Stephen M. Southwick

Leaf and canopy level photosynthetic responses of French prune (Prunus domestica L. ‘French’) to stem water potential based deficit irrigation

Summary The effects of full irrigation and two levels of water stress on tree physiology were investigated in a French prune orchard. The control treatment received approximately 100% of the estimated seasonal crop water requirements throughout the season. The moderate and severe water stress treatments were managed to decline from a fully watered status in late spring to a midday stem water potential of –1.5 and –2.0 MPa, respectively, by harvest. Water stress caused a clear reduction in stem water potential throughout the daylight period, as well as reductions in leaf conductance and photosynthesis, and all of these reductions were more severe with increasing water stress. In a moderate and severe stress treatment, the photosynthetic rate of fully exposed leaves was reduced to 90% and 81% respectively, of the control treatment. There was a linear decline in light saturated photosynthesis with a decline in stem water potential. However, the stress treatments also influenced leaf orientation and the resulting incident light distribution on leaves within the canopy, such that light was more uniformly distributed in canopies under water deprivation than it was under full irrigation. The overall effect of water stress was that the moderate and severe stress treatments had 101% and 93% respectively of calculated canopy photosynthesis compared with the control. This may account for the observed tolerance of prune production to deficit irrigation. A midday depression in assimilation was observed on most days in all treatments, but was not associated with feedback inhibition resulting from carbohydrate accumulation in leaves.

Detection of genetic diversity among populations of sweet cherry (Prunus avium L.) by AFLPs

Summary The high degree of polymorphism of AFLPs provides an efficient system for identification and genome analysis of sweet cherry (Prunus avium) cultivars and selections. The cultivars of sweet cherry have usually been characterized by assessment of phenotypic and pomological traits. AFLP markers were employed to identify 38 sweet cherry accessions and estimate the genetic diversity among this material. Ten of 18 tested primer combinations were informative with up to 80 bands per primer combination. Seven to 33% of the amplfied bands were polymorphic depending upon primer combination. Allcultivars and selections tested could be clearly identified. The objective of this work was to demonstrate the usefulness of molecular markers in revealing the genetic diversity among different sweet cherry genotypes.

Identi®cation and characterization of plum and pluot cultivars by microsatellite markers

Summary Pluots are putative hybrids between plums (Prunus salicina Lindl.) and apricots (P. armeniaca L.). The capability to distinguish among plum and pluot cultivars is important in breeding and cultivation. We investigated the genetic diversity among 14 plums, 6 pluots and one plumcot representing commercial cultivars in California, with 28 microsatellite markers. We also tested seven apricot cultivars as a reference to ®nd evidence of apricot in the ancestry of pluots and plumcot. The parental material used in the original cross that produced the pluot and plumcot was not available. Of the 28 SSR markers, 25 were from sweet cherry (Prunus avium L.) and three from peach (Prunus persica L.). Approximately 80% of the cherry primers generated ampli®cation products in plum and pluots, showing transportability between these Prunus species. One to eight putative alleles per locus were displayed by the tested SSRs in plums and pluots. In plum and pluot samples a total of 100 alleles were identi®ed with an average of 4.3 alleles per primer combination. The SSR markers were successfully used for the discrimination of all tested cultivars. In pluots, 76 alleles were found in which 63 (83%) were speci®cally coming from plum, 9 (12%) were common in plum, pluots and apricot while no allele in the pluots was observed that was contributed from apricot. In plumcot, 49 alleles were observed in which 25 (51%) were from plum, 18 (36%) were speci®cally from apricot and 6 (12%) were common in plum, plumcot and apricot. Relationships among the 28 plum, pluot and apricot cultivars were represented by a dendrogram, constructed on the basis of 168 SSR markers. The dendrogram showed the plums and pluots form a cluster distinct from the apricots, with pluot cultivars interspersed among plum cultivars and more closely related to plum than to apricot. Plumcot made a separate branch and was placed between the plum and apricot cluster. These results suggest that the SSR markers are valuable tools for identi®cation of cultivars and diversity analyses in plum.

