ABSTRACT
Climate change is a major stressor that would adversely affect tropical agriculture, which is largely rainfed. Available evidence shows that associated with climate change is an increasing trend in temperature and in some locations, decline in rainfall leading to repeated droughts during the growing season. In this study, the effects of increased temperature and drought on soybean, a C3 plant, was investigated under greenhouse conditions. An understanding of how soybean would respond to climate change effect is a major key to improving food security for the global population and continues to be of research interest.
This research was conducted in a greenhouse in the year 2018 with the purpose of determining the effect of climate variables such as temperature, relative humidity (RH), vapour pressure deficit (VPD) and soil water (W) on the phenology, biomass and grain yield of the plant. The research also aimed at developing and testing a simple temperature and water stress model for simulating the effect of these climate variables on the growth and yield of soybean. The experiment was set in a Split Plot Design with three average environmental conditions as main plots: E1 (36 oC, RH = 55 %), E2 (34 oC, RH = 57 % ) and E3 (33 oC, RH = 44 %) resulting in VPD values of 2.7, 2.5 and
3.0 kPa for E1, E2 and E3, respectively. Additionally, there were three water treatments: W1 (near saturation), W2 (Field capacity) and W3 (Drought) and two soybean varieties (Afayak and Jenguma) were used in the study. These treatments were replicated nine times.
The results showed that high temperature environment (E1) accelerated soybean development particularly towards flowering. The days from emergence to flowering were 37, 38 and 40 for Afayak (V1) for environments E1, E2, and E3, respectively. In
the case of Jenguma (V2), the days from emergence to flowering were 39, 40 and 41 for E1, E2 and E3, respectively. The cumulative evapotranspiration (ET) were 224, 208 and 185 mm for the environments E1, E2 and E3, respectively. Biomass and yield were drastically reduced under the combined effect of high temperature (E1) and drought (W3) compared to combined ambient temperature (E3) and well-watered condition (W1).
The water treatment W3 (drought) had the lowest mean pod weights of 1.29, 1.54 and
3.35 g/plant for E1, E2 and E3 respectively, while W1 (near saturation) had the highest mean pod weight. The interactive effect of environment and drought treatment (W3) was most severe under E1 and E2 giving relatively lower grain yield of 0.45 and 0.53 g/plant compared to the ambient environment E3 which had mean weight of 1.54 g/plant.
The varieties differed statistically in their responses to drought in both E1 and E2 environments with Jenguma significantly having higher yields than Afayak.
The model developed performed quite well, correctly predicting the time-course of the total dry weight (TDW) of both soybean varieties under the range of temperature and soil water conditions. The final seed weights were also well predicted. In general, the agreement between the predicted and observed TDW was good, with R2= 0.74 and Willmott d -index =0.9.
It was concluded that increasing environmental stresses associated with climate change would adversely affect the productivity of soybean in general, but some varieties may be more resilient. Breeding efforts should be directed to improving not only drought but also temperature tolerance.
CHAPTER ONE
1 INTRODUCTION
Background
Tropical agriculture depends largely on weather and is expected to remain so for many years, because irrigation development rate continues to be slow. Therefore, climate change, defined as persistent changes in climate variables (IPCC, 2014), is likely to affect crop growth and productivity. The available evidence shows that the average air temperatures have increased all over the world with the mean ambient temperature universally projected to rise between 1.4 and 5.8 oC in the 21st century (United Nations Environmental Programme, 2006). This may have the potential to negatively impact important agronomic crops, including soybeans (Hatfield et al., 2011). It is expected that climate change would also lead to the reduction in mean rainfall as well as increased frequency of droughts in many locations (IPCC, 2014), which together with increasing temperatures would have large negative effects on crops (Schlenker and Lobell, 2010; Roudier et al., 2011). When temperature increases beyond the optimum for crop growth and development leading to negative effects on crops, the phenomenon is referred to as heat stress (Zrobek-Sokolnik, 2012), which in combination with other factors would alter crop’s lifecycle and impact on growth and development. The negative impact of increase in temperature on plant development and growth can be attributed to several reasons. First, the development rate is accelerated, reducing the overall life cycle of the plants leading to reduction in size, shorter reproductive duration and reduced yield (Hatfield & Prueger, 2015). Second, plant respiration rate increases with temperature (Paembonan et al., 1992) and would lead to the reduction of net
assimilate accumulation. The combination of these two effects, even when other factors are non-limiting would decrease the overall growth of plants.
