The role of ear photosynthesis in grain filling was studied in a number of durum wheat (Triticum turgidum var durum L.) landraces and varieties from the Middle East, North Africa, and from the collections of ‘Institut National de la Recherche Agronomique’ (INRA, France) and ‘Centro International de Mejora de Maiz y Trigo’ (CIMMYT, Mexico). Plants were grown in the field in a Mediterranean climate. Flag leaves (blade plus sheath) and ears were kept in the dark from 1 week after anthesis to maturity which reduced grain weight by 22.4% and 59.0%, respectively. In a further experiment, the carbon isotope discrimination ratio (Δ) of ear bracts, awns and flag leaves was measured on samples taken at anthesis and on mature kernels. The mean value of Δ for the water soluble fraction of bracts (17.0‰) and awns (17.7‰) were lower than those of leaves (19.5‰) and fairly similar to those of kernels (17.4‰) averaged across all genotypes. Data indicate that most of the photosynthates in the grain come from ear parts and not from flag leaves. In addition, a higher water use efficiency (WUE) of ear parts than of the flag leaf is suggested by their lower Δ values. Gas exchange in ears and flag leaves was measured during grain filling. Averaged over all genotypes, CO2 diffusive conductance was about five times higher in the flag leaf than in the spike (with distal portions of awns outside the photosynthetic chamber) 2 weeks after anthesis. In absolute terms, the dark respiration rate (Rd) was greater than the net photosynthesis rate (Pn) by a factor of 1.74 in the spike, whereas Rd was much smaller, only 22.1, 65.7 and 24.8% of Pn in blade, sheath and awns, respectively. Data indicate that photosynthesis, and hence the water use efficiency (photosynthesis/transpiration), is greatly underestimated in ears because of the high rates of respiration which diminish the measured rates of net CO2 exchange. Results of 13C discrimination and gas exchange show that genotypes from North Africa have higher WUE than those from the Middle East. The high Rd values of ears as well as their low diffusive conductance suggest that CO2 from respiration may be used as source of carbon for ear photosynthesis. In the same way, the anatomy of glumes, for example, supports the role of bracts using internal CO2 as source of photosynthesis. In the first experiment, the Δ in mature grains from culms with darkened ears compared with control culms provided further evidence in support of this hypothesis. Thus, the Δ from kernels of control plants was 0.40 higher than that from ear‐darkened plants, probably because of some degree of refixation (recycling) of respired CO2 in the grains.
A study was conducted with 48 species of the amount of 14CO2 released during the first minute of dark following fixation of 4CO2 in the light. Light fixation periods varied from 5 to 60 seconds. The species examined included both monocots and dicots and represented C4, C3, and Crassulacean acid metabolism (CAM) photosynthetic types.Total C02 uptake was determined as the sum of soluble "4C (ethanol and water), insoluble 14C, and "4CO2 released during the first minute of dark. Insoluble 14C made up a very small percent of the total. The percent of soluble '4C increased with fixation time in C4 species, while the percent of "4CO2 released in the dark decreased. C3 species released a very small percent of "4CO2 in the dark, and this percent remained relatively constant with time of 14CO2 fixation whereas C4 species released a higher percent initially and this percent declined with time of fixation. CAM species were similar to C4 species.The species are grouped according to the 14CO2 released during the first minute of dark with light labeling times of 5 and 60 seconds. These comparisons are shown to separate C3 from C4 and CAM species. Further, the data suggest that the origin of the C02 in the dark release of C4 plants is quite different from that of C3 plants.The rapid efflux of CO2 immediately after darkening of illuminated leaves has been viewed as a remnant of photorespiration (7,9,12,13 PIB, has been correlated with anatomical characteristics and initial products of "4CO2 fixation (3, 8).It has been concluded from work with C3 plants that the source of CO2 in the PIB is the same as that for CO2 evolution in the light. For example, Atkins and Canvin (1) observed a rapid efflux of "4CO2 from sunflower discs into C02-free air upon transfer from light to darkness. The discs were labeled for 15 min in the light. "CO, evolution decreased from 32 jug dm-2 hr-' at 0.5 min in the dark to 8 ,ug dm-2 hr-' after 2 min of darkness and 4 /jg dm-2 hr-1 at 10 min. Specific radioactivity of the "4CO2 evolved relative to the "CO2 fed decreased from about 65% at 0.5 min to 25% at 10 min of darkness. The high relative specific activity of CO2 evolved at 0.5 min indicates that most of the substrate for the PIB was formed during the 15 min of labeling. The initial specific activity was similar when leaves were subsequently held in light and in darkness, indicating a common substrate for CO2 evolution in the light and for the PIB, and thus a correlation of the PIB and photorespiration. Tregunna et al. (13) concluded that the CO, in the PIB does not come from initial CO2 fixation products. When tobacco leaves were exposed to "CO2 in light for 40 sec, no "4CO2 evolution was observed in the first 75 sec of dark. There was some release after 3 min.Little is known about the source of CO2 in the PIB of C; plants. However, because some C4 plants lack a PIB and the PIB of C4 and C2 plants respond differently to 02 tension, it is thought that the CO2 evolved in the two groups arises via different metabolic pathways. As a precursor to...
