Raphael A. Stern

Sequential introduction of honeybee colonies and doubling their density increases cross-pollination, fruit-set and yield in ‘Red Delicious’ apple

Summary The ‘Red Delicious’ apple exhibits full self-incompatibility, therefore its fruit production depends totally on cross pollination, especially by honeybees, which are the ultimate apple pollinators. In the present study, the technique of sequential introduction of honeybee colonies and doubling their density was applied to determine effects on yield. In three consecutive season it was found that sequentially increasing the density of colonies in the apple orchards from 2.5 colonies per ha., as recommended previously, to 5.0 colonies per ha, and introducing them sequentially (half at the 10% full bloom (FB) and half at FB), increased the number of bees per tree, their mobility among the rows, and the proportion of “topworkers” compared with “sideworkers” . It seems that a large number of foragers per tree directly increases the amount of pollination, high bee mobility between rows increases the amount of cross-pollination, and a high proportion of “topworkers” increases pollination efficiency. All the above effects were expressed in higher fruit set and higher yield (50–100%) in the treatment plots.

The RHV region of S-RNase in the European pear (Pyrus communis) is not required for the determination of specific pollen rejection

In the gametophytic self-incompatibility system, growth of self-pollen tubes in the style is inhibited in a haplotype-specific manner by S-RNase. The mechanism by which S-RNase confers its specificity is unknown. However, a hypervariable region (RHV in Rosaceae and HVa-HVb in Solanaceae) that differs among the many cloned S-RNase alleles has been proposed to be involved in conferring the S-haplotype specificity of the S-RNase. Region swapping experiments between S-RNases and crystallography of the enzyme support this assumption. However, the deduced amino acid sequences of Sn-RNase and Si-RNase alleles from the European pear (Pyrus communis) were recently found to have an identical RHV. In the present study it is shown that Sn-RNase does not prevent fertilization by Si-pollen haplotype, thus presenting a case in which RHV is not required for the determination of specific pollen rejection by S-RNase, and implying that other regions in the enzyme may be sufficient for this specificity.

Sequential introduction of honeybee colonies increases cross- pollination, fruit-set and yield of 'Spadona' pear (Pyrus communis L.)

Summary The ‘Spadona’ pear exhibits full self-incompatibility, therefore its fruit production depends entirely on cross pollination, especially by honeybees, which are the ultimate pear pollinators. In the present study, the effect on yield of sequential introduction of honeybee colonies and doubling their density was studied. It was found (in three consecutive seasons, 2001–2003) that increasing the density from 2.5 colonies per ha to 5.0 colonies per ha in one introduction at 10% bloom, did not increase bee activity on the trees and did not improve fruit set and yield. However, introducing the colonies sequentially, (1.25 colonies per ha at 10% bloom and 1.25 colonies per ha at full bloom = FB), increased the number of bees per tree and their mobility among the rows, and consequently increased fruit set and yield by 50–80%.

Fertilisation efficiency of semi- and fully-compatible European pear (Pyrus communis L.) cultivars

Summary In Israel, European pear (Pyrus communis L.) is grown in warm, sub-optimal climate conditions and the yield is low. Commercial orchards consist of ‘Spadona’ as the main cultivar, and ‘Coscia’, ‘Gentile’ and ‘Spadochina’ as its pollinators. In a previous study, it was found that these three pollinators were semi-compatible with ‘Spadona’. In the present study, the fertilisation efficiency of the cultivars was compared using hand- and open-pollination. Differences in fertilisation efficiency characteristics (including seed number per fruit) were inconsistent, apart from an increase in ‘Spadona’ fruit diameter when pollinated by ‘Spadochina’. Hence, these results indicate that ‘Spadochina’ exerted metaxenia, a phenomenon which has not been described previously in European pear. However, this difference is not sufficient for the desired increase in yield. Therefore, a comparison between a fully- and two semi-compatible pollinators was carried out. Since ‘Spadona’ could not serve as a female parent in this kind of experiment, due to the S-RNase allele content of the cultivars, ‘Coscia’ served as the female parent, ‘Gentile’ as the fully-compatible pollinator, and ‘Spadona’ and ‘Spadochina’ as the semi-compatible pollinators. As this pear-couple composition is not included in orchard designs, only hand-pollination experiments could be carried out. The trial revealed a high and significant increase in fruit set and seed set by fully-compatible compared to semi-compatible pollinators.

CPPU and BA increase fruit size of ‘Royal Gala’ (Malus domestica) apple in a warm climate

Summary The effects of the synthetic cytokinins BA and CPPU on fruit size of the small fruited ‘Royal Gala’ apple were examined during four consecutive years (1999-2002) in the warm climate of Israel. CPPU at a concentration of 10 ppm and BA at a concentration of 50 ppm caused an appreciable (>50%) and significant increase in fruit size when applied two weeks after full bloom. The large increase in fruit size occurred with (BA) or without (CPPU) some thinning. Therefore, the main effect was attributed to the direct stimulation of fruit cell division. Both cytokinins increased fruit size without affecting fruit shape and seed number, and with no reduction in the return bloom or yield of the following year. Our findings demonstrate that BA and CPPU have a significant potential to improve fruit size of ‘Royal Gala’ apple without any negative effects on fruit quality at harvest or in storage of 0°C. Preliminary results with ‘Red Delicious’ apple indicate that these cytokinins may be used for other cultivars as well.

