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Article Excerpt
IN SEMIARID AREAS of the world with a Mediterranean climate, rainfall decreases and soil evaporation increases in spring when bread wheat (Triticum aestivum L.) enters the grain-filling period. Wheat crops often experience water deficit and heat stress during grain growth and development, which limit productivity (Ehdaie et al., 1988; Ehdaie and Waines, 1989).
Grain growth and development in wheat depend on C from three sources: (i) carbohydrate produced after anthesis and translocated directly to the grains, (it) carbohydrate produced after anthesis but stored temporarily in the stem before being remobilized to the grains, and (iii) carbohydrate produced before anthesis stored mainly in the stem and remobilized to grains during grain filling (Gallager et al., 1975; Daniels et al., 1982; Kobata et al., 1992).
Wheat crops grown in dryland areas may depend more on stem reserves for grain filling than crops grown under well-watered conditions. Under terminal drought, there is a rapid decline of photosynthesis after anthesis that limits the contribution of current assimilates to the grain (Johnson et al., 1981). The wheat canopy respires rapidly during grain filling (Gent and Kiyomoto, 1985; McCullough and Hunt, 1989). Flag leaf photosynthesis alone cannot support both respiration and grain growth under terminal stresses (Rawson et al., 1983). Therefore, a substantial amount of the carbohydrates used during grain filling in wheat must come from reserves assimilated before anthesis (Gent, 1994).
The estimated contribution of stored assimilates to grain yield in wheat depends on the genotype, experimental conditions, and the method of measuring stored carbohydrates. Stored reserves and their contribution to grain can be estimated by measuring postanthesis changes in internode dry matter (Hunt, 1979; Pheloung and Siddique, 1991; Borrell et al., 1993; Shakiba et al., 1996; Cruz-Agado et al., 2000), or/and changes in internode water-soluble carbohydrate content during grain-filling period (Davidson and Chevalier, 1992; Kiniry, 1993; Blum et al., 1994; Shakiba et al., 1996), or estimated by difference in canopy dry weight at anthesis and at maturity excluding the grains (Takami et al., 1990; Flood et al., 1995; Ehdaie and Waines, 1996).
In temperate cereals, the importance of stem reserve-utilization for grain filling under terminal drought and heat is derived mainly from preanthesis stem-storage capacity (Bonnett and Incoll, 1992; Borrell et al., 1993). Storage increases with longer stems and greater specific weight (Blum et al., 1997). The Rht1 and Rht2 dwarfing genes of wheat were found to reduce reserve storage by 35 and 39%, respectively, as a consequence of a 21% reduction in stem length (Borrell et al., 1993). However, mobilization efficiency, defined as the percentage of maximum stem weight mobilized, was lower in tall than in dwarf genotypes under favorable conditions. The Rht1 allele in 'Maringa' wheat reduced the amount of stored reserves contributed by the peduncle and penultimate internode to grain by 56 and 40%, averaged over both well-watered and droughted treatments, as a result of 18 and 16% reduction in the internode length, respectively (Shakiba et al., 1996).
There are two component traits involved in the extent of contribution of stored reserves to grain yield in wheat (Ehdaie and Waines, 1996). The first component is the ability to store assimilates in the stem and the second component is the efficiency with which the stored reserves are mobilized and translocated to grain. The second component is a function of the genotype's sink strength, which is dependent on the number of grains per spike and grain weight.
Little is known about the extent of genotypic variation for stem storage capacity and of the efficiency with which stored reserves are mobilized and transported to grain in wheat. Knowledge of the relationship between stem reserves and utilization with plant morphological characteristics, grain growth, grain yield, and its components could be used to develop wheats more adapted to harsh environments.
In this study, we evaluated the hypothesis that internode length, weight, and specific weight of wheat genotypes affect accumulation and mobilization of stem reserves. Ten diverse bread wheats along with a spring durum wheat (T. turgidum L. var. durum) were examined for postanthesis changes in internode dry weight, water-soluble carbohydrate content, and grain growth under well-watered and droughted field conditions across 2 yr to estimate genotypic variation in accumulation and mobilization of internode reserves.
This paper estimates the magnitude of mobilized dry matter and the mobilization efficiency in different internodes of the main stem. The data also provide the basis for a physiological and genotypic study of variation in internode water-soluble carbohydrate content and concentration and the relationship between internode changes with grain growth and grain yield, which will be reported in further papers of this series.
MATERIALS AND METHODS
Ten diverse bread and a durum wheat cultivars were evaluated under two water treatments. The spring bread wheat cultivars included 'Chinese Spring', a tall landrace from China; 'No. 14' and 'No. 49', two tall landraces from southwestern and central eastern regions of Iran, respectively, 'Ramona 50', a tall older cultivar previously grown in California; 'Maringa', a tall cultivar from Brazil; 'Express', 'Anza', and 'Yecora Rojo', two semidwarf and a dwarf cultivar, respectively, all CIMMYT-derived wheat cultivars grown locally in California; and a winter type of each of Yecora Rojo and Anza. The durum wheat cultivar was 'Westbred Turbo', a semidwarf spring cultivar grown locally in California. The winter type of Yecora Rojo, which is called 'Wincora', was derived from 'Phoenix'/Yecora Rojo * 5 at the University of California, wheat breeding program (L.E Jackson, personal communication, 2002). Phoenix wheat carries a vernalization gene vrnl (Pugsley et al., 1985). Wincora inherited the vrnl gene from Phoenix. Anza (winter) was developed similarly. Wincora and Anza (winter) required a mild vernalization and were expected to be different in phasic development than Yecora Rojo and Anza (spring), respectively.
Field experiments were planted on 19 Dec. 1997 and on 15 Jan. 1998 in a Ramona Type A sandy loam soil (fine-loamy, mixed, thermic Type Haploxeralfs) at the Moreno Farm of the University of California Agricultural Experiment Station, Moreno Valley, CA. The 11 genotypes were planted in a split-plot design with four replicates (blocks). The main plots consisted of two irrigation treatments, namely well-watered and droughted treatment. The split-plot consisted of the genotypes. Plants in well-watered treatment were irrigated with sprinklers to minimize water shortage until they reached physiological maturity. Irrigation was terminated for plants in droughted treatment when plants in 50% of plots reached late booting stage on 23 Mar. 1998 and on 15 Apr. 1999. In the 1997 season, plants in well-watered treatment received 496 mm of water (132 mm irrigation + 364 mm rain) and those in droughted treatment received 430 mm of water (66 mm irrigation + 364 mm rain). In the 1998 season, plants in well-watered treatment received 332 mm of water (278 mm irrigation + 54 mm rain) and those in droughted treatment received 270 mm of water (216 mm irrigation + 54 mm rain). After irrigation was terminated in the droughted treatment, 63 and 30 mm of rain fell during early grain-filling period in the 1997 season and in the 1998 season, respectively.
Each plot consisted of six rows, 5 m in length. Interrow spacing was 20 cm and interplant spacing was 3 cm. The land was fallowed the previous year and 112 kg [ha.sup.-1] urea fertilizer was incorporated into the soil before planting.
In each plot, 30 to 40 main tillers from the two middle rows next to the guard rows were tagged as spikes emerged from the flag leaf sheaths. Three main tillers were harvested at random at anthesis and at 10-d intervals after anthesis until maturity. The main tillers were harvested from the soil surface. After each harvest, leaf blades were removed and main tillers were immediately dried in a forced-air drier at 80[degrees]C for 48 h. Then, each main tiller was divided into spike and stem; then leaf sheaths were removed from the stem. Each stem was divided into three segments, namely peduncle (first internode below the spike including the distal node), penultimate internode (the...
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