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Abstract: Plant competition for light is a commonly occurring phenomenon in natural and agricultural vegetations. It is typically size‐asymmetric, meaning that slightly larger individuals receive a disproportionate share of the light, leaving a limited amount of light for the initially smaller individuals. As a result, size inequalities of such stands increase with competition intensity. A plant's ability to respond morphologically to the presence of neighbour plants with enhanced shoot elongation, the so‐called shade avoidance response, acts against the development of size inequalities. This has been shown experimentally with transgenic plants that cannot sense neighbours and, therefore, show no shade avoidance responses. Stands of such transgenic plants showed a much stronger development of size inequalities at high plant densities than did wild type (WT) stands. However, the transgenic plants used in these experiments displayed severely hampered growth rates and virtually no response to neighbours. In order to more precisely study the impact of this phenotypic plasticity on size inequality development, experiments required plants that have normal growth rates and reduced, but not absent, shade avoidance responses. We made use of an ethylene‐insensitive, transgenic tobacco genotype (Tetr) that has wild type growth rates and moderately reduced shade avoidance responses to neighbours. Here, we show that the development of size inequalities in monocultures of these plants is not affected unambiguously different from wild type monocultures. Plots of Tetr plants developed higher inequalities for stem length than did WT, but monocultures of the two genotypes had identical CV (Coefficient of Variance) values for shoot biomass that increased with plant density. Therefore, even though reduced shade avoidance capacities led to the expected higher size inequalities for stem length, this does not necessarily lead to increased size inequalities for shoot biomass.

Plant competition for light is a commonly occurring phenomenon in natural and agricultural vegetations. It is typically size-asymmetric, meaning that slightly larger individuals receive a disproportionate share of the light, leaving a limited amount of light for the initially smaller individuals. As a result, size inequalities of such stands increase with competition intensity. A plant's ability to respond morphologically to the presence of neighbour plants with enhanced shoot elongation, the so-called shade avoidance response, acts against the development of size inequalities. This has been shown experimentally with transgenic plants that cannot sense neighbours and, therefore, show no shade avoidance responses. Stands of such transgenic plants showed a much stronger development of size inequalities at high plant densities than did wild type (WT) stands. However, the transgenic plants used in these experiments displayed severely hampered growth rates and virtually no response to neighbours. In order to more precisely study the impact of this phenotypic plasticity on size inequality development, experiments required plants that have normal growth rates and reduced, but not absent, shade avoidance responses. We made use of an ethylene-insensitive, transgenic tobacco genotype (Tetr) that has wild type growth rates and moderately reduced shade avoidance responses to neighbours. Here, we show that the development of size inequalities in monocultures of these plants is not affected unambiguously different from wild type monocultures. Plots of Tetr plants developed higher inequalities for stem length than did WT, but monocultures of the two genotypes had identical CV (Coefficient of Variance) values for shoot biomass that increased with plant density. Therefore, even though reduced shade avoidance capacities led to the expected higher size inequalities for stem length, this does not necessarily lead to increased size inequalities for shoot biomass.

Methods are described for the vegetative propagation of Begonia venosa Skan. Young flower buds are capable of producing callus which, contrasting to callus from leaves of adult plants, is very organogenic. For callus induction are required: BA and NAA at a conc. of 0.5 mgl-1, 21 °C and low irradiance. Subculture of organogenic callus is optimal on a medium with 0.1 mgl-1 BA and 2% glucose, whereas NAA is ommitted. Shoot development from preformed adventitious buds is enhanced by lowering the BA and glucose conc. Optimal rooting of excised shoots is obtained on a medium without regulators and a low glucose level. No visible mutations can be detected in the plant material, even not at the flowering stage.

Abstract: The origin of bulblets formed on excised twin scales ofNerine bowdenii was investigated anatomically. At the start of the in vitro experiments pre-existing meristems were present in the axils of the bulb scales. However, in the axils of the scale explants there were also groups of meristematic cells which developed in vitro rather quickly by mitotic divisions into thickenings and ultimately into bulb meristems. It is concluded that the formation of bulblets on excised twin scales is the result both of the outgrowth of pre-existing axillary meristems and of the regeneration of adventitious bulblets in the axils of the scales. Because more bulblets were initiated than ultimately developed, further studies to optimalize the outgrowth of primordia are needed. From an anatomical point of view there is a large potential source of in vitro bulblets.

Abstract: Agars with different performance in bioassays were analysed for physical and chemical properties. Agars with the highest gel strength had the best performance. Good performance was also related to a low pH of a suspension of agars and to a low sulphur content. The diffusion rate of ions in gels differed between agars, but could not explain differences in agar performance. The time of autoclaving had a marked effect on the gel strength, however, without affecting the performance. Chemical analysis revealed large differences between agars. The best agars had a relatively low salt content. One of the best agars had an extremely high content of trace elements. Agar impurities, especially of the trace elements were tightly bound to the agars. Up to 30% of the Murashige and Skoog (MS) salts were also immobilized in the gel. At pH 4.2, less nitrogen and phosphate were available from the MS medium than at pH 5.7. Chlorine could be washed out completely and appeared to be a good marker for agar quality and purity. With AgNO3, chlorine contamination could easily be visualized.