The Effect Of Heat Stress

Published Date: 02 Nov 2017

Cody Pettit

Nate Lawrence

Seth Hubbell

IB 103

4/08/2013

The Effect of Heat Stress on Respiration Rate and Final Dry Biomass of Varying Growth Stages of Brassica rapa

Abstract:

This study was done to find the correlation between heat stress and growth stage of Brassica rapa (B. rapa). B. rapa plants with a genetic mutation that allows them to complete their life cycle in roughly 40 days were used for this experiment. Three sets of four individual plants were separated for the experiment as control, early and late heat shocked plants. Each set of plants were exposed to heat stress at approximately 30°C (room temperature at 26 °C) at each different life stage for a week. The significant data was that the early heat shocked plants resulted in greater dry biomass of all three sets and the respiration rates increases as the heat shock was delayed in the life stage. Some major findings were when the dry biomass of the early shock stage actually outweighed the control, the respiration rates were measured as assumed. To conclude this study, data shows that the early heat shocked plants of B. rapa performed better overall in dry biomass and came in second to the control for respiration rates.

Introduction:

The ability of plants to be resistant to heat stress is becoming a major component in agriculture today. With growing temperatures and more intense levels of drought and flooding, crops are at risk to be able to cope with their surrounding environment. Some attempts to develop heat-tolerant genotypes have been successfully done through traditional breeding methods (Wahid, Gelani, Ashraf & Foolad, 2007). Traditional breeding methods also take a long time as compared to genetically modified crops which seems to be the way of solving the heat-tolerant issue. Recently advanced techniques of genetic engineering have provided additional methods to better employ heat-tolerant plants (Wahid, Gelani, Ashraf & Foolad, 2007). All species have different upper and lower developmental thresholds which are the areas of temperature in which a plant will grow the best, when exposed to temperatures above and below these thresholds a plant will use cellular and sub-cellular techniques in order to still be able to thrive, grow and avoid senescence. These techniques can be more effective at different stages of a plants life, thus making heat stress more detrimental at different stages of a plant’s life. Proper conditions during the germinating, seedling and flower stages of a plants life are imperative to a plants growth. A study conducted in 2002 conducted on all Brassica species showed that there were severe yield losses at temperatures 29.5°C during the flowering stage (Morrison & Stewart, 2002). The study also found that high mean maximum temperatures during vegetative growth resulted in a lower flower numbers overall (Morrison & Stewart, 2002). A study in 2007 indicated that heat stresses during early seed development may delay germination and seedling growth, but in this study heat shock was represented at stages past seedling growth and development. In this study we predict heat stress induced on early vegetative Brassica rapa will have a less detrimental effect on both dry biomass, and CO2 respiration, than heat stress induced in latter vegetative stages.

Materials and Methods:

Brassica rapa plants

Lights for temperature control

Carbon-dioxide meter

Temperature difference of 4 Celsius (26° - 30° Celsius)

12

1 set (30°C)

1

Exposed to each set of plants

12 different Brassica rapa (B. rapa) plants that are genetically mutated to complete its life cycle in approximately 40 days were each exposed to a heat shock of 30°C. 3 sets of 4 individual plants were categorized into control, early and late heat shocked plants. Each set of plants were allowed 2 weeks of growing period before heat shock was introduced to any of them. The 4 plants labeled as the "control" set were grown at a constant temperature of 26°C for the entire duration of the study. At the second week 4 plants (1 set), labeled as "early shocked plants", were exposed to heat stress conditions (30°C) for one week. After a week this set was taken out and put back into normal temperature conditions (26°C). At the fourth week 4 plants (1 set), labeled as "late shocked plants", were exposed to heat stress conditions for one week. This set was then taken out after one week and grown in normal conditions. At the end of 6 weeks all the plant’s respiration rates were measure with the carbon-dioxide meter. The plants were then left out for a week to let them completely dry. Dry biomass was then calculated for each set of plants.

Results:

Different sets of plants exposed to heat shock (30°)

Groups

CO2 Rates measured (ppm/s)

Control (No shock)

.15

Early shock

.19

Late shock

.20

Different sets of plants and measured dry biomass (mg)

Groups

Dry biomass (mg)

Control (No shock)

868.081

Early shock

931.184

Late shock

679.304

The data shows that the lowest CO2 rate of the 3 groups was the control followed by the early shock and then the late shock. The final dry biomass was the greatest for the early shock plant and then followed by the control and the late shock plants.

Discussion:

The results found show the heat shock during the early stages of vegetative development show an advantage over the latter stages. The respiration rates of the early stages were less than that of the latter, showing that the early stages were more energy efficient and healthier. The final dry biomass of the early stages was also greater than that of the latter showing that more vegetative growth was produced by the early stage in its time of recovery. This study shows that the early stages of growth had more time to recover from the heat stress, thus producing more biomass while giving off less respired CO2 rates. The latter stages of growth seemed to be effected more because the time to recover was much less and the heat stress was expressed at a critical time in development as stated in previous studies. One important error in the data was that the final dry biomass of the early shocked plants was greater than the control even though the control had less respiration rates when we measured it. This could have been the result of a small sample size, few tests, shade from other student’s plants, or imprecise heat shock apparatus. In conclusion our overall results indicate that later heat shock is more detrimental than earlier heat shock  and that the heat shock decreases metabolic rate, but early heat shock may have limited impact on dry biomass.