Factors limiting the distribution of submerged waterplants in the lowland River Vecht (The Netherlands)

• 1 The significance of light climate and water movement for the absence of submerged vegetation in the largest part of the River Vecht was investigated at three stations along the river during the summer of 1991.
• 2 The underwater light climate was found to be quite adverse for the development of waterplants due to turbidity and periphyton development. Attenuation coefficients of the water were found to be high: on average 2.6, 2.8 and 3.0 m^?1 for the southern, central and northern stations, respectively. In spring most of the light attenuation (90–100%) was due to the water layer; in summer periphyton and water were equally important. The light climate limited the potential occurrence of submerged macrophytes to a zone shallower than 1m. The observed distribution of the waterplants along the river's length, however, could not be explained by the light climate alone. Presence of submerged vegetation is restricted to the southern section of the river where there is relatively little boat traffic.
• 3 Wave action caused by the (mainly recreational) boat traffic was observed to negatively affect plant growth in an in situ experiment with a breakwater. This points to physical damage by wave action caused by passing boats as the main factor determining waterplant distribution. An alternative explanation is the much higher density of houseboats in the northern section. When present, houseboats effectively shade out the zone shallower than 1 m, thus preventing the development of a submerged vegetation.

     

Beet luteovirus coat protein sequence variation

The sequences of cDNA clones covering the coat protein genes of 29 isolates of beet mild yellowing virus from sugar beet and beet western yellows virus mainly from oilseed rape were compared. The sequences could be partitioned into seven distinct clusters falling into three main groups. Group 1 isolates were found both in oilseed rape and sugar beet mainly from north Europe; group 2 isolates were from hosts other than sugar beet in England and France; group 3 isolates were beet-specific and found from northern Italy and Iran. The factors affecting this variation and its significance in relation to coat protein-mediated protection are discussed.