Proso millet (Panicum miliaceum L.) is an annual thermophilic, drought-resistant, short-season C4 grass that is suitable for growing under the present changing meteorological conditions. However, water shortage can reduce plant production and hinder mineral nutrition in plants, including silicon, which is of crucial importance for grasses. Ultraviolet radiation can mitigate the impacts of water shortage, and therefore we examined the effects of moderate water shortage and reduced ultraviolet radiation on different leaf traits, including leaf levels of silicon and other elements, and plant biomass production. Moderate water shortage and ambient ultraviolet radiation did not affect the contents of photosynthetic pigments, while they reduced stomata density. Water shortage significantly decreased leaf light reflectance in the ultraviolet and violet regions. Leaf silicon, calcium, phosphorus, and sulphur levels were significantly lower with reduced water availability and significantly higher with reduced ultraviolet radiation. Leaf silicon levels ranged from 1.5% to 2.5% of leaf dry mass, with leaf calcium levels from 0.3% to 0.6%. Except for chlorine and potassium, the levels of these elements in the leaves were significantly positively related (p ≤ 0.05) to soil moisture levels. Water availability, but not ultraviolet radiation, significantly reduced living leaf biomass.