Pauliina Palonen

Influence of photoperiod and duration of short-day treatment on vegetative growth and flowering of strawberry (Fragaria3ananassa Duch.)

Summary In Junebearing strawberry cultivars, flowering is induced by short photoperiod, which also reduces vegetative growth. The loss in vigour can lower yield if plants are not cold treated to restore vegetative growth. The aim of this study was to find a photoperiod treatment that induces flowering but does not reduce vegetative growth in strawberry ‘Korona’. Plants were subjected to different photoperiods (12, 13.5 or 15.h) for varying durations (21, 35 or 49.d). After treatments, effects on plant development were recorded during forcing at 18 h daylength. Floral induction was comparably successful in 12 and 13.5.h photoperiods and the number of flowers and yield were increased by lengthening the treatment. Induction failed in many plants treated in a 15.h photoperiod and flowering was poor regardless of duration of treatment. Shorter photoperiod increased the number of branch crowns and reduced runner production, and these effects were enhanced by lengthening the treatment duration. Reduction in vegetative growth measured by petiole length was most obvious in 12.h photoperiod in all treatment durations, although the differences between treatments rapidly disappeared during forcing. Twelve and 13.5.h photoperiods were equally efficient in producing yield, but more vigorous vegetative growth was maintained during treatment in a 13.5.h photoperiod.

Current state of cold hardiness research on fruit crops

This article gives an overview of the current state of cold hardiness research in fruit crops by reviewing the recently published studies on cold hardiness of both tree fruit and berry crops. Topics discussed include cold hardiness of fruit species, cultivars and different plant organs, biophysical and biochemical aspects of hardiness, evaluation of hardiness, as well as endogenous, cultural and environmental factors affecting cold hardiness in these species. Lack of cold hardiness is a major limiting factor for production of fruit crops in many regions of the world and improved cold hardiness one of the major objectives in numerous breeding programs and research projects. Screening cultivars or selections for cold hardiness is commonly done, and different methods applied to the evaluation of hardiness are discussed. The physical limit of deep supercooling may be a restricting factor for expanding the production of some fruit crops, such as Prunus species and pear. As for biochemical aspects, a relationsh...

Crown branching and cropping potential in strawberry (Fragaria ananassa Duch.) can be enhanced by daylength treatments

Summary When strawberries (Fragaria ananassa Duch.) are produced in a greenhouse, usually two crops a year are obtained; in the fall and in the spring. To increase productivity, new cultivation techniques are needed. The aim of this study was to examine the effect of daylength treatments on the performance of strawberry plants to improve plant production protocols. The possibility of obtaining two successive crops during forcing was explored by exposing plants of cultivar Korona to two successive short day (SD) treatments followed by cold storage. The following daylength treatments were studied: 3 wk SD + 2 wk long day (LD) + 3 wk SD (3SD2LD3SD), 3 wk SD + 4 wk LD + 3 wk SD (3SD4LD3SD), and 10 wk SD (10SD). In addition, crown branching was studied in plants subjected to SD treatments. Two successive SD treatments enabled two successive flowering and cropping periods during greenhouse forcing after eight weeks of cold storage.The first SD treatment caused crown branching and induced flowering in the apical meristem of the main-crown and in the oldest axillary meristem(s), whereas younger axillary meristems were induced during the second SD treatment. Marketable yield and the number of inflorescences were comparable in 3SD4LD3SD and 10SD, but considerably lower in 3SD2LD3SD. However, this study demonstrated the high cropping potential of artificially SD treated plants, which makes them a potential alternative for greenhouse strawberry cultivation. Multi-crowned plants of ‘Korona’ can be produced by subjecting young plants to a three week SD treatment, and crown branches can be induced to flowering by a new SD treatment resulting in a very high cropping potential. Induced plants can be stored at –1°C for later forcing.

