Negative Effects of Increased Temperature and O3 Offset Positive Effects of CO2 in Oilseed Rape (Brassica napus L.)

CO₂ in the atmosphere is steadily increasing and is predicted to be 500–1000 ppm by the end of the century. Emissions of other greenhouse gases are also increasing and are expected to raise the surface temperature 1.8–4.0^o C, along with increasing emissions of ozone (O₃) from human activity. These forces, and many others, naturally act together and have an important effect on agricultural productivity and climate change. It is more useful and practical to study the effects of multiple, layered factors of climate change than to study one factor in isolation. For example, increased CO₂ alone will increase the photosynthetic rate in plants, increasing biomass production, and result in positive growth for plants. However, this increase in biomass does not necessarily lead to an increase in crop yield. It is important to combine and test the effects of various factors of climate change on crops to determine potential crop yield because with a growing human population, maximizing crop yield is highly desirable. Plants do not have many natural adaptations for living in conditions with increased CO₂, so it is important to study how they react in order to better breed and genetically prepare plants for climate change.—Taylor Jones

Frenk, G., Van der Linden, L., Mikkelsen, T. N., Brix, H., Jorgensen, R. B., 2011. Increased [CO₂] does not compensate for negative effects on yield caused by higher temperature and [O₃] in Brassica napus L. European Journal of Agronomy 35, 127–134.

          Frenk and colleagues controlled the ambient conditions of four cultivars of oilseed rape (Brassica napus L.) of different ages and origins and exposed each cultivar to a different combination of increased CO₂ (700 ppm), increased temperature (+5^oC), and increased O₃ (60 ppb). The plants were raised in growth chambers and at the end of maturation, ten plants from each cultivar were selected at random to study. The pod, stem height, and stem width were recorded along with seed yield, stem weight, and biomass. The Thousand Seed Weight (TSW) and Harvest Index (HI) were determined for each sample.

          Increased temperature alone generally reduced the seed yield by 38–58%, the total number of seeds, and the mass of seeds and pods. Despite these general trends, variability among cultivars only produced a significant difference in seed yield for two of them. Stem biomass was not significantly different with increased temperature, and only one cultivar showed a difference in stem weight. The low total seed yield also reduced the HI. The authors predict the decrease in biomass typically associated with increased temperatures is due to reduced rates of photosynthesis, quick development, increased respiration, and decreased organ development. Plant breeding today is often focused on yield, so these new plants will likely be the most susceptible to climate change and the negative effects of increased temperature.

          Increased CO₂ alone resulted in a general increase in total seed yield (only significant for one plot) and the total number of seeds. Stem height increased for all cultivars and biomass increased in general, but was only significant for one cultivar. Frenk et al. predict that the effects of increased CO₂ can be offset over time because the plant does not have enough storage organs and has reduced carbon sink capacity.

          Increased O₃ alone had no effect on plant yield or stem weight, but combined with temperature, O₃ further reduced the positive effects of increased CO_2, and further decreased yield. When CO₂ and temperature both increased, they equalized the effects of one another and the sample resembled the control in biomass growth and yield. According to Frenk and colleagues, no study to date has examined the combined effects of the three factors discussed above on agricultural productivity and the results show significant changes in agricultural productivity and should be combined with more abiotic and biotic factors in the future to determine the full effects of increased CO₂.