The relationship between floral structure and honeybee pollination eficiency in ‘Jonathan’ and ‘Topred’ apple cultivars

Summary Apple (Malus domestica) has a gametophytically determined self-incompatibility (SI) system, which limits inbreeding. As a result, apple fruit-set is dependent on effective cross-pollination, which may be enhanced by honeybees (Apis mellifera) Excess pollination results in over-cropping, leading to many small fruit, regarded as of low quality. On the other hand insufficient pollination is also possible, and is manifested by low crop production. In the present work, characteristics of nectar reward and floral morphology revealed that ‘Jonathan’ and ‘Topred’ flowers had similar nectar constitutions. However, honeybee behaviour was totally different in the two cultivars. ‘Jonathan’ flowers attracted fewer honeybees but, due to their anther arrangement, more of the flowers were approached from the top by honeybees collecting nectar than in ‘Topred’. This finding demonstrates the importance of determining the pollination efficiency to achieve optimum honeybee management, in terms of the number of honeybee colonies required for sufficient pollination and minimum fruit thinning.

GA3 inhibits flowering, reduces hand-thinning, and increases fruit size in peach and nectarine

Summary Peaches and nectarines belong to a self-pollinating species and therefore tend to set a large number of small fruit. Chemical-thinning is not efficient in stone fruit. Consequently hand-thinning of fruitlets has become the commercial practice, although it is very expensive and not efficient at this stage. Over five consecutive years (2003 – 2007), application of 25 mg l−1 GA3 to the basal part of shoots during flower bud induction (60 d after full bloom ) reduced flowering of ‘Snow-Queen’, ‘Queen-Giant’, and ‘Arctic Mist’ nectarines, and ‘Scarlet-Snow’ peach in the following season. As a result, the time spent on hand-thinning was reduced by ca. 50%, and the yields of large fruit increased approx. two-to-three fold. Ripening of ‘Queen-Giant’ nectarine fruit was delayed slightly, and the incidence of internal breakdown following storage was reduced. Since the effect of the GA3 treatment lasted for only 1 year, it is suggested that it should be applied annually.

Xylem-sap zeatin-riboside and dihydrozeatin-riboside levels in relation to plant and soil water status and flowering in ‘Mauritius’ lychee

Abstract The effects of autumnal water stress on the levels of zeatine-riboside and dihydrozeatin-riboside in the shoot xylem-sap and on the intensity of flowering in the following year were studied in commercial lychee ( Litchi chinensis Sonn.) during the 1995/1996 season. Four irrigation treatments were initiated at the beginning of October coinciding with the end of the second vegetative flush after harvest as follows: 0.5, 0.25, 0.125 and 0 Class A pan evaporation coefficient, designated as 100, 50, 25 and 0%, respectively, were applied. Soil and midday stem water potentials were determined several times during October, after the beginning of treatment. Xylem-sap concentrations of trans -zeatin-riboside ( t -ZR) and dihydrozeatin-riboside (DHZR) were determined 20 days after the start of the irrigation treatments. Both soil and midday stem water potentials decreased with the decrease in the irrigation levels. Flowering intensities in the following spring were higher after the reduced irrigation treatments than in the control (100%). DHZR and t -ZR concentrations in the xylem-sap increased with decreasing irrigation level, down to the 25% irrigation level. Cessation of irrigation (0%) resulted in a steep decrease in the t -ZR concentration and a further increase of DHZR. The relationships between plant and soil water status, cytokinin xylem-sap content and flower bud differentiation reported herein suggest that moderate water stress is sufficient to induce fruit bud differentiation without any visible damage to the trees.

Multiple introduction of honeybee colonies increases cross-pollination, fruit-set and yield of ‘Black Diamond’ Japanese plum (Prunus salicina Lindl.)

Summary Japanese plum (Prunus salicina Lindl.) belongs to the Rosaceae family, which carries the S-RNase-mediated gametophytic self-incompatibility system, which prevents self-fertilisation, and thus promotes out-crossing. The plum cultivar ‘Black Diamond’® has become, one of the most important cultivars in Israel in the last decade, yet its yield is low in comparison with its known potential. Honeybees (Apis mellifera) are the most important pollinators for plums and several studies have demonstrated an apparent relationship between the number of honeybees and the size of the fruit crop. Therefore, in this study, we focussed on improving bee management in a ‘Black Diamond’ orchard. In four consecutive years of experiments, we examined the effects of increasing the density of bee colonies, and of making multiple introductions of colonies, on honeybee activity and on their effectiveness as pollinators of ‘Black Diamond’. We showed that four separate introductions, each of 0.625 colonies ha–1, every 2 – 3 d from 10% full bloom to 3 d after full bloom, with a total density of only 2.5 colonies ha–1, resulted in the greatest level of fruit set.

Honey bee (Apis mellifera) strains differ in apple (Malus domestica) pollen foraging preference

SUMMARY A large proportion of the honey bees from colonies placed in apple orchards collect pollen from competing flora. For four years we tested for a genetic component that would account for preferences for pollen from apple bloom. In the first two years we tested various genetic strains of bees and found significant differences among them in the proportion of apple pollen that they collected. In the final year we found that colonies that were progenies of colonies with high preference for apple pollen in the previous year (‘High strain’) tended to collect a higher proportion of apple pollen compared to colonies from a ‘Low strain.’ The genetic component for apple pollen preference that is evident from this study attests to the possibility of breeding a honey bee strain with high apple pollination effectiveness even under competition conditions.