Ola M. Heide

Long-day control of flowering in everbearing strawberries

Summary Photoperiod and temperature control of flowering in a number of perpetual-flowering or everbearing strawberry cultivars of widely varying pedigree has been studied in controlled environments. Flower bud initiation in the cultivars ‘Flamenco’, ‘Ridder’, ‘Rita’ and ‘Rondo’ was significantly advanced by long-day (LD) conditions at temperatures of 15°C and 21ºC; while, at 27ºC, flowering took place under LD conditions only. Some plants of the seed-propagated F1-hybrid ‘Elan’, raised at 21°C, also flowered under short-day (SD) conditions at 27°C, but reverted to the vegetative state after a few weeks when maintained under these conditions. When vegetative plants growing in SD at 27°C were transferred to LD conditions at the same temperature, they consistently initiated flower buds and started flowering after about 4 weeks. At such a high temperature, flowering could thus be turned on and off by switching between SD and LD conditions. This applied to all the cultivars studied. Also the cultivar ‘Everest’, which was tested only at 21°C, produced similar results. Night interruption for 2 h was effective in bringing about the LD response. At 9°C, flowering was substantially delayed, especially in ‘Flamenco’ and, at this temperature, flowering was unaffected by photoperiod. Runner formation was generally promoted by high temperature and SD conditions, but the photoperiodic effect varied between experiments. We conclude that everbearing strawberry cultivars, in general, whether of the older European-type or the modern Californian-type originating from crosses with selections of Fragaria virginiana ssp. glauca, are qualitative (obligatory) LD plants at high temperature (27°C), and quantitative LD plants at intermediate temperatures. Only at temperatures below 10°C are these cultivars day-neutral.

Physiology and genetics of flowering in cultivated and wild strawberries – a review

Summary Two main categories of flowering and fruiting habits exist in strawberry: 1) seasonal flowering (SF) genotypes which produce only one flush of flowering and fruiting in the Spring; and 2) recurrent flowering or everbearing (EB) genotypes which, in addition to a spring flush, produce more or less continuous flowering and fruiting throughout the growing season. Both types are represented in the octoploid cultivated strawberry, Fragaria × ananassa Duch., as well as in the diploid wood strawberry, F. vesca L. Independent of ploidy level, SF genotypes have proved to be basically short-day (SD) plants, while EB genotypes are basically long-day (LD) plants. However, flower induction is controlled in both groups, by a pronounced interaction of photoperiod and temperature, which has complicated resolution of the underlying response mechanisms. At low temperatures (< 10ºC), both SF and EB genotypes are essentially day-neutral, while, at higher temperatures, they exhibit contrasting photoperiodic responses. Significant progress has been made in elucidating the genetic factors and molecular mechanisms underlying the complex flowering responses of strawberry, using the diploid F. vesca as a model plant. Many of the central genetic components of the flowering pathways which are known in Arabidopsis thaliana have also been identified in strawberry. A major breakthrough was identification of the SFL (SEASONAL FLOWERING LOCUS) gene in F. vesca as a homologue of the Arabidopsis TERMINAL FLOWER1 (FvTFL1) gene, and the recent demonstration of its function as a genetic switch controlling the opposite photoperiodic responses of the SF and EB genotypes of strawberry.

Influence of postflowering temperature on fruit size and chemical composition of glen ample raspberry (Rubus idaeus L.).

The effects of postflowering temperature on the fruit chemical composition of Glen Ample raspberries were studied under controlled environment conditions. The berry weight decreased significantly with increasing temperature (12, 18, and 24 °C) and with progress of the harvest period. Because the moisture content increased in parallel with the berry weight, the antioxidant capacity (AOC) and the concentration of a range of bioactive compounds decreased with decreasing temperature and progress of the harvest season when expressed on a fresh weight basis in the conventional way. Under those circumstances, dry weight units are therefore preferable. However, despite the dilution effect of large berries, the concentration of ascorbic acid (vitamin C) increased with decreasing temperature, even on a fresh weight basis. Berry AOC was closely correlated with total phenolic concentration (r = 0.958), predominantly anthocyanins and ellagitannins. While a total of 10 anthocyanins were detected, cyanidin-3-sophoroside and cyanidin-3-(2(G)-glucosylrutinoside)-rutinoside accounted for 73% of the total, the former decreasing and the latter increasing with increasing growth temperature. By far, the most prevalent ellagitannins were lambertianin C and sanguiin H-6, both of which increased significantly with increasing temperature. It is concluded that the growth temperature has significant and contrasting effects on the concentration of a range of potentially bioactive compounds in raspberry.

