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In order to improve the basis for utilizing white clover (Trifolium repens L) in northern agriculture, we studied the effects of defoliation intensity on spring growth in a sub-arctic climate in relation to carbohydrate and nodule status. White clover plants (cv Snowy) were studied in a pot experiment in the field on the coast of northern Norway from spring 2001 until spring 2002. The experiment was repeated with some modifications from spring 2002 until spring 2003. During the growing season from summer to autumn, the plants were totally stripped of leaves down to the stolon, cut at four or seven cm height or left undisturbed. The plants were sampled destructively in autumn, early spring and late spring and sorted into leaves, stolons and roots. The plant material was weighed and analysed for carbohydrate reserves and nodule number. Defoliation during the growing season resulted in reduced concentration of reserve carbohydrates in autumn and reduced winter survival of the stolons. The most severe defoliation treatment reduced the herbage growth in spring. In contrast, the two milder defoliation treatments had no effect on herbage growth during spring or on total plant dry matter and nodule status in late spring. In conclusion, moderate defoliation during the growing season had no effect on herbage growth the following spring nor on carbohydrate, nodule or dry matter status of the plant in late spring. Moderate defoliation increased spring growth and thereby also the nitrogen demand of the plants. This probably enhanced nodule formation and development of inactive to active nodules. [ABSTRACT FROM AUTHOR]

Tree species adapted to the climatic conditions of the northern boreal and subarctic vegetation zones have a capacity to develop a very high level of frost hardiness, even to survive the temperature of liquid nitrogen in midwinter. Proper timing of hardening, as well as of dehardening, is crucial for winter survival of these species. In northern tree species, cessation of apical elongation growth and bud set is a prerequisite for developmental and metabolic processes leading to hardening, and this chain of events is induced by photoperiod. The northern tree species are closely adapted to the local light climate and display photoperiodic ecotypes. The critical photoperiod is under genetic control and increases with increasing latitude of origin of the eco-type. The photoperiod is probably perceived by the phytochrome system, but the role of other pigment systems, like cryptochrome, has not been studied in woody plants. Phytochrome genes have been cloned from both conifers and deciduous species, but so far we do not have any information about possible differences between photoperiodic ecotypes at the phytochrome level. Northern and southern ecotypes have different responses to red:far red ratios, which could indicate differences in composition of their phytochrome systems, for example, the proportions of phytochrome A and B. Both phytochrome A and phytochrome B can be involved in photoperiodic responses. Experiments with transgenic hybrid aspen suggest that responses to photoperiod could be affected by the amount of phytochrome A present in plants. In deciduous species, the plant hormone gibberellin A1 (GA1) can completely substitute for a long photoperiod, and short day induced cessation of growth is preceded by a significant reduction of GA1 levels, particularly in the elongation zone. Photoperiodic control of GA metabolism is supported by several studies, but very little is known about the interaction between phytochrome and GA metabolism and/or responsiveness to GA1. Although our knowledge is still very fragmentary, available results suggest that cessation of growth and initiation of hardening in trees can be controlled both through the phytochrome and the GA mediated systems. Research with tree species is a tedious and slow process, but with the emerging new methods and approaches, we may expect exciting new results in the near future. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The purpose of this study was to determine the influence of temperature applied during short day-induced budset on induction of dormancy in six ecotypes of Betula pubescens Ehrh. and two ecotypes of Betula pendula Roth. Seedlings were grown in a phytotron at constant temperatures of 9-21°C under a 12 h photoperiod (SD) during dormancy induction. Induction of dormancy was monitored by following bud flushing and shoot growth after transfer to long photoperiod conditions (24 h) at 18°C. Chilling requirement was studied in seedlings exposed to 10 weeks of SD. In both species induction of bud dormancy developed most rapidly at 15-18°C, and both 9-12°C and 21°C delayed the induction of dormancy. Raising the temperature (from 9 to 21°C) applied during induction of dormancy significantly increased the chilling requirement. These responses were noted for all ecotypes tested, but in general the northern ecotypes entered dormancy more quickly than the southern ones. No such trend was recorded for chilling requirement, although a B. pubescens ecotype from Iceland and another from the coast of northern Norway appeared to require a longer chilling treatment than the other ecotypes. In conclusion, induction and depth of bud dormancy in birch are significantly affected by temperature conditions and these effects may explain some of the annual variation in dormancy and chilling requirement observed in nature. [ABSTRACT FROM AUTHOR]

Vegetative growth and freezing tolerance of two genotypes of white clover (Trifolium repens L.) collected at Svalbard (latitude 78°39' N, longitude 16°23' E) were studied in a daylight phytotron at controlled temperature and humidity. For comparison, two genotypes of a new Norwegian cultivar, Norstar, and one genotype of the cultivar AberHerald were included in the experiments. The genotypes from Svalbard gave higher dry-matter yields than the genotypes of Norstar, and one of them also exceeded the AberHerald genotype in dry-matter production at the temperatures studied, constant 12°C and alternating 15 9°C and 12 6°C (12 12 h). The genotypes from Svalbard were more tolerant of freezing than all the other genotypes. After a successive hardening treatment at 6°C and 0.5°C they obtained LT[sub 50] values (temperature for 50% survival) of -16°C. The origin of these genotypes is not known, but their high productivity and freezing tolerance may make them useful for further breeding of white clover for marginal conditions. [ABSTRACT FROM AUTHOR]