Furthermore, if rainfall reduces under climate change, soil water replenishment and availability is also reduced. This would negatively impact crop growth because the increased vapour pressure deficit and evapotranspiration demand associated with increased temperature cannot be met under low soil water availability. The plant will hence be water stressed.
The simultaneous occurrence of reduced precipitation and increased temperature has been speculated to be more extensive in the future leading to lengthened drought periods (Field et al., 2014).
Since crop yields are particularly sensitive to water availability at the reproductive growth stage (Merah, 2001; Kato et al., 2008), the occurrence of extreme droughts at this critical development period would be detrimental to crop growth and food security worldwide (Adams et al., 1998; Olesen and Bindi, 2002). The understanding of plants response to changes in climate is, therefore, vital for improving food availability to the global population and this continues to be of research interest.
Problem statement
Soybean (Glycine max L.) is ranked as the sixth widely cultivated agricultural crop globally (FAOSTAT, 2016). Soybean is grown in 102 countries all over the world, with an approximated total area of land of over 92.5 million hectares and more than 217.6 million metric tonnes of production (FAOSTAT, 2010). In Ghana, soybean is grown predominantly in the northern region with an average farm size of 1.4 hectares (Plahar, 2006) and is intended to be exported as a cash crop and at the same time to supplement
farmers’ food needs (Aoyagi, 2007). Soybean is universally recognised as a legume and oil seed crop, it is also a quality source of protein for human consumption and also used as biofuel and livestock feeding (Masuda and Goldsmith, 2009). According to El Agroudy, et al. (2011), soybeans contain 30 % oil which is cholesterol free, 40 % protein and also contain most essential vitamins required by human beings. Goldsmith et al. (2008) also reported that only 2 percent of the protein found in soybean is consumed by humans in the form of food products. Soybean crop has the potential of improving three important sectors of Ghana’s economy, viz; agriculture, health, and industry (Plahar, 2006).
Temperature is a key environmental factor that affects soybean development and growth. The intensity (temperature in oC), duration, and the rate at which temperature increases determines the severity of heat stress (Sung et al., 2003; Wahid et al., 2007), and also the stage of crop development (Prasad et al., 2008) with the reproductive stage being more prone to heat stress effect than the vegetative stage. Studies have shown that flowering and seed filling periods which represent the reproductive phase of crop development are the most sensitive growth stage of crops to heat stress (Singh et al., 2010; Teixeira et al., 2013). Research has also shown that there may be varietal differences in the tolerance to heat stress.
With respect to water availability, both greenhouse experiment and field studies have shown that water stress leads to a notable reduction (24 %–50 %) in soybean seed yield (Frederick et al., 2001; Sadeghipour and Abbasi, 2012). Considerable efforts have been made to enhance drought tolerance in soybean, with the primary goal being to enhance yield under drought conditions.
The combined effect of high temperature and drought has more detrimental effects on yield and grain number as compared to their individual effects (Prasad et al., 2011). Understanding the combined effects of heat and drought stress on plants is paramount since the future climate is projected to be characterized by frequent incidence of increased temperatures and reduced precipitation (Hartfield et al., 2011).
Associated with temperature and soil water variation is the changes in relative humidity. Under high soil water conditions, the relative humidity of the air is likely to rise, reducing transpiration rates and overall growth. On the other hand, dry spells would also reduce the relative humidity and increase the potential evapotranspiration rate, aggravating drought effects. The way the soybean crop would respond to such climate change induced environmental conditions remains largely unknown. Furthermore, possible differences in varietal response is not well documented.
Crop models offer opportunities for predicting the impacts of varying weather conditions on plant growth. Examples include DSSAT-Legume models (Jones et al., 2003), APSIM (McCowan et al., 1996), among others. Though these models have been validated under ambient temperature and rainfall conditions in Ghana (MacCarthy et al., 2017), the performance of these models under extreme temperature and rainfall conditions has not been investigated in Ghana. This study therefore seeks to derive temperature and water stress functions that can be included in crop models to improve their efficiency in modelling crop growth under such extreme weather conditions.
Given the paucity of information on the possible climate change effects on soybean production in Ghana, this study is designed to shed further light on the climate change- soybean productivity nexus, especially under extreme conditions.
Objectives
The purpose of this study is to evaluate the response of two soybean varieties to varying temperatures and soil moisture regimes. This study is designed to:
- determine the effect of increased temperature, relative humidity and reduced soil water on the phenological development of the crop,
- determine how temperature, relative humidity and soil water affect the biomass and grain yield of the crop, and
- develop a simple temperature and water stress model for simulating the effect of extreme weather conditions on the growth and yield of soybean.