Light quality regulates flowering in FvFT1/FvTFL1 dependent manner in the woodland strawberry Fragaria vesca

Control of flowering in the perennial model, the woodland strawberry (Fragaria vesca L.), involves distinct molecular mechanisms that result in contrasting photoperiodic flowering responses and growth cycles in different accessions. The F. vesca homolog of TERMINAL FLOWER1 (FvTFL1) functions as a key floral repressor that causes short-day (SD) requirement of flowering and seasonal flowering habit in the SD strawberry. In contrast, perpetual flowering F. vesca accessions lacking functional FvTFL1 show FLOWERING LOCUS T (FvFT1)-dependent early flowering specifically under long-days (LD). We show here that the end-of-day far-red (FR) and blue (B) light activate the expression of FvFT1 and the F. vesca homolog of SUPPRESSOR OF THE OVEREXPRESSION OF CONSTANS (FvSOC1) in both SD and LD strawberries, whereas low expression levels are detected in red (R) and SD treatments. By using transgenic lines, we demonstrate that FvFT1 advances flowering under FR and B treatments compared to R and SD treatments in the LD strawberry, and that FvSOC1 is specifically needed for the B light response. In the SD strawberry, flowering responses to these light quality treatments are reversed due to up-regulation of the floral repressor FvTFL1 in parallel with FvFT1 and FvSOC1. Our data highlights the central role of FvFT1 in the light quality dependent flower induction in the LD strawberry and demonstrates that FvTFL1 reverses not only photoperiodic requirements but also light quality effects on flower induction in the SD strawberry.

In vitro screening for cold hardiness of raspberry cultivars

Raspberry (Rubus idaeus L.) cultivars ‘Festival’, ‘Titan’ and ‘Willamette’ were cultured in vitro on three different media: (A) MS medium supplemented with 1.0 mg l-1 BAP and 0.1 mg l-1 IBA, (B) MS medium without growth regulators, and (C) MS medium with reduced sucrose (10 g l-1), and exposed to different low temperature acclimation treatments: (1) control, no acclimation, (2) 1 week at +15 °C, 1 week at +2 °C, 24 h at -2 °C and 3 days at +2 °C, and (3) 2 weeks at +15 °C, 2 weeks at +2 °C, 24 h at −2 °C and 3 days at +2 °C. After acclimation, shoot moisture content was measured, and cold hardiness (LT50) was determined by controlled freezing. Shoot moisture content was generally lower on culture medium B compared to the other media, but not affected by acclimation treatment. In non-acclimated plants, medium composition had no effect on cold hardiness and no cultivar differences in hardiness were observed. After acclimation, plants on culture medium B were on average more cold hardy than on the other media. Acclimation treatment 3 on media A and B allowed the best discrimination between the hardy cultivar ‘Festival’ and less cold hardy ‘Titan’ and ‘Willamette’. When acclimation treatments were tested further using 11 raspberry cultivars with different levels of cold hardiness, discrimination between cultivars was satisfactory only after acclimation treatment 3 on culture medium B.

Evaluation of three methods to assess winter hardiness of strawberry genotypes

Summary In northern regions, winter injury is a frequent problem in strawberry (Fragaria X ananassa Duch.) production. Controlled freezing tests provide an option for screening of winter survival potential in different genotypes. In this study, winter hardiness of strawberry genotypes was evaluated by three low-temperature procedures: an artificial hardening-dehardening-rehardening programme accompanied by freezing tests, prolonged freezing test, and laboratory cold hardiness measurements of field-grown plants. Rooted runner cuttings of five June-bearing strawberry cultivars and three new selections were tested in the course of two years. Relative winter hardiness of the genotypes could not be predicted by the hardening-dehardening-rehardening programme employed in the present study. Prolonged freezing at –6˚C yielded promising results, whereas storage at _4˚C and –8˚C failed in discrimination between genotypes. Field-grown plants were sampled for cold hardiness measurements in January, March and April. The genotypes were best differentiated in January, at a fully acclimated state. The results indicate that strawberry winter hardiness can be evaluated by testing differences in the ability to survive lengthy exposure to sub-lethal low temperatures, or by applying conventional freezing tests to measure cold hardiness in different genotypes after field acclimation.

Winter hardiness of micropropagated and conventionally propagated strawberry plants

Summary Enhanced vigour of micropropagated strawberry (Fragaria X ananassa Duch.) plants has given cause to suspect their winter hardiness to be inferior to that of conventionally propagated plants. This experiment was carried out to examine and compare cold hardiness and overwintering of three types of strawberry plants of cultivars Senga Sengana and Jonsok: 1) micropropagated virus-free elite plants, 2) certified plants (runner plants from elite plants), and 3) ordinary plants (runner plants of conventionally propagated plants from a strawberry farm). A special “double pot”-method was developed and used in planting so that the plants could be lifted for controlled freezing tests during winter. Additionally, winter survival was evaluated in the field during flowering. In controlled freezing tests, strawberry crowns survived -8°C without injury, suffered minor injury at -10°C and severe injury at -12°C. No consistent differences in cold hardiness among the three types of plants were detected during winter. Field evaluation did not reveal any differences in their winter survival, either. Micropropagated plants flowered more freely than did the plants produced through runners.

