مقایسه کمی و کیفی ژنوتیپ‌های گندم نان (Triticum aestivum L.) تحت تأثیر تنش رطوبتی انتهایی و محلول‌پاشی هیومیک اسید

Introduction
In order to achieve healthy food production, application of ecological inputs such as organic and biological fertilizers is inevitable. Organic fertilizers are fertilizer compounds that contain one or more kinds of organic matter. They can improve the soil ability to hold water and nutrients.
Humic substances (HS) are natural organic compounds comprising from 50% to 90% of the organic matter of peat, lignites, sapropels, as well as non-living organic of soil and water ecosystems. According to the classical definition, HS are "a general category of naturally occurring heterogeneous organic substances that can generally be characterized as being yellow to black in color, of high molecular weight and refractory". They are a mixture of complex organic compounds that are usually separated into three fractions based on aqueous solubility: humic acids (HA), the fraction of HS that is not soluble in water under acidic conditions (pH < 2) but is soluble at higher pH values; fulvic acids (FA), the fraction of HS that is soluble in water under all pH conditions; humins, the fraction of HS that is not soluble in water at any pH value. HS cannot be described by unique, chemically defined molecular structures.
Production of drought tolerant wheat varieties which can produce acceptable yield with minimum irrigation requirement has special significance in breeding programs. Use of humic acids as soil amendments and for plants nutrition purposes enhances plants tolerance to the abiotic stresses specially drought stress. Humic acid is an eco-friendly fertilizer that improves the physical, chemical and biological properties of soil. This nutritional source has hormonal compounds and exerts a positive effect on elements absorption, quality and yield of plants.
The aim of this research was to determine the effects of humic acid on quantity and quality of bread wheat against end drought stress conditions.

Materials and methods
In this study, in order to determine the effect of potassium humate on wheat tolerance to late season drought and also quantity and quality of its grain yield, an experiment was carried out on six selected wheat genotypes including Gascogne, Sabalan, 4057, Ruzi-84, Gobustan and Saratovskaya-29. The experiment was carried out as split plot factorial based on completely randomized block design with three replications in research farm of Islamic Azad University, Ardabil branch, Iran during 2008-2009 growing season. Main plot include stress levels ( irrigation cut at anthesis stage and non-stress) and potassium humate (humate application), and sub plot included as genotypes. Treatment composition of non-stress, non humate and Gascogne wheat was considered as a control treatment. Treatments were stress levels (stress, non-stress); humate levels (humate, non-humate) and genotypes.
Applied liquid humic fertilizer (potassium humate) was derived from peat. Humic treatments were done at several times (spraying on seeds after planting, foliage spraying at tillering, stem elongation and grain filling stages). Grain yield, biological yield and protein content of grains were recorded. After grain milling by Laboratory Mill-3100, protein content of flour measured by Inframatic-8600. Data analysis was done by using SAS software and means were compared by least significant difference test (LSD).

Results and discussion
Potassium humate reduced stress intensity by 23% and also increased biological and economic yield in drought conditions. In late season drought conditions potassium humate increased the yield from 2.8 to 3.6 tons per hectare (around 26%). Humate increased seed yield of the varieties Gascogne (1.8 tons per hectare), Gobustan, Saratovskaya-29 and Sabalan (0.54 to 0.67 tons per hectare) and Ruzi-84 (0.27 tons per hectare). Seed protein content in Saratovskaya-29 was highest in both humate application conditions and without it, but the yield was low. The variety Sabalan had the highest grain yield and average protein content. The 4057 genotype and Sabalan had the highest grain yield in both stress and no-stress conditions.

Conclusion
Generally, the findings of this investigation showed that potassium humate as a natural preparation can improve quality and quantity of produced wheat grains against end seasonal drought condition.