Use of gibberellins on ‘Patterson’ apricot (Prunus armeniaca) to reduce hand thinning and improve fruit size and firmness: Effects over three seasons

Summary‘Patterson’ apricot (Prunus armeniaca) trees were sprayed with an air blast sprayer over three seasons (1992, 1993, and 1994) in three application timings prior to and after harvest, with single sprays of 50, 75, and 100 mg l−1 gibberellic acid (predominantly gibberellin A3 (GA3); Release LC, Abbott Laboratories, North Chicago, IL). A single pre-harvest timing (first week of June) and two concentrations (50 and 100 mg l−1) were consistent across all three years and the effects of these treatments on flowering and fruit development were compared. In all years following treatment with 100 mg l−1 GA, flower numbers per cm of limb length and circumference were significantly reduced. Fruit set was not affected by GA sprays. June 1993 GA sprays of 50 and 100 mg T1 resulted in total yields per tree in 1994 that were equivalent to hand-thinned control trees with 36% and 46% reduction in hand thinning, respectively. Individual fruit weights of fruits treated with 100 mg l−1 GA sprays were equal to those of ...

Flowering and fruiting in ‘Patterson’ apricot (Prunus armeniaca) in response to postharvest application of gibberellic acid

Abstract In July or August of 1988 and 1989 which was approximately 2 or 6 weeks, respectively, after fruit harvest, cultivar ‘Patterson’ apricot (Prunus armeniaca) trees were sprayed with a single spray of either 10, 50 or 100 p.p.m. gibberellic acid (gibberellin A3, GA). GA sprays of 100 p.p.m. applied in early July reduced flower number per centimeter of limb length in the year following treatment. Flower number per centimeter of limb circumference was reduced by sprays of 50 and 100 p.p.m. GA applied in July. Fruit set was not affected by GA sprays. The yield and fruit number of hand thinned trees was equivalent to that found on trees treated with 50 and 100 p.p.m. GA sprays in July. Individual fruit weight (size) was increased by GA sprays of 50 and 100 p.p.m. in July compared to hand thinned trees. Fruit maturity was advanced when yields were reduced by GA sprays. In July, GA sprays of 10 p.p.m. resulted in increased individual fruit weight without reduced total yields per tree compared to non-thinned control trees. Results showed that the use of GA sprays the year before flowering (July) decreased flower numbers, eliminating the need for chemical or hand fruit thinning of ‘Patterson’ apricot.

The effects of Apogee® on shoot growth, secondary flowering, fire blight, fruit quality, yield and return bloom in ‘Bartlett’ pear growing in California

Summary Apogee® sprays were applied to mature ‘Bartlett’ pear (Pyrus communis L.) trees as single or multiple sprays during the growing seasons of 1999 to 2003 at concentrations of 420 g ha–1 (125 mg –1) or 840 g ha–1 (250 mg–1): both reduced vegetative shoot growth. Two sprays of 840 g ha–1 applied 3–3.5 weeks apart were most effective and reduced shoot growth by approximately 40%. An application made 2 d after fall bloom was ineffective and led to more vegetative shoot growth. Single sprays of Apogee® made within 9–28 d after full bloom (DAFB) were equally effective in reducing vegetative shoot growth. Bourse shoot growth was reduced 43% by 3 April, 2001 (8 DAFB) treatment of 840 g ha–1 when bourse and vegetative shoots ranged from 2.5 to 7.5 cm in length. Subsequent single applications of 840 g ha–1 Apogee® (on April 12 or 23, 2001) did not reduce bourse shoot growth significantly. Vegetative shoot growth was also reduced by 12% (9 d after treatment) and by 38% (approximately one month after treatment) by 840 g ha–1 Apogee® applied on 3 April, 2001, (8 DAFB). In 2002, bourse shoot growth was controlled most effectively by split applications (840 + 420 g ha–1 or 840 2 g ha–1, 9 April and 24 May) and by 840 g ha–1 Apogee®, applied at 2.5 to 7.5 cm shoot growth (9 April). These split applications, as well as girdling, decreased shoot strikes in 2002, the year after treatment. Split applications of 840 g ha–1 Apogee® made in April to early May were most effective in controlling both vegetative and bourse shoot growth in most years, although single applications of Apogee® were effective in some years. Subsequent flushes of shoots tended to be controlled by split applications with the second application timed to the emergence of new shoots. Apogee® was not consistently effective in reducing the incidence of Type I or Type V secondary flowers in these trials. Single or multiple sprays of Apogee® had no measurable or consistent effect on return bloom, fruit size or shape, firmness, percentage soluble solids, or yield per tree over the period of these experiments. Apogee® appears to have promise as a shoot growth control agent in ‘Bartlett’ pear growing in California, USA, without deleterious effects.

