Introduction

The global mean surface temperature increased by 0.56-0.92 ^°C between 1906 and 2005. In addition, it has been predicted that the global average surface temperature will increase by 0.6-2.5 ^°C over the next fifty years, and by 1.1-6.4 ^°C during the next century, although it is also predicted that there will be significant regional variation in these temperature changes.¹ In the last 100 years, the mean surface temperature in China has shown an increase of 0.4-0.6 ^°C,² and it is predicted that the average surface temperature in China will rise by 1.7 ^°C in the next thirty years and by 2.2 Cover the next fifty years.³

Increases in temperature could have many effects on crop growth and ecosystems. For example, increasing temperatures will result in increased evaporation, which is likely to cause a reduction in soil moisture in many regions. As a result, the growth stages of many crops will be shortened, albeit to different extents.^4–567 Indeed, there is now strong evidence that crop yields will decrease by 5%-10% in China by 2030 as a result of climate changes.^8,9

Global warming will likely aggravate the effects of diseases and pests on agricultural production. Specifically, the occurrence of wheat rust, wheat sharp eyespot and powdery mildew, as well as armyworms, beet webworms, cotton bollworms, wheat aphids, green wheat mites and other diseases and pests are all expected to respond to climate change. Warm winters increase the winter survival rates of crop and forest diseases and pests, which can in turn aggravate crop production. Warmer conditions in the spring are also advantageous to the occurrence and reproduction of diseases and pests, while the aridity and infrequent rains in spring are extremely beneficial to the reproduction of wheat aphids, green wheat mites and other similar insect pests.¹⁰

The pea (Pisum sativum L.) is a major rotation crop in semiarid regions of China, and its sown area has reached approximately 20% of the total crop area. The pea is often incorporated in rotation sequences such as pea–spring wheat–potato, pea–winter wheat–potato, spring wheat–pea–spring wheat–potato, and winter wheat–pea–potato–millet. Crop rotation may be more effective than monoculture for minimizing soil erosion, improving water use efficiency and maintaining high yields.^11,12 In addition, crop rotation could affect the water availability for crop growth by changing infiltration and evaporation rates.¹³ Although the benefits of crop rotation on the quantity and quality of soil organic matter are well documented,^14–1516 it is possible that a wide range of adaptations to climate change exist in crop rotations. Such adaptation may enable farmers to maintain or increase crop yields under future climate change.¹⁷ Consequently, farmers are able to respond to changes in environmental conditions by choosing the most favorable crops, cultivars, and cropping systems. Therefore, assessment studies should indicate which strategies have the best chance of success in the future and which specific climate conditions represent a threshold for adaptation.¹⁸ However, in the last 20 years, climate change has already impacted the growth and development of the pea.¹⁹ Therefore, it is vital for us to identify the most likely impacts of climate warming so that practical and effective counter-measures can be applied to ensure the continued efficiency of future crop production.

Here, we present the relationship between the growth stages and yields of pea crops and temperature increases as part of a long-term research effort aimed at development of sustainable agricultural practices. The results of this study indicate that it is necessary to identify the potential consequences of global warming and the methods best suited to maintaining pea production. The purpose of the present study was to evaluate the effects of temperature increases (0.6-2.2 ^°C) on pea yield and to predict the potential changes in crop yields that will occur in response to global warming.

Materials and Methods

Experimental Design and treatments

A field experiment on pea was conducted at Tongwei Experimental Station during 2007-2008. The experiment was subjected to four levels of temperature increase and two levels of supplementary irrigation, which resulted in eight different experimental treatments (Table 1). Each plot in all treatments was 6.0 m long and 2.4 m wide with 16 rows of pea. Each treatment was replicated in three randomized complete blocks. In this experimental field, available nitrogen (N) is 36.8 mg kg^-1 and total N is 89.3 mg kg^-1. Available phosphorus (P) is 5.22 mg kg^-1 and total P is 28.4 mg kg^-1. Organic matter content is about 10 g kg^-1.

