions 1. Outline how sorghum is adapted to growing in hot, dry conditions (6) 2. The diagram shows a transverse section through the leaf of a C4 plant. epidermis/epidermal cell Markschemes bundle sheath (cell) Acknowledgements: This Factsheet was researched and written by Kevin Byrne. Curriculum Press, Bank House, 105 King Street, Wellington, Shropshire, TF1 1NU. Bio Factsheets may be copied free of charge by teaching staff or students, provided that their school is a registered subscriber. No part of these Factsheets may be reproduced, stored in a retrieval system, or transmitted, in any other form or by any other means, without the prior permission of the publisher. ISSN 1351-5136 mesophyll (cell) 5