Overcoming Abiotic and Biotic Stress

Stress in plants can be of two types:

(1) Abiotic - arising from fluctuations in their physical or chemical environment (such as cold, heat, excessive light, elevated salt levels in soil, ozone in air, oxygen deficiency, flooding or drought, or mechanical damage); or

(2) Biotic - imposed by insect pests, weed and pathogens (fungi, viruses, bacteria).

Most yield losses, however, are attributed to abiotic stress.

The response of Trimax-treated (Trimax is an optimized imidacloprid formulation) barley to pure abiotic stress stimuli was investigated in detail in order to elucidate the underlying physicochemical and biochemical mechanisms. In this case, a significant leaf-area growth improvement following imidacloprid soil application could be shown after short-term drought stress. Plants from these trials were analysed at different elapsed time intervals using DNA microarrays (barley chip consisting of 400 000 expressed sequence tags with information on 22 840 barley genes), a tool for profiling gene expression in plants. After imidacloprid treatment, results from the DNA microarray experiments showed three different plant reactions to drought stress:

(1) The expression level of drought-stress marker genes in barley was significantly delayed, suggesting a mitigation of drought stress;

(2) Photosynthesis-related genes were simultaneously expressed at a higher level (energy production was ongoing), whereas in untreated plants photosynthesis declined more rapidly; and

(3) In contrast to non-treated plants, numerous pathogenesis-related proteins were found to be overexpressed, explaining field observations of synergistic fungicidal and bactericidal effects.

Imidacloprid treatment led to a considerable reduction in yield losses by drought stress compared with other neonicotinoids, such as thiamethoxam, as established in a pepper field in Georgia.102

Since most field-grown cotton is sensitive to abiotic or biotic stress at various times throughout the seasons, the application of neonicotinoids may have a widespread benefit if upcoming stress could be timely detected in field crops to allow for targeted applications.

In 2004, water-deficit field studies confirmed the potential of Trimax to moderate water stress in plants with an average lint yield increase in cotton of 10 percent.103,104

Later on, in field trials with broad strip side-by-side aerial applications of Trimax Pro/Provado (imidacloprid) and Centric (thiamethoxam), the abiotic stress (drought, salinity and hypoxia caused by flood irrigation) reducing potential of both neonicotinoids and their impact on lint yield were elucidated

Figure 4.12 Field trials with broad strip side-by-side aerial applications of Trimax Pro/Provado (imidacloprid) and Centric (thiamethoxam); assessment of plant vigor by remote sensing of Near Infrared (NIR) images and Normalized Difference Vegetation Index (NDVI) images analysis: (A) typical NDVI variability (vigor legend: blue = very high, green = high, yellow = medium, red = low, pink = very low); and (B) typical field plot design (adapted from Zelinski and Thielert, 2008).105

Figure 4.12 Field trials with broad strip side-by-side aerial applications of Trimax Pro/Provado (imidacloprid) and Centric (thiamethoxam); assessment of plant vigor by remote sensing of Near Infrared (NIR) images and Normalized Difference Vegetation Index (NDVI) images analysis: (A) typical NDVI variability (vigor legend: blue = very high, green = high, yellow = medium, red = low, pink = very low); and (B) typical field plot design (adapted from Zelinski and Thielert, 2008).105

using remote sensing of reflectance of Near Infra Red (NIR) images and Normalized Difference Vegetation Index (NDVI) images analysis (see Figure 4.12).105

The NIR (800 nm) and red light (660 nm) can be used to measure plant stress levels. So-called healthy cotton plants reflect very little red light due to absorption by their chlorophyll. Therefore, the normalized ratio of NIR to red light; can be a used as a measure of plant stress.

Data suggest a superior stress mitigation performance of Trimax Pro/Provado in cotton over treatments by Centric. This statistical design allows users to investigate plant growth effects across a range of stress levels both quickly and accurately.

All results clearly indicate that, in addition to its insecticidal activity, a stress shield MoA (mode of action) of imidacloprid supports plants in moderating the effects of abiotic and biotic stress by its interaction with the NAD-salvage pathway. As a trigger of these effects of imidacloprid, its major metabolite, 6-chloro-nicotinic acid (6-CNA; see section 4.3.1; Figure 4.2), is discussed as a systemic plant inducer which possibly causes physiological changes in the plant that result in stress protection.

On the other hand, thiamethoxam-treated plants are more tolerant toward abiotic stress factors. They can grow more vigorously under suboptimal conditions. However, this vigor effect leading to abiotic stress mitigation is only a sub-set of the whole stress shield (mitigation of abiotic and biotic stress). The bioisosteric metabolite, 5-chloro-1,3-thiazole carboxylic acid (2-CTA; see section 4.3.1; Figure 4.2), could be a trigger of these effects as well.

Finally, the interaction of neonicotinoids like imidacloprid (Trimax) with plants to moderate abiotic and biotic stress leads to the conclusion of a second MoA on top of the well known direct neurotoxic MoA against insect pests, supporting plants to achieve higher yields and better quality under adverse growth conditions.

The use of the abiotic and biotic stress mitigation potential of neonicotinoids, especially during critical crop periods (e.g. emergence, transplanting, flowering, fruiting), and the additional synergies with fungicides/bactericides and virus reduction offers sustainable solutions in modern crop protection.

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