Effect of zinc supply on growth and some metabolic characteristics of safflower and sunflower plants

The effect of five Zn levels, viz. 0, 2.5, 5.0, 7.5 and 10.0 mg kg⁻¹ of soil (designated as Zn₀, Zn_2.5, Zn_5.0, Zn_7.5, Zn_10.0 respectively) on the specific leaf area (SLA), dry matter production, and some related metabolic parameters of safflower (Carthamus tinctorius L.) and sunflower (Helianthus annuus L.) were studied. The first visible symptoms of Zn toxicity were observed as reduction in both specific leaf area (SLA) and dry matter yield especially of shoots along with decreased chlorophyll and carotenoids content. These symptoms were more expressed in safflower than in sunflower plants. Zinc toxicity in safflower was apparent above Zn _7.5, while in sunflower only at the highest level Zn_10.0. Net photosynthetic rate decreased at higher zinc doses. In contrast, the respiration rate in both plant species increased progressively with increasing Zn doses. Zinc content in the two tested species increased more in roots than in shoots with increased application of Zn. Zinc applied affected accumulation of other nutrients, but marked differences were noted in Fe and Mn contents of shoot and root in both plants. Also, membrane permeability increased with increasing Zn doses especially in safflower plants.