Light effects on root formation in aspen and willow cuttings

The effect of light on rooting of leafy cuttings of aspen (Populus tremula × tremuloides) and a willow hybrid (Salix caprea × viminalis) was investigated under controlled conditions in water culture. Two levels of irradiance were used, 40 and 8 W m^?2. The lower level gave the best rooting of aspen cuttings, both when applied to the stock plants before the cuttings were taken and when given to the cuttings during the rooting period. Irradiation of the cutting base during the rooting period inhibited rooting almost completely in aspen and decreased the number of roots formed in the Salix hybrid. Net photosynthesis in the cuttings of Salix decreased considerably after excision and increased again after formation of roots. Indirect evidence indicated that photosynthesis was even more affected in aspen cuttings. The possible roles of carbohydrates and inhibitors in the light effects are discussed.     

Photosynthetic efficiency of Betula pendula acclimated to different quantum flux densities

Abstract. Seedlings of Betula pendula were grown in a controlled environment chamber at quantum flux densities of 50, 250 and 600 μmol m⁻² s⁻¹. The relationship between the flux densities of absorbed CC₂ and quanta was determined for shoots of whole seedlings. Rates of both light-saturated and in situ (measured under the growing conditions) net photosynthesis were determined and the pholosynthetic quantum yields under light-limiting conditions were calculated. Anatomical leaf characteristics, chlorophyll contents and sizes and densities of the photosynthetic units (chlorophyll/P700) were determined. Chloroplasts were isolated and their rates of 2,6-dichlorophenol indophenol photoreduction were measured together with their pool sizes of the electron transport carriers plastoquinone and cylochrome ƒ.
Although acclimated to different quantum flux densities, the three birch populations showed the same quantum yield of net photosynthesis. This was approximately 0.028 in normal air (21.2 kPa oxygen) and about 0.040 when photorespiration was largely inhibited in 2.0 kPa oxygen. In addition, the in situ net photosynthesis rates were limited by the absorbed quantum flux density for low, intermediate and high light grown seedlings. It was concluded that birch acclimated to the three light regimes at different levels of organization (metabolic and anatomical). Thus, the quanta which were absorbed in situ could be transferred into chemical equivalents at an optimal and constant efficiency. The use of different reference bases for expressing rates of net photosynthesis are also discussed.     

Photosynthetic efficiency during ontogenesis of leaves of Betula pendula

Abstract. Net photosynthesis, photosynthetic electron transport, and leaf area density of photosynthetic units have been studied in developing, mature, and old leaves of seedlings of Betula pendula . The photosynthetic quantum yield under light-limiting conditions and the leaf area related rale of light-saturated net photosynthesis were lower in developing than in mature and old leaves. Developing leaves also had more oxygen inhibition of photosynthesis, a lower pool size of plastoquinone in the electron transport chain, a lower chlorophyll content and a lower leaf area density of photosynthetic units than mature and old leaves. The photosynthetic properties of The oldest leaves resulted partly from acclimation to shade and partly from a different ontogeny to that of younger leaves.     

Effects of artificial frost hardening and winter stress on net photosynthesis, photosynthetic electron transport and RuBP carboxylase activity in seedlings of Pinus silvestris

Net photosynthesis of seedlings of Pinus silvestris has been measured and compared with the activities of photosynthetic electron transport and extracted RuBP carboxylase. The effects of prolonged frost hardening (photoperiod 8 h, + 3°C) followed by winter stress at subzero temperatures were analysed. There was a parallel effect of frost hardening and winter stress on the photosynthetic properties of both intact seedlings and isolated chloroplast thylakoids. The activity of extracted RuBP carboxylase was less affected by the treatments. In relation to earlier works we conclude that the decay of net photosynthesis in winter climate is determined by the electron transport properties of the chloroplast thylakoids, i.e. by the pool sizes of photosynthetically active plastoquinone. The results of this work justify the definition of two phases in the response of conifers towards autumn and winter climates: I. Frost hardening occurs at temperatures slightly above zero and it does not affect the efficiency of photosynthesis as defined by the quantum yield at rate limiting light absorption. II. Winter stress occurs at subzero temperatures and it is characterized by a suppression of the photosynthetic efficiency as a result of damage within the photosynthetic apparatus.     

An apparatus for measuring photosynthetic quantum yields and quanta absorption spectra of intact plants

Abstract An apparatus is described which consists of an assimilation chamber mounted in the centre of a light-integrating Ulbricht sphere, irradiated through two fibreoptic light pipes. This arrangement provides totally diffuse radiation in the sphere. The quantum flux density in die sphere is measured by an integrating quantaspectrometer connected to the sphere by eight fibreoptic light pipes. The quantaspectrometer gives the spectral and the total quantum flux density in the sphere. By measuring the quantum flux density in the sphere before and after the insertion of a plant the number of absorbed quanta per unit time and plant area can be determined. In addition, the spectral distribution of the absorbed quantum flux density is obtained. CO₂-assimilation per unit time and plant area is determined in the same apparatus. The totally diffuse radiation within the sphere minimizes mutual shading between branches and leaves. Hence, well-defined light-responsecurves of photosynthesis can be obtained by plotting the flux density of C0₂-assimilation as a function of the absorbed quantum flux density, and the quantum yields of intact plants can be calculated. Examples of photosynthetic quantum yield determinations and quanta absorption spectra are given for some plant species.