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Introduction

Durum wheat (Triticum turgidum L. var. durum Desf.) is the preferred class of wheat for the manufacture of pasta products. Assessing the cooking quality of durum wheat pasta is difficult because it is a perceived quality. Factors influencing cooking quality of spaghetti are complex and poorly understood, consequently various tests are performed to obtain reliable quality assessments (Kovacs 1985).

The cooking test, however, is time consuming and requires a large sample size. Hence, developing acceptable quality cultivars at a successful breeding program in durum wheat depends on a number of physical and chemical tests used on experimental lines to predict the preferred cooking quality of durum wheat pasta (Dexter and Matsuo 1977; Matsuo and Dexter 1980; Kovacs 1985; D'Egidio et al. 1990).

Weight per unit volume, or test weight is used widely as a primary specification in wheat grading. Dexter et al. (1987) reported highly significant relationships between test weight and grain characteristics, milling performance and spaghetti quality in durum wheat. Differences in pasta cooking quality are believed to depend on both protein quantity and protein quality (D'Egidio et al. 1990). Durum wheat has greater grain hardiness than any other wheat classes. This is an advantage in the production of semolina, because the endosperm is less friable than that of softer wheats, allowing a higher yield of coarse-ground stock (semolina) and less durum flour (Finney et al . 1987). Hence, grain hardiness can be used to assess the semolina yield of experimental lines.

Protein content is the most important quality characteristic influencing cooking quality. Generally, as protein content increases, pasta becomes firmer, less sticky and its cooking score increases (Marchylo and Dexter 1997). Gluten content and gluten strength, which are related to protein quality, are universally acknowledged as the important prerequisites for production of quality pasta (Marchylo and Dexter 1997). The sodium dodecyl sulphate (SDS)-sedimentation volume (the Zeleny sedimentation test modified by addition of SDS), as a measure of gluten strength, is used when screening a large number of breeding lines in durum wheats (Dick and Quick 1983; Blanco et al. 1998). The sedimentation volume is well correlated with gluten strength and spaghetti cooking quality (Dexter and Matsuo 1980; Dick and Quick 1983).

Under field conditions, temperatures of 35 to 40C are common in many wheat-producing areas of the world (Gusta and Chen 1987), some of which accompany with a hot and dry wind at grain filling period. The responses of wheat plants to high temperature stress at anthesis and grain filling period differ on at least one major account. Heat stress at anthesis causes both male and female sterility (Gusta and Chen 1987), whereas heat stress at grain filling period causes only grain shriveling (Arzani unpubl). The difference between heat stress and water stress at anthesis is that water stress causes only male sterility (Gusta and Chen 1987). Thus in the present study, grain set or grain number per spike was not reduced, but grain weight was drastically reduced (Arzani unpubl).

The objectives of this study were to evaluate grain quality characteristics of durum wheat germplasm with exotic and native origins grown under irrigated field conditions at two locations, and to determine the relationship of a combination of heat and drought stress at grain filling period to end-product quality.

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