The rate of net CO2 assimilation (A), the stomatal (g,) and residual (g,) conductances to C02, the intercellular CO2 concentration, the CO2 compensation points at 21% 02 (r2l) and at 2% 02 (r2) and the amounts of dry matter, nitrogen, and carbohydrates were determined, from anthesis through grain filling, in the flag leaf blade and sheath of spring wheat (Triticum aestirum L. cv Kolibri). The nitrogen content and the rate of net CO2 assimilation declined slowly until the onset of senescence in both orpns, about 3 weeks after anthesis. During senescence the reduction of A in both organs was not primarily caused by a decrease in g;, the main factor is the decrease in g,. From values of r2, and r2 it is suggested that the rate of respiration in the light contributing to the CO2 compensation point is higher in sheaths than in blades irrespective of the 02 level considered. The role of sheaths storing and later transporting assimilates to the developing grains seems to be more important for shoot yield than that of sheaths functioning as photosynthetic organs after the onset of senescence occurs. It is suggested that accumulation of carbohydrates in leaves might somehow trigger senescence in the flag leaf blade and sheath simultaneously.In wheat, the main source of assimilates for the grains is the flag leaf (8). The flag leaf has two components: the blade and the sheath. Most authors usually refer only to the flag leaf blade as the total flag leaf, thus neglecting the role of the flag leaf sheath. However, morphological characteristics of the flag leaf sheath are highly correlated to shoot yield, even more than characteristics of the flag leaf blade (19). The role of the sheath in the final shoot yield could be of particular importance in spring crops, which develop their grain filling period under warm and dry conditions such as those of a Mediterranean climate. This is based on three considerations. First, throughout the grain filling period the flag leaf sheath is more protected than the flag leaf blade from adverse environmental conditions. This could accelerate the senescence of the blades. Ledent and Moss (19) suggested the high ranking of sheaths in determining final shoot yield may be due to their functioning as photosynthetic organs during the latter part of the grain filling stage, when the flag leaf blade is senescing. There are some experimental results supporting this hypothesis (25). Second, within the canopy it appears that the photosynthetic contribution of the upper parts of the shoot (the ear, peduncle, and flag leaf blade) is relatively less important at lower latitudes, whereas the contribution of the flag leaf sheath increases with increasing elevation of the sun (22). Third, the flag sheath could store assimilates (produced by itself and the blade) and later transport them to the developing grain after the initiation of flag senescence. This may be most important in crops with a short grain filling period such as spring crops under warm conditions (29).Even though the characteristics an...

We examined changes in diet composition during the breeding period for the endangered Lesser Kestrel (Falco naumanni). Pellets were collected weekly from the last week of March until the first week of July in a colony located in southwestern Spain. Diet composition was evaluated in terms of frequency of occurrence of different prey, mean prey weight, and prey richness of each pellet. Generalized additive models were used to analyze the predictive ability of calendar week (as an index to prey availability), reproductive week (as an index to breeding demands), and nest identity (as a proxy for individual preferences/abilities) on the above dietary metrics. Primary prey species were the mole cricket (Gryllotalpa gryllotalpa) during courtship, the saddle-backed bush cricket (Ephippiger ephippiger) during incubation, and the migratory locust (Locusta migratoria) and the white-faced bush-cricket (Decticus albifrons) during the nestling phase. Small mammals were only important during some particular weeks at the beginning of the nestling period. Mean prey weight increased as the breeding season advanced, while species richness tended to decline. This was mainly due to the decreasing contribution of small prey items such as beetles to the diet and the greater incidence of large prey species. Calendar week and reproductive week had similar abilities to predict diet composition, and were better predictors than nest identity. Models fitted to particular prey species had greater explanatory power than models fitted to prey groups. Likewise, models fitted to those prey species that we considered ''preferred'' fitted better than those considered ''refuge'' prey.

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Summary 21We studied foraging habitat selection by Lesser Kestrels throughout the breeding period 22 in South-Western Spain by means of transects in which foraging observations were 23 recorded. We focused on the effect of habitat and crop type, but also on the effect of 24 vegetation structure and the presence of agricultural activities in the field on Lesser 25Kestrel's use. We considered both the accumulated use of the foraging area during the 26 breeding season and the instantaneous foraging habitat selection by kestrels. Se estudió la selección del hábitat de caza del cernícalo primilla a lo largo del ciclo 48 reproductivo en el suroeste de España mediante transectos en los que se registró el 49 comportamiento de los individuos observados. Se evaluó el efecto de los usos del suelo, 50 tipo de cultivo, estructura de la vegetación y presencia de actividades agrícolas en el uso 51 acumulado y la selección instantánea del hábitat de caza de los cernícalos. La selección 52 del hábitat de caza demostró ser muy dinámica en función del desarrollo de los cultivos 53 y las actividades agrícolas. Casi todos los cultivos herbáceos mostraron una selección 54 positiva por parte de los cernícalos en algún momento del ciclo reproductivo. El uso 55 acumulado mostró relaciones positivas con el trigo y el algodón y una selección 56 negativa de hábitats o cultivos permanentes que o bien son pobres en presas o son 57 rechazados por la especie por su estructura, como las áreas forestales, los cultivos 58 leñosos o las zonas urbanas. La estructura de la vegetación parece que juega un papel 59 preponderante en la selección instantánea del hábitat de caza. Los cernícalos 60 seleccionaron áreas con vegetación baja y con cobertura intermedia. También cazaron 61 sobre lindes y allí donde se estaban llevando a cabo actividades agrícolas, como el arado 62 o el cosechado, que facilitan el acceso a las presas. Nuestros resultados contribuyen a 63 esclarecer las aparentes controversias entre estudios previos, subrayando la importancia 64 de la heterogeneidad del paisaje agrícola alrededor de las colonias (cultivos en 65 diferentes estados de crecimiento que ofrecen variabilidad en la altura de la vegetación 66 y cobertura a lo largo de todo el ciclo reproductivo) así como el efecto de las 67 actividades agrícolas que podrían estar facilitando el acceso a las presas por parte de los 68 cernícalos. Más allá de la aproximación específica, nuestro trabajo incentiva la 69 utilización de variables objetivas como la altura y la cobertura de la vegetación --en 70 -5 -lugar de clasificaciones de interés humano como el tipo de cultivo--que reflejan mejor 71 la disponibilidad de presas y el gran dinamismo estructural de los paisajes agrícolas. 72