It is difficult to establish agronomic practices for wheat (Triticum aestivum L.) production in Mediterranean regions because of high annual variability in rainfall. Plant density is a factor of particular importance in wheat production systems because it can be controlled. This study was conducted to determine the optimum seeding rates of Mediterranean types of wheat in irrigated Mediterranean systems. Field experiments were conducted under irrigation at two locations of the Ebro Valley, Spain, during two growing seasons, 1999–2000 and 2000–2001. Six seeding rates were compared: 150, 175, 250, 300, 400, and 500 seeds m−2 with four adapted wheat varieties including a hybrid wheat. Seeding rate affected grain yield and yield components in three of the four environments, but its effect varied with the environment. The plant densities giving the highest yields were at least 400 to 500 plants m−2 for most of the varieties studied. The results suggest that the rate of seeding under irrigation for Mediterranean areas might be higher than those used in other wheat‐growing areas.

Four triticale (xTriticosecale Wittmack) genotypes were grown under rainfed conditions with limited irrigation support in Lleida in northeast Spain. For each variety, samples consisting of 10 tillers with half-sterilized spikes were taken three times from anthesis to maturity. Carbon isotope ratios (513C) were then determined in water extracts from ear bracts (glumes, paleas, and lemmas), awns and flag leaves, and in powdered kernels. For the half-sterilized spikes, carbon isotope analysis was carried out separately in bracts and awns from fertile and nonfertile spikelets. The 613C in the water-soluble fraction of awns, glumes, and glumells from fruitless spikelets was significantly higher than that from fertile spikelets sampled at mid-grain filling. Differences in 613C among sterile and fertile spikelets were not significant in samples taken a few days after anthesis or at maturity. These results are in accordance with some degree of refixation by awns and ear bracts of the CO2 respired by grains during grain filling. There was progressively higher 63C from flag leaf blades to awns, glumes, and glumells. This variation in 613C along plant parts may be caused by differences in the ratio of assimilation rate to C02-diffusive conductance. Values of 613C of mature kernels were between the values at anthesis and mid-grain filling for the water-soluble fraction of flag leaves and inner bracts and were fairly similar to those of glumes and awns. Conservation of respired CO2 by an efficient recycling mechanism in fruit could provide a significant source of C for yield productivity (2). Cereal crops have received the greatest attention in this regard (4, 9, 16). For example, Kriedemann (9) studied the refixation of respired carbon dioxide in whole ears by measuring CO2 evolution of ears in a C02-free environment in the light and dark, reporting that evolution was lower in the light and suggesting that refixation was taking place. However, evidence of bracts and awns refixing the intemal CO2 released from growing grains remains to be elucidated.Carbon isotope ratios of plant tissue have been proposed as an indicator not only of water use efficiency or photosynthetic metabolism (5) but also of recycling of respired CO2 in