Temperature rather than photoperiod controls growth cessation and dormancy in Sorbus species

Environmental regulation of growth and dormancy of four Sorbus genotypes was studied in controlled environments. Emphasis was placed on assessment of the presence and nature of the deficient photoperiodic dormancy regulation system that has previously been reported for some woody Rosaceae species. Two genotypes of Sorbus aucuparia L. maintained indeterminate growth for 8 weeks and 9 weeks at temperatures of 15 °C and 21 °C in both 20 h and 10 h photoperiods, while at 9 °C, in the same photoperiodic conditions, they immediately ceased growing. At the higher temperatures, initiation of new leaves (nodes) was unaffected by photoperiod, while internode elongation was significantly enhanced by long days (LD). However, even after prolonged exposure to 9 °C, most plants resumed growth when moved to high temperature and LD, indicating a shallow state of dormancy. Seedlings of Sorbus intermedia (J. F. Ehrh.) Pers. and micro-propagated plantlets of S. commixta Hedl. ‘Dodong’ were also unaffected by photoperiod during primary growth, but failed to elongate and gradually became dormant regardless of temperature and day-length conditions. However, after chilling and breaking of dormancy, the plants elongated vigorously but changed to a determinate mode of growth. Furthermore, a temperature of 9 °C was found to be fully effective for breaking dormancy in S. intermedia plants. It is concluded that deficient photoperiodic dormancy control seems widespread in the Rosaceae and that, in such plants, both dormancy induction and release is brought about by low temperature. The potential impacts of climate change on such trees are discussed.

Quantitative long-day flowering response in the perpetual-flowering F1 strawberry cultivar Elan

Summary Perpetual-flowering strawberry cultivars are commonly classified as photoperiodically day-neutral, even though early investigations demonstrated long-day (LD) regulation. An important reason for this inconsistency is that these freely flowering plants are difficult to establish in a true vegetative state, and experiments have therefore often been started using runner plants with pre-formed inflorescences. In order to circumvent this problem, we have used the perpetual-flowering F1-hybrid ‘Elan’ that is propagated by seed, and is thus not pre-conditioned by its earlier life history. The results demonstrated a marked quantitative LD response across a range of temperatures from 9° – 27°C. Seedlings were responsive to the LD stimulus at an early stage, and early flowering required LD exposure almost from germination. The critical daylength for the early flowering response was about 15 h at 18°C. Because of this threshold LD response, it is concluded that regulation of flowering is truly photoperiodic in nature, and not merely an effect of additional light. Flower development was also slightly advanced by LD conditions. Stolon formation was strongly enhanced by short-day conditions in combination with high temperature. Thus, not only flowering, but also runnering, was oppositely affected by photoperiod in ‘Elan’ compared with mono-flowering cultivars. The results are discussed in relation to the photoperiodic classification of strawberries.

Long-day rather than autonomous control of flowering in the diploid everbearing strawberry Fragaria vesca ssp. semperflorens

Summary The environmental control of flowering in the perpetual-flowering (everbearing) diploid strawberry Fragaria vesca ssp. semperflorens cultivars ‘Rügen’ and ‘Baron Solemacher’ has been studied in controlled environments. Seedpropagated plants were exposed to 10-h short-day (SD) and 24-h long-day (LD) conditions at temperatures ranging from 9° – 27°C. The results revealed a quantitative LD response of flowering that increased in strength with increasing temperature, to become almost obligatory at 27°C. Occasional runner formation was observed in SD at high temperature, conditions which were inhibitory to flowering, demonstrating that runnering ability is not completely lost in these genotypes. A comparison with the perpetual-flowering octoploid F. ananassa ‘Elan’, in one experiment, demonstrated an identical LD temperature interaction in the two species. The results are discussed in relation to available information on the genetics of flowering habits in the two species. Since seasonal flowering types of F. vesca and F. ananassa have also been shown to share a principally identical flowering response, controlled by SD and low temperature, it is concluded that a remarkably similar flowering control system is present in the diploid F. vesca and the octoploid F. ananassa. Despite the large genetic differences between the two species, and regardless of the origin of the cultivars, the seasonal flowering types are all SD plants, while their perpetual-flowering counterparts all appear to be LD plants. In both cases, there is a pronounced interaction with temperature; the photoperiodic responses increasing with increasing temperature, in both cases. This raises the question whether a common genetic flowering control system is present in both species.

TERMINAL FLOWER1 is a breeding target for a novel everbearing trait and tailored flowering responses in cultivated strawberry (Fragaria × ananassa Duch.)

