Effect of Plant Growth Regulator on Wheat & Barley Production
Background
Wheat is the second most important strategic food security crop in Zimbabwe after Maize. Wheat has become a staple crop given high demand of bread other of its products by the urban population. Wheat farming is Zimbabwe’s major winter cropping activity. Wheat contributes about 4% to the GDP of Zimbabwe and the country has a consumption requirement of around 400,000 tonnes annually against an average annual production of 60,000 tonnes in the last 10 years. Given the strategic importance of the crop in food security, the continued decline in production from the peak of 325,000 tonnes in 2001, is a cause for concern. It is therefore critical to look at all factors that affect productivity and the use of Plant Growth regulators (PGRs) is one of them.
Wheat growers in Zimbabwe have potential to achieve yields greater than 8t/ha in good seasons. This level of yield and associated plant growth can cause lodging in many wheat varieties. PGRs have been used in high yielding environments to minimise the risk of lodging by reducing plant height and increasing stem strength. Research has confirmed that responses to PGRs can be variable depending on seasonal conditions, timing of applications, individual varietal responses, seeding rates, and fertilizer application rates.
PGRs are widely used in many countries to control lodging of winter wheat especially where it is grown with high rates of applied nitrogen fertiliser. There is also evidence from research that timely application of PGRs increases yield irrespective of whether lodging has occurred. Cycocel (Chlormequat) is one of the PGRs that have been widely evaluated and their effects on height and yield of winter wheat are well documented. Cycocel (CeCeCe) as a synthetic growth retardant has been recommended for wheat since 1960s. Growth retardants are natural or synthetic chemical substances which are directly applied to crops to alter some structural processes. It is expected that these alterations modify hormonal balance and growth leading to increased yield, improved crop quality or facilitated harvesting. Cycocel inhibits gibberellin biosynthesis via blocking ent-kaurene synthesis in the metabolic pathway of gibberellin production, resulting in reduced amounts of active gibberellins and consequent reduction in stem elongation. The stem shortening effect of Cycocel in cereals such as wheat and Barley result in reduction in crop lodging.
Notable Benefits of Cycocel on Wheat and Barley:
- Stimulation tillering.
- Redistribution of biomass resulting in increased root growth.
- Reduction of plant stature and increase in stiffness of straw that limits the risk of lodging.
- Cycocel applied at 4-5 weeks after germination (GS 30) thickens and reduces height of wheat stems.
- Results of trials indicate that generally Cycocel applied at GS 30 increase wheat yield by 10-25%.
Summary results of Field Trials and Studies
Influence of Cycocel on Grain Yield and its Components
The positive role of Cycocel on yield components such as greater fertile tillers, spike number, fertile spikelets, grain number and in some cases mean grain weight has been shown in studies evaluating the production potential of cereals; however, numerous studies have revealed that the grain number has been the main important component significantly associated with enhanced grain yield in response to Cycocel application. The major impact of Cycocel on grain yield is mediated via initiation of more fertile tillers per plant resulting in a greater number of grains (Emam and Moaied, 2000). Work on winter wheat and barley by Shekoofa and Emam (2008) on confirmed that the increased grain yield was the result of higher grain number. Various applications of Cycocel enhanced formation, growth and survival of head bearing tillers of barley (Cartwright and Waddington, 1981; Waddington and Cartwright, 1986; Ma and Smith, 1991; Ma and Smith 1992a), wheat (Khan and Spilde, 1992; Latifkar et al., 2014). When Cycocel is applied close to anthesis, a likely explanation for increased grain number is the decline in spikelet, floret and grain abortion (Ma et al., 1994; Rajala, 2003). This implies that the potential to modify grain number in cereals is more likely to result from reduced abortion rate prior to and at early grain filling, rather than from further increase in the number of already abundant spikelets and florets (Craufurd and Cartwright, 1989, Peltonen-Sainio, 1997). This suggested that adequate assimilate would be supplied by enhanced photosynthesis.
Enhancement of Stress Tolerance
Cereal crop production in arid or semi-arid regions is usually restricted by soil moisture deficit as well as soil salinity. Water deficit coupled with salinity in irrigation water is the major limiting factor where cereals are subjected to extreme water deficit. Enhanced stress tolerance in cereals can be achieved by application of some PGRs, such as Cycocel. Application of Cycocel can reduce some of the harmful effects of drought and salt stress and in some cases, compensate losses or damages caused by these stresses (Ashraf et al., 2008). It has been reported that the application of Cycocel partially compensated the reduction in growth, yield and some biochemical traits. Such compensatory effects of Cycocel could be due to various reasons such as stomatal closure, increased chlorophyll content and intercellular CO2 concentration, and stimulatory changes in other physiological and biochemical attributes (PirastehAnosheh et al., 2012). Cycocel can also stimulate root growth, reduce transpiration, increase water use efficiency, and prevent chlorophyll destruction (Rajala, 2003; Wang et al., 2010). Increased levels of soluble protein, free proline and antioxidant enzyme activities in plants under stress conditions are natural responses, which can help plants better tolerate the stress.
