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The relationship between dry matter production and solar radiation interception by plant canopies is well known for several crops including sunflower. The effects of temperature environment on sunflower development have also been subject of much research. However, under seasonal rainfall, management strategies, such as the choice of sowing dates and of plant densities may change crop responses to temperature and radiation, preventing the use of standard relationships. The object of this work is to study the influence of two sowing dates on crop development and two plant densities on light interception and growth of sunflower crops grown on stored soil water in the semi-arid Mediterranean region of southern Portugal.

The cultivar "Florassol" was sown at Evora, Portugal on 13 March 1997 (S1) and on 5 May 1997 (S2) and in each sowing date at the densities of 11.4 plants m(-2) (D-1) and 4plants m-2 (D-2) Leaf number, leaf area and above-ground dry matter were measured weekly throughout the crop cycle. Meteorological parameters and canopy light transmission were measured continuously.

Crop development was slower in S1 than in S2 because of cooler temperatures during crop cycle in S1 but thermal requirements for each development phase were similar. Temperature, though affecting the duration of phenological phases, did not affect maximum leaf area per plant, which was similar for the same plant density in both sowing dates.

For the first sowing, significant differences in dry matter production and light interception were found between the two densities. Crop dry matter production and solar radiation interception were smaller in D-2 than in D-1 owing to a greater leaf area index in D-1, although leaf area and above-ground dry mass per plant were higher in the lower density (D-2) Leaf number, canopy light extinction coefficient and the radiation use efficiency were similar in both densities. (C) 2001 IMACS. Published by Elsevier Science B.V. All rights reserved.