Steven A. Weinbaum

Heteroblasty in Eucalyptus globulus (Myricales: Myricaceae) Affects Ovipositonal and Settling Preferences of Ctenarytaina eucalypti and C. spatulata (Homoptera: Psyllidae)

Abstract Heteroblasty describes plants whose juvenile and adult vegetative shoots differ morphologically. In Eucalyptus, heteroblasty is a common source of within-plant variation and an expression of ontogenetic aging that affects the within-tree distribution of psyllids. Using Eucalyptus globulus Labillardiere as a model system, we studied the reproductive behavior of adults of two Australian psyllid species (Ctenarytaina eucalypti Maskell and C. spatulata Taylor) on the glaucous, simple-shaped juvenile leaves and the glossy, sickle-shaped adult leaves under field conditions. We compared the ovipositional preferences and mating site preferences of the psyllids in caged pairs of juvenile and adult shoots, as well as the behavior of the psyllids after they landed on both types of shoots. Ctenarytaina eucalypti oviposited only on juvenile shoots and C. spatulata oviposited only on adult shoots. Ctenarytaina eucalypti mated primarily (88% of the time) on juvenile shoots, and C. spatulata mated only on adult shoots. After landing on both shoot types, C. eucalypti remained longer than C. spatulata on juvenile shoots, but C. spatulata remained longer than C. eucalypti on adult shoots. This is the first experimental evidence that heteroblasty in Eucalyptus affects insect reproductive behavior. These results are discussed in relation to subsequent studies on psyllid performance in this system that found that the epicuticular wax (present only on the juvenile leaves) plays a primary role in the preferences of these psyllid species for the juvenile versus adult shoots.

Alternate bearing influences annual nutrient consumption and the total nutrient content of mature pistachio trees

The influence of alternate bearing on nutrient utilization and total tree nutrient content was investigated in mature pistachio (Pistacia vera L. cv Kerman trees). Removal of N, P and Zn in fruit and abscised leaves of cropping (‘on’) trees averaged 5, 6, and 2 times, respectively, the removal in abscised leaflets of the non-fruiting, ‘off’ year trees. One hundred and thirty-five kg N, 131 kg K, 86 kg Ca, 39 kg Mg and 18 kg P per hectare were removed in fruits and abscised leaves in ‘on’ year trees. Tree nutrient contents and, presumably, the size of nutrient storage pools in dormant trees varied between ‘on’ and ‘off’ years. There was 22% and 14% more N and P, respectively, in dormant trees following ‘off’ than ‘on’ years. The greater N and P accumulation in ‘off’ year trees is depleted in support of the large fruit demand for N and P during ‘on’ years. In contrast to N and P, there was greater K and Ca accumulation in perennial tree parts during ‘on’ years than during ‘off’ years. The greater K accumulation in perennial tree parts and approximately 30% greater removal of K in annual organs during ‘on’ than ‘off’ years suggests that K uptake could be 4 times higher in ‘on’ year trees than in (non-cropping), ‘off’ year trees.

Stylet penetration and survival of three psyllid species on adult leaves and ‘waxy’ and ‘de-waxed’ juvenile leaves of Eucalyptus globulus

The stylet probing behavior and survival of Ctenarytaina eucalypti Maskell, C. spatulata Taylor and Glycaspis brimblecombei Moore (Homoptera, Psyllidae) on adult leaves and ‘waxy’ (untreated) and ‘de‐waxed’ (rubbed) juvenile leaves of the heteroblastic plant Eucalyptus globulus Labillardiere were compared. Psyllids were confined within clip‐on cages and after 15 days the leaf tissue under each cage was sectioned and stained to characterize and quantify stylet tracks. Across all leaf treatments and psyllid species 1438 stylet tracks were observed in 7239 leaf sections and 571 of these stylet tracks reached the vascular tissue. Survival and the number of stylet tracks of C. eucalypti on the different leaf types did not differ. In contrast, C. spatulata survived significantly longer and produced significantly more stylet tracks on adult leaves and ‘de‐waxed’ juvenile leaves than on ‘waxy’ juvenile leaves. Although G. brimblecombei survived equally on all treatments, it produced more stylet tracks on adult leaves and ‘de‐waxed’ juvenile leaves than on ‘waxy’ juvenile leaves. For all three psyllid species, there was a positive correlation between survival and number of stylet tracks reaching the vascular tissue. Stylet tracks of all species were generally intercellular from the leaf epidermis to the vascular tissue. Oil glands were avoided by all species, as indicated by the repeated branching and formation of stylet tracks at the periphery of the glands. In ‘de‐waxed’ juvenile leaves, C. spatulata and G. brimblecombei produced stylet tracks that entered the leaf at the adaxial surface and exited at the abaxial surface. This pattern was rarely observed on adult leaves, and never occurred with C. eucalypti in any leaf type. We conclude that (1) the epicuticular wax on juvenile leaves reduced stylet probing by C. spatulata and G. brimblecombei, (2) there were apparently no internal physical barriers in either juvenile or adult leaves to prevent the stylets of any of the psyllid species from reaching the vascular tissue, (3) the psyllids avoided oil glands in both leaf types, and (4) C. spatulata and G. brimblecombei may lack cues to orient their stylets towards the vascular tissue in the juvenile leaves.