The pea (Pisum sativum L.) is an annual plant that is sown in the middle ten days of March and harvested in the middle ten days of July every year in this region. In general, the growing period from seedling emergence to harvest is 110∼120 days. The pea is a multi-flower and multi-pod plant, with each individual plant containing 8∼10 pods and each pod containing 5∼7 seeds.

Dingwan 2 is a typical variety of pea that was designed to be sown on March 15 in 2007 and 2008 at a density of 600,000 seeds ha^-1. After sowing, an electric wire was arranged on the soil surface between two rows of peas to induce a temperature increase of 0.6-2.2 ^°C, which was maintained from sowing to harvest. The increase in mean daily temperature was monitored continuously through a Computer-Temperature Control System. In addition, 60 mm of supplementary irrigation was applied during the sowing and flowering stages using a drip irrigation system. Furthermore, 1000 kg ha^-1 of ammonium nitrate (NH₄NO₃) was applied during the sowing and flowering stages. Finally, soil samples were collected from individual plot using an auger (2.5 cm diameter and 20 cm deep) and then analyzed for the soil water content. The pea plots were harvested from July 11 to 28 in 2007 and 2008.

Table 1.

The eight treatment combinations in the open-top chamber (OTC) pea experiment at Tongwei Experimental Station.

Results

Duration of the Growth Stage

The results of treatments T1-T4 revealed that an increase of 2.2 ^°C led to a decrease in the duration of the growth stage of the pea crop of approximately 17 days. In addition, an increase in the mean daily temperature of 1.4 ^°C led to duration of the growth stage of the crop being shortened by 10 days. Taken together, these findings indicate that a temperature increase will shorten the duration of the growth stage of pea crops (Table 2).

The results from treatments T5-T8, which receive 60 mm of supplementary irrigation, revealed that a 2.2 ^°C increase in temperature led to a 17 day reduction in the growth stage of the pea crop. In addition, an increase in the mean daily temperature of 1.4 ^°C combined with 60 mm of supplementary irrigation resulted in the duration of the growth stage of the crop being reduced by 11 days. Taken together, these findings show that application of 60 mm of supplementary irrigation will not change the duration of the growth stages of pea crops (Fig. 1).

Table 2.

Changes in the growth stage durations of peas in response to different temperature increases for all treatments (days).^a,A

Yields and Their Yield Components

Grain outputs from the pea crops in T3 and T4 were 11.0% and 17.5% lower than those in T1, respectively (p < 0.01). When compared with T5, the yields in T7 and T8 were 11.1% and 14.1% lower, respectively (p < 0.01) (Table 3). These findings indicate that an increase in temperature will change the pea yield.

The pea yield in treatments T1-T4 was lower than that of treatments T5-T8. In addition, the results of this study indicate that the addition of 60 mm of supplementary irrigation combined with a 0.6-2.2 ^°C increase in mean daily temperature would result in an 8.3%-12.8% improvement in crop yields. These findings indicate that supplementary irrigation can play an important role in improving the pea yield when the temperature increases.

Figure 1

Changes in the growth stages of peas in response to different temperature increases in treatments T1-T4 and T5-T8.

Taken together, the aforementioned results indicate that the pea yields are affected by both an increase in temperature and supplementary irrigation. To further evaluate these effects, we applied regression analysis to the relationship between temperature increase and yield decrease. The results of this analysis revealed that a non-linear negative relationship exists between the yield decrease (Y) and temperature increase (X) for treatments T1-T4 (Y = 54.05e^0.64X, r=0.98). In addition, a non-linear negative relationship also exists between the yield decrease (Y) and temperature increase (X) for treatments T5-T8 (Y = 83.45e^0.38X, r = 0.94) (Fig. 2).

The Number of Stems with Root-Rot Sickness

The ratio of Sr and Sn in T3 and T4 was 23.3% and 50.6% higher, respectively, than in T1 (p < 0.01). Additionally, The ratio of Sr and Sn in T7 and T8 was 24.4% and 51.7% higher, respectively, than the ratio of Sr and Sn on plants in T5 (p < 0.01) (Table 4).

