Physiological responses of sunflower to water stress under different levels of zinc fertilizer

To investigate the physiological responses of sunflower (Helianthus annuus L., Alstar hybrid) to water stress under different levels of zinc fertilizer, an experiment was conducted at the Isfahan Agricultural Research Center, Isfahan, Iran, during 2008 and 2009 using a randomized complete block design within a split plot layout with three replications. Five irrigation treatments used in this experiment to impose water stress were IR1 (irrigation after 70 mm cumulative evaporation from class A evaporation pan (CE) during the entire growth cycle as control treatment), IR2 (irrigation after 120 mm CE during the entire growth cycle), IR3 (the same as IR1, except withholding one irrigation at initiation of peduncle elongating (R2)), IR4 (the same as IR1, except withholding one irrigation at the beginning of flowering (R5.1)) and IR5, (the same as IR1, except withholding one irrigation at 70 to 80% flowering (R5.7-8)). Irrigation treatments were allocated to main plots and three zinc fertilizer levels (0, 30 and 60 kg ha-1 of zinc sulfate) to subplots. Water stress reduced leaf relative water content (LRWC), chlorophyll a (CHLa) and b (CHLb), chlorophyll a/b (CHLa/b), total chlorophyll (CHLt), leaf area index (LAI), leaf dry weight (LDW) and head dry weight (HDW), but increased proline (PR) content of leaves. Sixty Kg ha-1 zinc sulfate fertilization could partly prevent deleterious effects of water stress at some occasions. This level of zinc sulfate application might be recommended under conditions similar to this experiment which sufficiency of soil zinc content to cope with water stress is in doubt.