As a member of the Sunflower family, this plant can quickly accumulate nitrates from the soil (especially if fed with artificial fertilisers). Nitrates are known to be toxic to cattle, so although the plant itself is safe, it is not one to offer to your tortoise on a regular basis.
Although the tubers are regularly eaten by humans, the leaves and flowers are the only parts you should feed to your tortoise. See also Sunflower.
Do not confuse with Globe Artichoke, which is a completely different plant.