Table 3.

Pea yields and their yield components for all treatments.^a,A

These results indicate that a temperature increase will lead to a significant increase in the number of stems with root-rot sickness.

The number of stems with root-rot sickness in treatments T1-T4 did not differ significantly from that of plants in treatments T5-T8. This result indicates that supplementary irrigation will not play an important role in improving the number of stems with root-rot sickness if the temperature increases. In addition, the results of this study also indicate that the number of stems with root-rot sickness is associated with an increase in temperature as well as supplementary irrigation. To further evaluate these effects, we applied regression analysis to the relationship between temperature increase and the number of stems with root-rot sickness. The results of this analysis revealed that a non-linear negative relationship exists between the number of stems with root-rot sickness (Y) and the increase in temperature (X) for treatments T1-T4 (Y = 1.74e^1.28X, r = 0.92). Furthermore, a non-linear negative relationship also exists between the number of stems with root-rot sickness (Y) and temperature increase (X) for treatments T5-T8 (Y = 3.87X² + 0.30X-1.58, r = 0.96) (Fig. 3).

Figure 2

Relationship between yield decreases of peas and temperature increase in treatments T1-T4 and T5-T8.

Input-Output Analysis

The input-output ratios of T3 and T4 are 1.11 and 1.20, respectively, which is higher than that of T1. Furthermore, the results from treatments T1-T4 indicate that if the mean daily temperature increases by 0.6-2.2 ^°C, the In/Ou ratio of the pea group will be greater than 1.0. Taken together, these results show that an increase in temperature will change the In/Ou ratio of pea plants (Table 5).

Table 4.

Number of stems with root-rot sickness for all treatments.^a,A

The input-output ratios of T7 and T8 are 1.08 and 1.12, respectively, which is higher than that of T5. These findings indicate that if the daily temperature increases by 0.6-2.2 ^°C and crops are irrigated with 60 mm of supplementary irrigation, the In/Ou ratio of pea plants will be greater than 1.0. Taken together, these results demonstrate that application of 60 mm of supplementary irrigation will not decrease the In/Ou ratio of pea plants that are being subjected to a 0.6-2.2 ^°C increase in temperature.

Discussion

EPIC (The erosion/productivity impact calculator) and DSSAT (The decision support system for agro-technology transfer) model projections suggest that, although increased temperature and decreased soil moisture will act to reduce global crop yields by 2050, the direct fertilization effect of rising CO₂ will offset these losses.²¹ However, the CO₂ fertilization factors used in models to project future yields were derived from enclosure studies conducted approximately 20 years ago. In addition, most information regarding crop responses to elevated CO₂ and temperature has been obtained from studies conducted in greenhouses, laboratory controlled-environmental chambers, and transparent field chambers, where CO₂ concentrations and temperature may be easily controlled. Free-air concentration enrichment (FACE) technology has now facilitated large-scale trials conducted to evaluate major grain crops subjected to elevated CO₂ levels under fully open-air field conditions.²² Open-top chamber (OTC) facilities are less costly than FACE experiments and have been used to evaluate plant responses to ozone and other air pollutants.²³ More recently, OTCs were also used in multi-site experiments to evaluate the combined effects of elevated CO₂ and ozone concentrations and/or changes in water supply related to changes in weather on wheat.²⁴ However, OTCs modify growing conditions, primarily by increasing the air temperature and reducing the incident solar radiation.^25,26 In this study, the field experiment was used to evaluate the effects of increases in air temperature on water use and the yields of pea plants.

Figure 3

Relationship between the number of stems with root-rot sickness and temperature increase in treatments T1-T4 and T5-T8.

Table 5.