Performance of adult psyllids in no‐choice experiments on juvenile and adult leaves of Eucalyptus globulus

The presence of glaucous juvenile leaves and glossy adult leaves in the canopy of the heteroblastic plant Eucalyptus globulus Labill., is a source of within‐tree variation that affects herbivory by three psyllid species (Ctenarytaina eucalypti Maskell, C. spatulata Taylor and Glycaspis brimblecombei Moore) (Homoptera, Psyllidae). In no‐choice field experiments with adult female psyllids in clip‐on cages, we compared (1) leaf‐settling, honeydew production, and survival of the Ctenarytaina species on juvenile and adult leaves of E. globulus, (2) leaf‐settling and survival of G. brimblecombei on juvenile and adult leaves, (3) leaf‐settling and survival of the three species on waxy (untreated) juvenile leaves and de‐waxed (rubbed) juvenile leaves, and (4) leaf‐settling and survival of C. spatulata on juvenile leaves from which wax was repeatedly removed. Leaf‐settling by C. eucalypti was significantly greater on juvenile than adult leaves, however, honeydew production and survival were equal on both leaf types. In contrast, leaf‐settling, honeydew production, and survival by C. spatulata were significantly greater on adult than juvenile leaves. Similarly, leaf‐settling and survival by G. brimblecombei were significantly greater on adult than juvenile leaves. Wax removal from juvenile leaves significantly increased leaf‐settling and survival of C. spatulata and G. brimblecombei. Although wax removal decreased leaf‐settling by C. eucalypti, it did not affect survival. Repeated removal of regenerating wax from juvenile leaves did not affect leaf‐settling or survival of C. spatulata. These results suggest that the epicuticular wax of juvenile leaves plays a role in resistance to C. spatulata and G. brimblecombei, however other unknown factors are probably also involved.

Psyllid Responses to Colored Sticky Traps and the Colors of Juvenile and Adult Leaves of the Heteroblastic Host Plant Eucalyptus globulus

Abstract Heteroblasty describes plants with juvenile and adult vegetative shoots that differ morphologically. Virtually nothing is known about the affects of this source of within-plant variation on host-plant selection by herbivorous insects. This is the first study on the use of visual cues by insects that specialize on different shoot types of a heteroblastic plant. Two psyllid species (Ctenarytaina eucalypti Maskell and C. spatulata Taylor) specialize on glaucous juvenile and glossy adult shoots of Eucalyptus globulus Labill., respectively. We compared their responses to artificial colors and to the colors of expanding and expanded juvenile versus adult leaves. Sticky traps were used to compare psyllid response to seven artificial colors. A “color tube” was developed to compare psyllid responses to leaf color. Both psyllid species preferred yellow traps over all other colors (red, blue, green, white, and clear), and color preference was independent of species. Both species were also more attracted to the colors of expanding and expanded juvenile leaves than to adult leaves. Although the psyllids perceived differences between the colors of juvenile and adult leaves of E. globulus, their responses suggest that leaf color alone is not the basis of discrimination between foliage types.

Foliar application of urea to almond and olive: Leaf retention and kinetics of uptake

Abstract Regression equations relating volume of urea solutions retained on leaves initially and leaf area were developed to assess urea deposition nondestructively and facilitate measurement of urea uptake by leaves of olive (Olea europaea L.) and almond [Prunus dulcis (Mill.) D. A. Webb], Foliar uptake of urea was slower in olive than it was in almond, but uptake in both species was proportional to the concentration of the urea applied. Foliar uptake of urea was not influenced by previous applications. No phytotoxicity was apparent in almond and olive following single applications of 0.5% w/v urea and 4% (w/v), respectively. Approximately 15 times more urea could be applied per cm of an olive leaf than per cm of an almond leaf at the threshold of phytotoxicity. Leaf N content in olive was increased 47% with minimal phytotoxicity following 5 successive foliar applications of 2% urea within ten days.

