Stress magnitude matters: different intensities of pulsed water stress produce non‐monotonic resistance responses of host plants to insect herbivores

Abstract

1. Water stress may increase or reduce the suitability of plants for herbivores. The recently proposed 'pulsed stress hypothesis' suggests consideration of stress phenology (pulsed vs. continuous stress) to explain these conflicting effects of plant water stress on herbivore performance.2. This hypothesis was tested for the effect of differing stress intensity on performance and preference of insect herbivores belonging to different feeding guilds, namely leaf-chewing insects ( Spodoptera littoralis caterpillars) and phloem-feeding insects ( Aphis pomi aphids), on apple plants ( Malus domestica ). The plants were non-stressed or exposed to a low or high intensity of pulsed water stress.3. Plant responses to the different stress levels were generally monotonic. Growth, stomatal conductance ( g s ), leaf water, and old-leaf nitrogen concentration decreased, whereas young-leaf nitrogen concentration and leaf mass per area (LMA) increased with increasing stress intensity. The stable isotope composition of foliar carbon ( ␦ 13 C) responded non-monotonically to the drought treatments. The ␦ 13 C values were highest in low-stress plants, intermediate in high-stress plants, and lowest in non-stressed plants.4. The preference and performance responses of the caterpillars were also non-monotonic. Non-stressed plants were intermediately, low-stress plants least, and high-stress plants most attractive or suitable. Aphid population growth was highest on non-stressed plants and lowest on low-stress plants.5. The results highlight the importance of water stress intensity for the outcome of interactions between herbivores and drought-affected plants. They show that pulsed water stress may enhance or reduce insect herbivore performance and plant resistance, depending on stress intensity.

Within-plant distribution of induced resistance in apple seedlings: rapid acropetal and delayed basipetal responses

“…It thereby provides the first record of systemically induced direct resistance responses to insect herbivores for apple plants and contributes to completing the picture of factors affecting direct apple resistance to insect herbivores (Stoeckli et al 2008;Mody et al 2009;Stoeckli et al 2009). Induced resistance showed a clear temporal pattern and strong within-plant variation.…”

Species-specific responses of herbivores to within-plant and environmentally mediated between-plant variability in plant chemistry

“…The remaining leaf material was immediately flash frozen and used for chemical analysis. Leaf discs were offered to single (L2-L3) insect larvae in triple-choice or dual-choice feeding tests and followed the procedure described by Mody et al (2009). Choice tests were conducted under standardized greenhouse conditions in plastic Petri dishes of 9 cm diameter, containing a moist filter paper on top of a 3-mm thick styrofoam layer.…”

Growth, herbivore distribution, and herbivore damage of timber trees in a tropical silvopastoral reforestation system

“…Leaflets were ground separately into a homogenous powder using a Retsch ball mill (MM 200; Retsch GmbH, Haan, Germany) to quantify total nitrogen content and δ 15 N ratios, which served to test whether the timber tree T. rosea accumulates nitrogen fixed by the companion tree G. sepium (Daudin and Sierra 2008). Additionally, δ 13 C ratios were measured to assess drought stress experienced by T. rosea (Mody et al 2009). Nitrogen content, δ 15 N values, and δ 13 C values were determined using a Flash EA 1112 Series elemental analyzer (Thermo Italy, Rhodano, Italy) coupled to a Finnigan MAT Delta plus XP isotope ratio mass spectrometer (IRMS; Finnigan MAT, Bremen, Germany) via a six-port valve and a ConFlo III, following the procedure described by Werner and Brand (2001).…”

“…C values increase along with increasing water shortage (Gutbrodt et al 2012) unless drought intensity is very severe (Mody et al 2009). Moreover, leaf mass per area (LMA) is positively related to irradiance (DaasGhrib et al 2011).…”