PLANT ALLOCATION TO DEFENSIVE COMPOUNDS: INTERACTIONS BETWEEN ELEVATED C02 AND NITROGEN IN TRANSGENIC COTTON PLANTS

Authors: COVIELLA, CARLOS - UNIV. OF CA-RIVERSIDE; Stipanovic, Robert - Bob; TRUMBLE, JOHN - UNIV. OF CA-RIVERSIDE

Submitted to: Journal of Experimental Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/21/2001
Publication Date: N/A
Citation: N/A

Interpretive Summary: The world is currently experiencing higher levels of carbon dioxide than in the recent past. How this may effect the interaction between insects and plants is not clear. We compared cotton plants grown at normal and high levels of carbon dioxide and at low and high levels of nitrogen fertilizer. Transgenic plants that produce the Bt toxin used to control cotton insects were compared to non-transgenic plants. We found less of the Bt toxin in plants grown at high levels of carbon dioxide when high levels of nitrogen fertilizer were used; at low levels of nitrogen fertilizer the amount of Bt toxin was not different. Compounds that occur naturally in the plant to deter insect feeding were higher at low nitrogen levels at both normal and high levels of carbon dioxide. Insects feeding on the plants grew slower when carbon dioxide levels were higher and nitrogen fertilizer levels were low. These results show that at low nitrogen levels insects will not grow as well as carbon dioxide levels increase.

Technical Abstract: Patterns of plant allocation to defense against herbivory are crucial to understanding plant-insect interactions. We examined plant allocation to defensive compounds grown at elevated and normal atmospheric CO2 in combination with two levels of nitrogen availability. The effect on defensive allocation patterns in transgenic and non-transgenic plants was determined. The carbon-based defensive chemistry was assessed, including total phenolics, condensed tannins, and terpenoid aldehydes. In addition, half the plants were from a transgenic line expressing a gene for the Bacillus thuringiensis (Bt) toxin and the other half were from a near isogenic line without the Bt gene. We included transgenic plants to assess allocation to a nitrogen-based compound (Bt toxin) which did not evolve in cotton, and to determine how such transgenic constructs might affect total defense allocation. All resulting combinations of plants were bioassayed against the herbivore Spodoptera exigua (Hubner). Plants had lower N concentrations and higher C/N ratios when grown in elevated CO2. All carbon-based defensive compounds increased under elevated CO2, low N availability, or both. The increase in carbon-based compounds under elevated CO2 and low N availability, adversely affected growth and survival of S. exigua. The production of the nitrogen-based toxin was affected by a significant interaction between CO2 level and N availability; elevated CO2 decreased N allocation to Bt toxin, but this could largely be mediated by the addition of additional N to the soil. The herbivore bioassay showed that the significant decrease in Bt toxin production was not enough to differentially affect non-target lepidoptera such as S. exigua.