Effect of temperature-photoperiod interaction on growth and winter bud development in Norway spruce (Picea Abies)

Abstract

Woody species growing in temperate and boreal regions, like Norway spruce (Picea abies) have to enter dormancy to survive winter and freezing temperatures, while simultaneously maximizing their growing period. Dormancy is a temporary inability to resume growth, even though the plant experiences favourable growth conditions. Dormancy is usually initiated by growth cessation and bud set. Many species, including Norway spruce, use the daylength, also called photoperiod, as a signal to induce seasonal life events. Such plants respond to photoperiods shorter than a certain daylength, called the critical daylength with growth cessation, bud set and further dormancy development in the autumn. In such plants photoperiods longer than a certain daylength, called the critical daylength, sustain growth. In some species, a long photoperiod is also required for bud break and re-growth. The length of the critical daylength varies both between species, but especially between provenances. Provenances are local populations that have adapted to local climatic conditions and daylength. Provenances from higher latitudes usually have a longer critical daylength for growth than those from lower latitudes. Temperature is also an important environmental factor affecting both dormancy and regrowth. With temperature both increasing in the past and predicted to increase further in the future, it is highly relevant to study the effect of temperature on the phenology in plants. Several studies have been conducted on plants responses to temperature and short days (SD) and contradictory results have been found: Studies on several species conducted in growth chambers have found that bud set occurs earlier when the plants are exposed to warmer compared to colder temperatures, while a number of field studies have found opposite results, with colder temperatures resulting in faster bud formation. In growth chamber studies, the plants have commonly been placed directly to SD shorter than the critical daylength for growth under constant temperature or alternating day and night temperature involving rapid changes. Such daylength and temperature regimes may possibly stress the plants since daylength and temperature changes are gradual in nature. The aim of this MSc thesis has been to study the effect of temperature on seedlings from the Halden (59°N) and Rana (66°N) provenances (both from Norway) of Norway spruce exposed to different bud set-inducing SD conditions. Specifically, it was tested whether the growth cessation and bud set response to temperature differed in plants exposed to gradually decreasing daylengths (24 h to 12 h) and plants exposed to constant SD conditions of 12 h photoperiod. The temperature regimes were either a) constant temperature of 12, 18 or 24°C under SD of 12 h and LD of 24 h photoperiod for comparison, or b) 12 or 18°C under gradually decreasing daylength, or c) gradually changing, alternating day and night temperatures of 18/12°C or 24/18°C day/night temperature in combination with gradually decreasing daylengths. In addition, the effect of the different temperatures on various other growth parameters was studied. Afterwards, all plants were re-transferred to LD and 18°C to study the after-effect of the temperature and daylength treatments on bud break and re-growth. The results showed that both the plants given decreasing daylengths (2 h per week down to 12 h photoperiod) and plants exposed to SD of a 12 h constant photoperiod, ceased growth and showed earlier bud set when grown at warmer temperature. The plants given decreasing daylengths had a delayed bud set response, compared to the constant 12 h SD, and plants from the northern provenance (Rana) showed faster bud set than the plants from the more southern provenance (Halden). In addition, more growth was generally observed (for most growth parameters) when the plants were kept at warmer temperatures, and plants from Halden generally grew more than those from Rana. Under constant daylengths, the plants that were exposed to 24°C did not differ from those at 18°C as much as plants at 12°C differed from those at 18°C, both with respect to growth and bud set. Furthermore, in plants exposed to alternating, gradually changing day and night temperature and decreasing daylength, bud set was more rapid under 24/18°C day/night than 18/12°C, with the Rana-plants showing earlier bud set than the Halden-plants. In both daylength treatments (combined with constant temperatures), bud break after subsequent transfer to LD and 18°C was the fastest in the plants that had been exposed to 12°C, indicating less deep dormancy in these plants compared to those from the higher temperature regimes. However, re-growth in plants from 12°C was only faster in the plants that were exposed to the decreasing daylengths. In conclusion, the response to the different tested temperature regimes was similar with earlier bud set at the highest temperature both under gradually decreasing daylengths and constant SD of 12 h photoperiod. Thus, the specific daylength regimes tested did not affect the overall bud set response to temperature.

Trearter som vokser i tempererte og boreale strøk slik som gran (Picea abies) må gå i vinterhvile for å overleve vinteren og minusgrader, mens de samtidig skal maksimere sin vekstperiode. Vinterhvile er en midlertidig manglende evne til å gjenoppta vekst, selv om planten opplever gunstige vekstvilkår. Vinterhvile er vanligvis innledet av vekstavslutning og knoppsetting. Mange arter, inkludert gran, bruker daglengden, også kalt fotoperiode, som et signal for å indusere sesongavhengige livshendelser. Disse plantene responderer på fotoperioder kortere enn en viss daglengde, kalt kritisk daglengde, med vekstavslutning, knoppsetting og videre hvileutvikling på høsten. Er fotoperioden lengre enn en viss daglengde, opprettholdes veksten. I noen arter er en lang fotoperiode også nødvendig for knoppbrytning og for å gjenoppta vekst. Lengden av den kritiske daglengden varier mellom arter, men særlig mellom provenienser. Provenienser er lokale populasjoner som har tilpasset seg til lokale klimatiske forhold og daglengder. Provenienser fra høyere breddegrader har vanligvis en lengre kritisk daglengde for vekst enn de fra lavere breddegrader. Temperatur er også en viktig miljøfaktor som påvirker vinterhvile og gjenvekst. Siden temperaturer både har økt i fortiden og forutsees å øke mer i fremtiden, er det høyst relevant å studere effekten av temperatur på fenologien i planter. Flere studier har blitt utført på planters respons på temperaturer og korte dager (KD) og her har motstridende resultater blitt funnet: Studier av flere arter utført i vekstkamre har funnet at knoppsetting skjer tidligere hvis planten utsettes for varmere, sammenlignet med kaldere temperaturer, mens noen feltstudier har funnet motsatte resultater, det vil si at kaldere temperaturer resulterer i raskere knoppdannelse. I vekstkammerstudier har planter vanligvis blitt plassert direkte til KD kortere enn den kritiske daglengden for vekst, under konstante temperaturer eller vekslende dag- og natt-temperaturer med raske endringer. Slike daglengde- og temperaturregimer stresser muligens plantene siden daglengden og temperaturen endrer seg gradvis i naturen.