Cue hierarchy for host plant selection in Pieris rapae

During oviposition, female insects utilize a wide variety of cues to find an optimal host for their offspring. These cues may be encountered simultaneously or sequentially, and females must rely on a hierarchical categorization to rank cues for optimal ovipositional choice. During alighting, cues are often encountered at different distances, which may influence hierarchical order. Cues that are observed at longer distances and are experienced first may be more influential on oviposition choice than those experienced at shorter distances. We tested the effects of two long‐distance cues, plant size and habitat structure, and two short‐distance cues, plant nitrogen level and predator presence, on Pieris rapae L. (Lepidoptera: Pieridae) oviposition choice through a series of two‐ and four‐choice tests in both greenhouse and field settings. We found that the long‐distance cues of plant size and habitat structure had the largest impact on P. rapae oviposition in laboratory and field settings, with females preferring large plants without habitat structure. Plant nitrogen level had a smaller impact on oviposition, and the presence of predators did not affect oviposition choice. However, plant size and plant nitrogen level had a synergistic relationship, with more eggs laid on large high‐nitrogen plants compared to large low‐nitrogen or small high‐nitrogen plants, suggesting that optimal cues throughout the entire searching process may be important for P. rapae in choosing a host plant.     

Daily Irradiance and Feedback Inhibition of Photosynthesis at Elevated Carbon Dioxide Concentration in Brassica oleracea

The fundamental cause of down-regulation of photosynthesis at elevated carbon dioxide concentration (EC) is thought to be a slower rate of utilization of saccharides than their stimulated rate of production, but there are few studies directly supporting this idea under field conditions. We hypothesized that within Brassica oleracea, down-regulation would not occur in kohlrabi because it has a large sink for saccharides in an enlarged stem, but would occur in collards, which lack this sink. Field tests were consistent with this hypothesis. In collards, the degree of down-regulation of photosynthesis in plants grown at EC varied depending on the daily integral of photosynthetically active radiation (PAR) of the day prior to the measurement of photosynthetic capacity, as did leaf saccharide content. However, EC did not result in lower leaf contents of chlorophyll, soluble protein, ribulose-1,5-bisphosphate carboxylase, or nitrate in collards, nor was there any evidence of a triose phosphate utilization rate limiting photosynthesis. Experiments in controlled environment chambers confirmed that there was a threshold response for the down-regulation of photosynthesis in collards at EC to the PAR of the previous day, with down-regulation only occurring above a minimum daily integral of PAR. Down-regulation of photosynthesis could be induced in plants grown at ambient carbon dioxide by a single night at low temperature or by a single day with high PAR and EC. In the controlled environment study, the degree of down-regulation of photosynthesis was highly correlated with leaf glucose, fructose, and sucrose contents, and less well correlated with starch content. Hence down-regulation of photosynthesis at EC in collards in the field represented feedback inhibition from the accumulation of soluble saccharides and day-to-day variation in its occurrence was predictable from the weather.     

A comparative study of low-temperature-induced ultrastructural alterations of three species with differing chilling sensitivities

Abstract. The effects of light and water stress upon chilling injury of chloroplasts have been assessed by electron microscopy in seedlings of three species known to differ in their chilling susceptibility. Chilling injury to chloroplasts was first manifested by distortion and swelling of thylakoids, reduction in starch granule size, and the formation of small vesicles of the envelope, called the peripheral reticulum. More prolonged treatment produced accumulations of lipid droplets, increased staining of the stroma, disintegration of the envelope, and mixing with cytoplasmic contents. Cotton, a notably chilling-sensitive plant, and bush bean, a somewhat less sensitive plant, showed damage within 6 h when exposed to both light and water stress at chilling temperatures (5°C). Even collard, a chilling-resistant species, exhibited signs of chilling injury to chloroplasts after 6 h when exposed to both light and water stress but the plastids remained intact throughout the 48 h of treatment. Comparable chilling injury does not occur in cotton until around 72 h if the plants are exposed to water stress or light separately. Bush bean was affected less by separate treatments of light and water stress. The least chilling injury occurred in all three species when they were kept in the dark at a high humidity.