Phenology and Source-Sink Dynamics of Carbohydrates in Relation to Management of Perennial Weeds: Cirsium arvense (L.) Scop and Tussilago farfara L

Abstract

Perennial weed infestations are putting severe constraints on organic and conventional farming. Cirsium arvense (L.) Scop. and Tussilago farfara L. have high vegetative regeneration capacity from underground organs. These organs can reach deep soil layers, which prevent them from being easily uprooted. In our research, we primarily addressed the importance of the source-sink dynamics of carbohydrates at different phenological stages and related results to eventual management strategies. Experimental work was carried out under greenhouse and/or growth chamber conditions. The first study spanned from roots/rhizomes planting to shoot establishment. The second study began after establishment and included the rosette to the flowering stages. The third study was about the storage of carbohydrates in the underground parts. And finally, a fourth study aimed at unraveling the effect of drought on new shoots of these perennials. To determine the depletion of carbohydrate storage associated with shoot development, we destructively sampled roots of C. arvense and rhizomes of T. farfara from which nonstructural carbohydrates were measured using HPLC. We found that fructan was highest at the planting time, but it decreased by releasing fructose as shooting growth progressed. However, before the released fructose was depleted, carbohydrates were reloaded by photo-assimilates. This was demonstrated by 14C labelling to track the commencement of the basipetal translocation of photo-assimilates. The conclusion was that appreciable basipetal translocation starts at 6 and 8 fully developed leaves for T. farfara and C. arvense, respectively. This falls between 500 and 600 degree days or around 21 to 23 days after emergence under our experimental conditions. After shoot establishment we measured the maximum net photosynthesis of disturbed and undisturbed clones over a period of 3 weeks, from the rosette stage to the flowering stage. Maximum net photosynthesis decreased over time, but there were no differences between disturbed and undisturbed clones. Our conclusion was that the physiological integration found in other clonal species seems to be absent in C. arvense and T. farfara, suggesting that shoots are autonomous. To understand carbohydrate storage, juvenile and mature plants were grown at different temperatures. The results showed that the polymerization of fructans was associated with low temperatures for C. arvense but not for T. farfara. Polymerization of fructans in roots/rhizomes was not significantly different between juvenile and mature plants. Only the dry weight of shoots from juvenile plants reflected the differences found in carbohydrate content. The conclusion is that the continuous growth of underground propagules facilitates the survival of these two perennials and thus complicates their control. Gradient soil water content during the establishment period of shoots of C. arvense, T. farfara and Elytrigia repens showed that E. repens is more tolerant to water stress than broadleaf T. farfara and C. arvense. T. farfara is more prone to water stress compared to C. arvense if we consider the relationship between the soil water content and shoot biomass. This susceptibility might give an opportunity window for managing broadleaf perennial weeds