Role of natural self-pollination in self-fruitfulness of almond

Abstract The capacity for self-fruitfulness in the almond cultivar ‘Le Grand’ is limited by (a) self-incompatibility and (b) insufficient capacity for natural self-pollination. Expression of self-incompatibility following self-pollination by hand reduced fruit-set by 67 and 89% in 2 crop-years in comparison with fruit-set percentages achieved following cross-pollination by hand. Pollen retention following natural self-pollination equalled about 8 pollen grains/stigma and reduced fruit set by 50% when compared with branches receiving non-limiting (> 300 grains) quantities of self-pollen. These data indicate that ‘Le Grand’ is (a) unsuited for planting in monoculture and (b) highly dependent on insect-mediated pollen transfer. Blossom observations made at anthesis indicated that the relative capacity for natural self-pollination among clones may be anticipated by the spatial relationship between dehiscing anthers and receptive stigmas.

Spur survival and return bloom in almond [Prunus dulcis (Mill.) D.A.Webb] varied with spur fruit load, specific leaf weight, and leaf area

Summary Variation in fruit load, leaf area, and light exposure among almond spurs was used to evaluate whether or not spurs were autonomous with regard to Winter survival and return bloom. Fruiting was associated with reduced spur survival over the subsequent Winter and reduced return bloom in the subsequent year.This resulted in a tendency for individual spurs to bloom and bear fruit in alternate years. Survival was high among all non-fruiting spurs, but survival of fruiting spurs was positively related both to leaf area per spur and specific leaf weight (SLW; an indicator of light exposure). SLW was a much stronger correlate for spur survival than leaf area per spur. The likelihood of flowering varied positively with spur leaf area the previous season on both fruiting and non-fruiting spurs, but was not related to spur SLW. Localisation of leaf area and shading effects within individual spurs created spur sub-populations with differing tendencies toward alternate bearing. The likelihood of flowering on spurs was enhanced when branch-wide carbohydrate demand by fruit was eliminated by early fruit removal the previous season, suggesting that almond spurs are not entirely autonomous with regard to carbohydrate supply during floral initiation and development. Nevertheless, our data are consistent with a high degree of spur autonomy regarding Winter survival and return bloom, with each spur apparently being strongly influenced by the ability of its own leaves to meet its carbohydrate demands.

Sensitivity of yield determinants to potassium deficiency in ‘Nonpareil’ almond (Prunus dulcis (Mill.) D.A.Webb)

Summary The objective of this multi-year study was to generate data linking potassium (K) deficiency to almond (Prunus dulcis (Mill.) D.A. Webb) productivity. A range of tree K status (as indicated by leaf K concentrations) which encompassed the currently-accepted K deficiency/sufficiency range was established to assess the relative sensitivity to K deficiency of yield components (flower number, percentage fruit set, fruit size, and spur mortality) and to establish the time frame between the onset of K deficiency and yield reduction. Although large, statistically-significant differences in leaf K concentrations were detected in the year following the start of differential K fertilizer application, yield was not affected significantly until the third growing season (i.e. two years after the start of differential K fertilizer applications). Inadequate K-availability did not affect percentage fruit set or fruit size. Spur mortality following fruit maturation was (highly) significantly increased on fruit-bearing spurs relative to vegetative spurs, but there was no evidence of an independent effect of K availability on spur mortality. Potassium-deficient spurs exhibited reduced flowering in the subsequent years. Effects of K-deficiency are likely to be cumulative, and especially pronounced following years of heavy cropping. There was a good linear correlation (R = 0.60, P = 0.0050) between leaf K concentration in 1999 and yield in 2000.

Differential low-temperature germination responses by pollen of Pistacia vera clones with different bloom dates

Abstract Pollen was collected from 7 male clones of Pistacia vera selected to cover the range of staminate bloom dates represented in an extensive collection of the species. Pollen was incubated at a range of controlled temperatures from 11 to 39°C, in 1–4°C increments, and analyzed for germination percentages. High rates of pollen germination occurred over a broad range of temperatures for all clones. Germination of pollen from the early-blooming clones showed greater tolerance for low temperatures. Curves for pollen germination vs. temperature in the early-blooming clones showed a broader peak at the low-temperature end.