Input–output analysis of pea production with different temperature increases for all treatments.^a,A

The effects of temperature increase on the growth and yields of crops have become a major research theme that has led to some significant breakthroughs and advances in the wider field of global climate change.^27–272930 Indeed, many studies have reported decreased dry matter production and decreased yields of annual crops in response to only an increase in temperature.^31,32 According to the results of these studies, a constant 16% and/or 49% decrease in crop yield occurs due to an average temperature increase of 1.4 ^°C and/or 4.2 ^°C. In addition, the growth stage of rain-fed spring wheat is known to decrease by 8-10 days in response to an average increase of 1.0 ^°C in temperature. Warming generally reduces wheat yield, probably due to a shorter grain filling period caused by more rapid development.³³ The aforementioned studies were conducted to evaluate the growth and yields of crops as a monoculture over the course of a year. However, the effects of global warming on the growth and yields of the pea over several years have not yet been reported.

Figure 4

Water use efficiency of pea plants in treatments T1-T4 and T5-T8. WUE is the water use efficiency. ^a,AMeans within columns followed by different letters are significantly different (p < 0.01). “a” is significantly different from “b” and “‘A” is significantly different from “B”.

In conditions where high temperature is accompanied by staggered drought, the population and regeneration rate of diseases and pests is showing an increasing trend, which often results in the mass occurrence of diseases and pests. Due to climate warming, the period during which diseases and pests occur and reproduce is prolonged, which results in the growth and development rates of eggs and ova being accelerated. Therefore, the duration required for one generation to reproduce is shortened, resulting in an increased generation rate.³⁴ In this study, when the mean daily temperature increased by 1.4 ^°C or 2.0 ^°C, the number of stems with root-rot sickness increased significantly, by 23.3%, or 50.6%, respectively. These results indicate that a temperature increase will significantly increase the number of stems on pea plants with root-rot sickness. This combination of an increase in root-rot and a decrease in yield will provide a new challenge for future pea production and crop rotation systems in the semiarid regions of China.

The response of agricultural systems to future climate change also depends on management practices, such as variations in the types and levels of water and nutrients being applied.³⁵ The annual mean rainfall decreased by approximately 60 mm from the 1950s to the 1990s in semiarid regions of China, during which time the evaporation also increased by 35-45 mm due to an increase in temperature.³⁶ Additionally, rainfall and available soil moisture during the complete growing stage of crops were reduced by approximately 100 mm in the 1990s when compared with the 1950s. Therefore, the study of soil moisture is an important theme in agriculture that addresses the effects and responses to temperature increase. However, it may be necessary to study the effects of micro-irrigation systems on crop yield under global climate change to find a practical and effective way for improving future crop production.³⁷ This study has shown that supplementary irrigation using micro-irrigation technology can improve pea yields when temperatures increase. Specifically, supplementary irrigation with 60 mm of water applied over the entire growth stage of the crop was found to improve crop yields by 8.3%-12.8% when coupled with an increase of 0.6-2.2 ^°C in mean daily temperature.

Conclusions

In this study, the mean daily temperature was increased by 0.6-2.2 ^°C over the entire growth stage of the pea. When the mean daily temperature was increased by approximately 2.2 ^°C, the water use efficiency (WUE) of the pea decreased by 30.4%, the duration of the growth stage was shortened by approximately 17 days, the crop yield was decreased by 17.5%, and the number of stems with root-rot sickness increased by 50.6%. When the mean daily temperature was increased by about 1.4 ^°C, the WUE decreased by 26.1%, the duration of the growth stage of the crop was shortened by 10 days, crop yields were decreased by 11.1%, and the number of stems with root-rot sickness increased by 23.3%. In addition, supplementary irrigation was beneficial to the improvement of pea yields when accompanied by a temperature increase. Specifically, application of 60 mm of supplementary irrigation during the complete growth stages of the crop along with an increase of 0.6-2.2 ^°C in mean daily temperature resulted in an increase in crop yields of 8.3%-12.8%. However, the results also revealed that application of 60 mm of supplementary irrigation will not decrease the number of stems with root-rot sickness.