Effect of alternate irrigation on root-divided foxtail millet (Setaria italica).(Clinical report)

Introduction

Forage crops have important role in protein production and food security. Foxtail millet is one of these crops. This plant is a [C.sub.4] plant and well-adapted to arid and semiarid areas of Iran. Millet has high WUE (Hatfield et al., 2001) and produces high quantity and quality of grains (Heidari Zooleh et al., 2006). Irrigation is an increasingly important practice for sustainable agriculture in semi-arid environment of Iran. Karaj region in Iran is particularly relies on the dwindling ground water resources, so traditional irrigation methods in this region have experienced significant improvements with introduction of new technologies over the years. A new method of irrigation proposed by Kang et al. (1998) is the alternate irrigation system, by which, water is supplied to alternate sides of the plants root system. This method induces some root signals, such as production of Abscisic Acid (ABA) in the xylem to trigger drought responses, reduced stomatal conductence that reduces transpiration rate and photosynthesis to a lesser extent (Sepaskhah and Ahmadi, 2010). Kang et al. (2002) investigated alternate watering in soil vertical profile with pot-grown maize plants and found water consumption fell by between 20% (moderate soil drying) and 40% (severe soil drying) depends on the length of the watering intervals. The response also differed whether the application was based on alternate watering (AW) or drying, on either part of soil column which largely keeps its biomass production under moderate soil drying.

Alternate watering results in higher water use efficiency (WUE), root to shoot ratio, photosynthesis rate, total nutrient uptake (N, K) and crop quality (Kang et al., 1998; Kang et al., 2000; Tang and Zhang, 2005). Water-stressed plants usually have higher water use efficiencies than well-watered plant. The increase in efficiency is due to a larger decrease in plant transpiration, because of decreased green leaf area which probably reduces evaporation from soil (Karam et al., 2003). There is a negatively partial correlation between water stress and plant pigments such as chlorophyll (Abdalla and El-khoshiban, 2007; Zaidi et al., 2008). The water stress can decrease relative water content (RWC) of plant (Siddique et al., 2000; Moussa and Abdel-Aziz, 2008). Webber et al. (2006) found that common bean (Phaseolus vulgaris) is not well suited to water scarce conditions and alternate furrow irrigation as green gram (Mung bean). So, it should be necessary to test every plant for water stress and alternate irrigation method in different conditions. There are only a few studies about agronomic traits of foxtail millet and its response to partial root zone drying. The main objectives of this study were to (i) evaluate morphological and physiological traits of foxtail millet under partial root zone drying and deficit irrigation (ii) determine the WUE and forage yield of foxtail millet under partial root zone drying and deficit irrigation.

Materials and methods

Plant materials and root division method

The pot experiment was conducted in 2009 at Research Greenhouse, Faculty of Agricultural Science and Engineering, University of Tehran, Karaj, Iran. Foxtail millet seeds (Setaria italica, cv. KFM9) were planted in 27 pots (20 cm in diameter, 20 cm in depth) on May 7th, 2009. The pots were filled with light loam soil. Seeds were densely sown 1 cm deep but after emergence seedling were thinned to 12 plants per pot. The inside of the pots was divided into two vertical halves separated with a sandy soil layer (3 cm in diameter) covered by thin layer (2-3 mm in diameter) of wax, such that water exchange between the two halves of root system was prevented. This layer can break the capillarity movement of water between two layers of the soil. Seeds were planted at the sandy soil layer. In order to supply nutrients for the seedling at the sandy soil layer, it was nourished with Hogland solution. Plants were initially well-watered and irrigation treatments were only imposed 44 days after sowing.

Experimental design and treatments

The study was involved a factorial experiment in a randomized complete block design (RCBD) with three replications. The treatments were different irrigation methods and intervals. There were three irrigation intervals: I1: Control, irrigated every 2 days. I2: Mild water stress, Irrigated every 3 days. I3: Sever water stress, irrigated every 4 days. There were three methods of water application, viz: Conventional irrigation (M1): the whole root system was relatively evenly dried. Fixed irrigation (M2): fixed irrigation group, by which, water was always applied to one part of root system during the whole experimental period. Alternate irrigation (M3): watering was alternated between two halves of root system of the same pot. The watered and dried halves of root system were alternately replaced each irrigation interval. Irrigation intervals were determined according to factors such as greenhouse temperature and humidity. At each irrigation event, enough water was allowed to be absorbed by the soil in each pot, and any excess water was allowed to drain. The pots were weighed before and after each irrigation event to determine the water consumption by the plant in each pot (Sivapalan, 2006).

Plant sampling and measurements

Relative water content (RWC) of leaf was estimated according to the method proposed by Turner and Kramer (1980): RWC (%) = (fresh weight - dry weight) / (turgid weight - dry weight). Chlorophyll content was measured using chlorophyll meter (SPAD-502, Minolta, Japan) (Bail et al., 2005). The upmost leaf per plant was selected for measuring RWC and chlorophyll content. Leaf relative water content was measured at 55, 67 and 87 days after sowing. Chlorophyll content was measured at 57, 63 and 86 days after sowing. Water use efficiency was computed as following equation (Viets, 1962):

WUE=forage yield (kg) / water used to produce the yield (lit). Root volume was measured by water volume changes in a graduated cylinder. To estimate root length (R), the roots were spread out on a flat surface (of area A), on which, there were sample lines (total length H), and the number of intersections (N) between roots and lines were counted. Then root length was estimated as following formula (Newman, 1966):

R = [pi]NA/2H, Root surface area was estimated as (Darra and Raghuvanshi, 1999): Root surface area ([cm.sup.2]) = 2[{[root volume, cc] x [pi] x [root length, [cm]}.sup.0.5], Leaf Area (LA) was measured using leaf area meter. LA and Leaf Dry Weight (LDW) were used to calculate Specific Leaf …