Use of gibberellin to delay maturity and improve fruit quality of 'French' prune.

Summary Gibberellin (GA) increased firmness of ‘French‘ prune (Prunus domestica L.) when applied preharvest at a rate of 31 or 62 mg 1‒1 in 1997,1998 and 1999. Most effective application dates corresponded with a fruit soluble solids (SS) of 11% to 13% in 1997 and 1998, but with 15% in 1999. The development of SS was temporarily delayed by some of the GA treatments, but was generally similar to the control by harvest time. In 1997, fruit firmness remained higher than the control and extended fruit maturity for up to 7 d by either 31 or 62 mg 1‒1 GA applied at 13% SS, or by 62 mg 1‒1 GA applied at 16% SS. In 1998, fruit firmness was increased by treatment with either 31 or 62 mg 1‒1 GA at 11.5% SS or 31 mg 1‒1 at 16% SS. In 1999, harvest was extended by 3 d due to greater fruit firmness by 31 mg 1‒1 GA applied at 11% or 13% SS, or by 62 mg 1‒1 GA applied at 13% or 15% SS. GA treatments did not affect fruit drop, dried fruit size, or yield. Bloom was delayed by application of GA in 1997 and 1998, particularly with later treatments and with the higher rate. Return bloom in 1998 was not affected by GA treatment in 1997. In 1999,31 mg 1‒1 GA at 11.5% SS resulted in greater return bloom than the control, but bloom of other treatments had similar return bloom as the control. Fruit set was unaffected by the application dates and concentrations tested.

Response of secondary bloom of ‘Bartlett’ pear (Pyrus communis L.) to pruning

Summary Type V secondary bloom of pear (Pyrus communis L.) is associated with pruning and is frequently infected by fire blight (Southwick et al., 2000). The goal of this research was to determine which pruning practices would most likely be associated with subsequent Type V secondary bloom. Pruning date, shoot age, type of pruning cut and shoot orientation were varied and effect on Type V secondary bloom assessed. In both 1998 and 1999, dormant pruning was more likely to result in Type Vs than summer pruning or pruning one month after harvest. Pruning one week after harvest in 1998 was as likely to lead to Type Vs as dormant pruning, but was less likely in 1999. The likelihood that Type Vs would occur was lowest from 49 to 89.d from full bloom (DFB) and at 175 DFB in 1998. The lowest likelihood of Type Vs in 1999 was found on shoots pruned at 83 or 185 DFB. The likelihood for Type Vs rose with increasing shoot age from one to four years, but there was no difference between three- and four-year old shoots. In 1998, stub cuts (severe heading) were more likely to have Type Vs than head cuts when pruning two year old shoots, but not for one year old shoots. In 1999, type of pruning cut had an inconsistent effect on the likelihood for Type Vs and was not the same for each shoot age. NAA applied to summer pruned shoots completely prevented Type Vs from occurring and almost completely stopped Type Vs when applied to dormant pruning cuts. Summer pruning from 50 to 90 DFB or pruning one month after harvest and the application of NAA to pruning wounds may be strategies for reducing the occurrence of Type V secondary bloom.

GA and Ethephon Have Limited Effect on Secondary Bloom in ‘Bartlett’ Pear

Abstract GA3, GA4+7 + BA or ethephon was applied in the spring or summer 1997, 1998 and 1999 to reduce secondary bloom in ‘Bartlett’ pear (Pyrus communis L.). Reduction of secondary bloom could reduce associated fire blight (Erwinia amylovora [Burrill] Winslow et al.) infections. The number of Type I secondary bloom was reduced by 400 mg·L−1 GA3, applied between 33 and 97 DAFB, dates which also reduced normal bloom and fruit set. Summer timings of GA3 at 100 mg·L−1 or GA4+7 + BA at 200 mg·L−1 reduced the number of Type V secondary bloom with no difference between GA3 and GA4+7. Ethephon applied one month after harvest in 1997 and 1998 reduced Type Vs in 1998 but not in 1999. Ethephon also reduced fruit set in 1999. Use of GAs to reduce Type V secondary bloom in a fire blight management program may be possible.