Photosynthetic Gas Exchange Characteristics of Wheat Flag Leaf Blades and Sheaths during Grain Filling

Abstract

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...

Comparison of flag leaf and ear photosynthesis with biomass and grain yield of durum wheat under various water conditions and genotypes

“…The relative water content values of 40% attained at 10% available soil water (ASW) corresponded to a very severe water stress. The values of gas exchange traits (A, g s and T) per unit leaf area of flag leaves at full pot capacity were also in the same range as those reported as typical for fully irrigated bread [1] and durum wheat [2, but only g s was reported], whereas the ear values were in general higher than those previously published in durum wheat [2] and in different cereals [5, but on detached organs]. An underestimation of the total photosynthetic area (and thus an overestimation of gas exchange rates) of the ear should not be disregarded in this study.…”

Effect of leaf structure and water status on carbon isotope discrimination in field‐grown durum wheat

“…However, N on an area basis showed a somewhat weaker correlation with leaf A than LDM (Table 2), which agrees with previous results in peanut (Nageswara Rao & Wright 1994), holm oak (Fleck et al 1996) and bariey (Araus et al \991h). Even when N on an area basis has been reported to be a strong indicator of the photosynthetic capacity on a leaf-area basis of wheat, the saturating pattern for such a relationship is attained for bread wheat under mediterranean conditions at values of N on area basis over 2-0 g m"' (Evans 1983;Araus & Tapia 1987). As this threshold of N on area basis is well within the range of values for THI (Table 1), any further increase in N on an area basis would have little effect on photosynthetic capacity.…”

“…Alternatively higher photosynthetic capacity per unit leaf area and lower/:>//:'., could be associated with a greater leaf nitrogen content (Araus & Tapia 1987;Araus et al 1989), which in turn would decrease A (see Nageswara Amaro, Bergareche & Araus 1995). However, N on an area basis showed a somewhat weaker correlation with leaf A than LDM (Table 2), which agrees with previous results in peanut (Nageswara Rao & Wright 1994), holm oak (Fleck et al 1996) and bariey (Araus et al \991h).…”

Identification of Ancient Irrigation Practices based on the Carbon Isotope Discrimination of Plant Seeds: a Case Study from the South-East Iberian Peninsula

“…When water status is evaluated, one possible area of confusion is that greater rainfall might produce the same effect on as irrigation. The likelihood of this is decreased if one considers that, except for very severe drought conditions, most of the assimilates stored in the grains of cereals come from photosynthesis during grain filling (Araus & Tapia, 1987; see also references in Hay & Walker, 1989). In the western Mediterranean basin this period (from anthesis to maturity) covers 4-6 weeks (in April and May) for barley, and 5-6 weeks (in May and June) for wheat, and is characterized by the lowest rainfall and the highest evapotranspiration of the entire plant cycle.…”