Summary The effects of daylength and temperature on flowering of the cultivated octoploid strawberry (Fragaria × ananassa Duch.) have been studied extensively at the physiological level, but information on the molecular pathways controlling flowering in the species is scarce. The flowering pathway has been studied at the molecular level in the diploid short‐day woodland strawberry (F. vesca L.), in which the FLOWERING LOCUS T1 (FvFT1)–SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (FvSOC1)–TERMINAL FLOWER1 (FvTFL1) pathway is essential for the correct timing of flowering. In this work, we show by transgenic approach that the silencing of the floral repressor FaTFL1 in the octoploid short‐day cultivar ‘Elsanta’ is sufficient to induce perpetual flowering under long days without direct changes in vegetative reproduction. We also demonstrate that although the genes FaFT1 and FaSOC1 show similar expression patterns in different cultivars, the regulation of FaTFL1 varies widely from cultivar to cultivar and is correlated with floral induction, indicating that the transcription of FaTFL1 occurs at least partially independently of the FaFT1–FaSOC1 module. Our results indicate that changing the expression patterns of FaTFL1 through biotechnological or conventional breeding approaches could result in strawberries with specific flowering and runnering characteristics including new types of everbearing cultivars.

Physiology of flowering and dormancy regulation in annual- and biennial-fruiting red raspberry (Rubus idaeus L.) – a review

Summary Recent research on how the structure and physiological development of red raspberry (Rubus idaeus L.) plants are controlled by genotype and the climatic environment is reviewed. Some older work, especially on plant structure relations, is also included. Physiological differences between annual- and biennial-fruiting plant types are highlighted. One major difference is the different requirements for flower formation. While biennial-fruiting cultivars have an absolute low temperature (≤ approx. 15°C) requirement for floral initiation, annual-fruiting cultivars readily initiate floral primordia at temperatures as high as a constant 30°C. Also, while biennial-fruiting cultivars are facultative short-day plants with a critical photoperiod of 15 h at intermediate temperatures, flowering is promoted by long photoperiods in at least some annual-fruiting cultivars. However, the essential difference that determines whether the shoot life-cycle becomes annual or biennial is that, in biennial-fruiting genotypes, floral initiation is linked to the induction of bud dormancy; whereas, in annual-fruiting cultivars, floral initiation is followed by direct flower development. Although this is genetically determined, it is a plastic trait that is subject to modification by the environment. Thus, at low temperatures and under short photoperiods, the majority of initiated buds also enter dormancy in annual-fruiting cultivars, with tip-flowering as a result. Practical applications are discussed, and it is concluded that our present physiological knowledge-base provides excellent opportunities for the manipulation of raspberry crops for out-of-season production and high yields. It also provides a firm platform for further exploration of the underlying molecular genetics of plant structures and response mechanisms.

Transfer of flower induction stimuli to non-exposed tillers in a selection of temperate grasses

The effects of time of tiller emergence on panicle formation and other flowering characteristics were studied in single plants of Scandinavian varieties of Bromus inermis, Festuca pratensis, Dactylis glomerata and Lolium perenne exposed to primary induction at 4°C/LD (24 h), 9°C/SD (10 h) or 15°C/SD. The need for primary induction varied from a predominantly SD requirement in Bromus inermis and Dactylis glomerata to a predominantly low temperature (vernalization) requirement in Lolium perenne and an almost pure low temperature requirement in Festuca pratensis. Although tillers emerging before or during primary induction had the highest chance of becoming reproductive in all species, up to 22%, 15% and 27% of the tillers emerging after termination of primary induction, and thus without direct exposure to SD, became reproductive in Bromus inermis, Dactylis glomerata and Lolium perenne, respectively. In an additional experiment with SD induction at 12°C, detillering during secondary induction increased the flowering capacity of non-directly exposed tillers from 17 to 41 in Bromus inermis. These results are discussed in the light of different hypotheses regarding control of flowering in temperate grasses. It is concluded that indirect induction, i.e. the transmission of flowering induction stimuli from mother to daughter tillers, is more likely after SD induction than after low temperature vernalization.

Production of high-yielding raspberry long canes: The way to 3 kg of fruit per cane

Summary Experiments were conducted on-Station and in growers’ nurseries with the aim of developing cheap techniques for the production of pot-grown raspberry long canes with high yield potential. In an earlier paper, we reported the feasibility of producing raspberry long canes with a yield potential of > 3 kg fruit per cane.The experiments presented here confirm that such high yields can be achieved on a regular basis, even when applying intensified and cheaper production techniques.Thus, the pot size could be reduced from the previous 3.5 l, to 2.5 l and the plant spacing could be decreased from 200 cm20 cm, to 100 cm×20 cm without loss of cane yield potential. Even at a plant spacing of 55 cm×20 cm, canes with a yield potential close to 3 kg per cane were produced. It was also shown that, during the cropping year, an additional one or two new canes with the same high yield potential could be raised concurrently with flowering and fruiting of the old cane, without a significant loss of yield on the old fruiting cane.When two canes were produced and cropped in each pot, either as two separate canes or as forked single shoots with two canes, yields of almost 4 kg per pot were achieved. However, as in our earlier experiments, long canes with such high yield potentials could only be produced under greenhouse conditions in the cool Norwegian environment.These experiments confirm the feasibility of producing raspberry long canes with yield potentials of ≥ 3 kg fruit per cane on a regular basis, even with intensified and cheaper production techniques.