Cropping potential of raspberry long-cane plants is affected by their growing conditions and duration of cold storage

SUMMARY Long-cane plants of raspberry (Rubus idaeus L. ‘Maurin Makea’) were raised in a tunnel and in an open field in order to study how nursery growing conditions affected their quality and cropping potential. The plants were then cold- stored at -1°C for different periods of time before forcing them in a greenhouse in order to determine how the duration of cold storage affected their carbohydrate status, cropping potential and performance after storage. Long- cane plants raised in the tunnel showed more vigorous growth, the canes were taller and heavier with more buds and higher carbohydrate reserves than those raised in the open field. They also produced longer laterals with more nodes during subsequent greenhouse forcing, and their cropping potential, estimated as the number of flowers per cane, increased by 28–40%, depending on the duration of cold storage. Extending cold storage from 4 weeks to 20 weeks promoted and advanced lateral bud burst, resulting in more lateral shoots per cane, and increased the proportion of reproductive nodes on the lateral shoots, resulting in increased cropping potential, although the carbohydrate reserves of canes declined during cold storage. We conclude that, under high-latitude conditions, the maximum cropping potential was achieved by raising raspberry long-cane plants in a tunnel. Moreover, while a cold storage period of 12 weeks was too short to overcome dormancy effects, 20 weeks of cold storage resulted in a high cropping potential in the cultivar studied here.

Growth and cropping of primocane and biennial raspberry cultivars grown under a film absorbing far-red light

Summary In an attempt to control vegetative growth in raspberry (Rubus idaeus L.) in protected cultivation, we studied the effect of a far-red (FR)-absorbing greenhouse film on growth, cropping, and fruit quality in four raspberry cultivars. The primocane cultivars ‘Autumn Bliss’ and ‘Polka’ were grown in tunnels covered either with a clear film or a FR-absorbing film until the end of their first-year-harvest season. The biennial cultivars ‘Glen Ample’ and ‘Tulameen’ were treated similarly during their period of primocane growth. The FR-absorbing film increased the numbers of flowers in the primocane cultivars, whereas the numbers of flowers were reduced in the biennial cultivars during forcing in the following year. The numbers of flowers per plant were 185 in ‘Autumn Bliss’ and 171 in ‘Polka’ under the clear film; but 202 and 203, respectively, under the FR-absorbing film. The number of berries per plant was also increased in ‘Polka’. The FR-absorbing film reduced cane growth in ‘Polka’, due to a reduced number of internodes, and in ‘Tulameen’, due to reduced internode length. The modified light spectrum had no effect on the berry contents of total anthocyanins, sugars, most mineral components, or on fruit colour in the primocane cultivars. However, levels of ellagic acid were increased by the FR-absorbing film, while the soluble solids content:titratable acidity ratio was slightly reduced. FR-absorbing films may therefore be useful when growing primocane raspberry cultivars in order to increase their yield potential, the contents of bioactive compounds in their fruit, and to decrease vegetative growth. However, such films may not be recommended for growing primocanes of biennial-type raspberry cultivars due to impaired flower development.

Flower Bud Cold Hardiness of ‘Muskoka’ Red Raspberry as Related to Water Content in Late Winter

Abstract ‘Muskoka’ red raspberry (Rubus idaeus L.) canes were sampled from a field (latitude 60°10′N) at 2- to 3-week intervals from February to April to examine whether the loss of cold hardening capacity in late winter is connected to the increase in bud water content, or to the increased ability of buds to absorb water. Bud base vascular tissue lost its hardening capacity earlier than floral primordia and was injured at warmer temperatures. Cold hardiness was more closely correlated with bud water content than with water potential. Both low water content and low water potential indicated the ability of tissue to harden at low temperatures. All determined parameters except hardening capacity were positively correlated with mean air temperature during a 2-day period preceding sampling. Loss of bud base vascular tissue hardening capacity coincided with increases in bud water content, maximal water content and water potential in the beginning of April, after diurnal mean temperature had risen above 0°C.