Application of Cycocel increased these traits and improved stress tolerance in plants. Furthermore, the enhanced antioxidant enzyme activities in response to Cycocel application may also protect their photosynthetic machineries against damages caused by Reactive Oxygen Species (ROS) during water-deficit conditions (Rajala, 2003; Ashraf, 2010; Wang et al., 2010; PirastehAnosheh et al., 2012). The role of Cycocel in improving leaf area has been well-documented (Ma and Smith, 1991; Leitch and Hayes, 1990; PirastehAnosheh and Emam, 2012a). By improving leaf area, Cycocel application can result in increased photosynthetic rate, leading to a higher grain yield. The positive effect of Cycocel under drought stress in improving the “stay green” trait in wheat and the role of stay green in reducing canopy temperature are also well documented (Shekoofa and Emam, 2008; PirastehAnosheh and Emam, 2012b). Thus, the application of Cycocel may improve plant performance under drought via lowering canopy temperature.
Cycocel sprayed plants produced more yield under optimal as well as moisture stress conditions. Cycocel treated plants also maintained a higher level of nucleic acids, proteins and chlorophyll at all levels of tissue dehydration, suggesting that they were less prone to degradative processes under soil moisture stress conditions. It appeared that less research has been conducted on possible beneficial effects of Cycocel application on cereals grown under saline conditions. Nevertheless, it has been reported that Cycocel could regulate plant adaptation to salt stress. Gabr et al. (1977a) found that the wheat grain yield and dry matter accumulation were increased by the application of Cycocel under saline conditions.
Time of Application and Concentration
The effectiveness of Cycocel depends upon several factors, such as concentration, time of application, sowing time, environmental conditions and nutritional status of the crop. To achieve the best efficiency of Cycocel application, time and concentration of foliar application should be carefully chosen. Time of Cycocel application must be selected in order to enhance its action in improving plant performance. Application time is considerably related to the objectives of use; for example, if Cycocel is applied for reducing the risk of lodging, its application at GS 30 is recommended since the lodging resistance is a direct function of the level of thickening of the stem tissues (Rodrigues et al., 2003). If Cycocel is applied for increasing salt stress tolerance, priming or application at seedling stage is recommended because germination and establishment are the most sensitive stages to salt stress (PirastehAnosheh et al., 2014a). Kurepin et al. (2013) believed that Cycocel is used on cereal grain crops at early vegetative growth stages to achieve higher grain yield. They concluded that Cycocel could improve grain yield primarily by reducing lodging caused by heavy rain or hail. On the other hand, too late application may, depending on conditions, have adverse effects on grain.
Toxicology, Environmental and Safety Profile
Human health effects and environmental fate and effects assessments were done by the Environmental Protection Agency (EPA) in the USA for Re-Registration Eligibility Decision for Chlormequat Chloride in 2007. The Agency did not identify any human health risks of concern based on current use patterns and label restrictions. However, the assessment revealed potential environmental risks of drift resulting from aerial application. As a result, the product has a Red Triangle in the Zimbabwe pesticide classification system and aerial application is prohibited. Cycocel pre-harvest interval is zero for grain cereals and 21 days if the crops are used for fodder. Re-entry period for fields treated with Cycocel is 12 hours and rain fastness is 6 hours.
Future Perspectives
The current registered recommendations for use of Cycocel in Zimbabwe is for reduction of rank growth in Cotton and reduction in plant height to prevent stem lodging in Wheat and Barley. However, some farmers have been using Cycocel in Maize planted at high plant populations of 60-80,000 plants per hectare to reduce stem length and prevent lodging with good results. Based on the available literature, it appears that more studies are needed to better understand the relationship between Cycocel effects and other chemical inputs. Concurrent use of Cycocel with herbicides, pesticides and fungicides or even liquid fertilizers also require further investigations. The effect of Cycocel on cereal grain quality, such as baking quality may be worth further research. Furthermore, studies on genes involved in Cycocel signalling pathways in cereals could be of interest to Zimbabwean Wheat and Barley breeders.
By: Dzingo Mafuvadze
PIVOTAL AGRO-SERVICES
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