Photosynthetic rates and related anatomical characteristics of leaves developed at three levels of irradiance (1200, 300 and 80 umol · m(-2) · s(-1)) were determined in the C4-like species Flaveria brownii A.M. Powell, the C3-C4-intermediate species F. linearis Lag., and the F1 hybrid between them (F. brownii × F. linearis). In the C3-C4 and F1 plants, increases in photosynthetic capacity per unit leaf area were strongly correlated with changes in mesophyll area per unit leaf area. The C4-like plant F. brownii, however, showed a much lower correlation between photosynthetic capacity and mesophyll area per unit leaf area. Plants of F. brownii developed at high irradiance showed photosynthetic rates per unit of mesophyll cell area 50% higher than those plants developed at medium irradiance. These results along with an increase in water-use efficiency are consistent with an increase of C4 photosynthesis in high-irradiance-grown F. brownii plants, whereas in the other two genotypes such plasticity seems to be absent. Photosynthetic discrimination against (13)C in the three genotypes was less at high than at low irradiance, with the greatest change occurring in F. brownii. Discrimination against (13)C expressed as δ (13)C was linearly correlated (r (2) = 0.81; P<0.001) with the ratio of bundle-sheath volume to mesophyll cell area when all samples from the three genotypes were combined. This tissue ratio increased for F. brownii and the F1 hybrid as growth irradiance increased, indicating a greater tendency towards Kranz anatomy. The results indicated that F. brownii had plasticity in its C4-related anatomical and physiological characteristics as a function of growth irradiance, whereas plasticity was less evident in the F1 hybrid and absent in F. linearis.
The genus Panicum has been shown to have species exhibiting a range of leaf digestibility. Leaf anatomy as well as differences in degradation of specific tissues have been implicated as factors influencing the digestibility. The objective of the present report was to examine microscopically specific cell wall types in leaves of upper and lower nodes for the pattern of attack and the microorganisms involved in cell wall degradation in Panicum species having different digestibilities. Lignified tissues (i.e., sclerenchyma and vascular tissue) were the most resistant, with xylem cells essentially not degraded and sclerenchyma cells degraded at the periphery. Mesophyll cells displayed a range in digestibility; bacteria did not adhere to the cells. The mesophyll in bottom lamina tended to be more rapidly degraded than in upper lamina of P. milioides Nees ex Trin. and P. laxum Sw. whereas differences were not observed between upper and lower lamina in other species. Epidermal cells were usually degraded after adherence by bacteria, which formed clearly defined zones of erosion in cell walls and separated the intact cuticle from the blades. However, cell walls of P. tricanthum often showed zones of erosion without adhering types of bacteria. Bundle sheath cells of C4 species were degraded mostly by adhering types of bacteria, with a thin electron dense layer often resisting digestion. Sheath cells in P. milioides and P. tricanthum were totally degraded. Two distinct morphotypes of bacteria, i.e., encapsulated cocci and irregularly‐shaped bacteria, were the predominant ones that adhered to and attacked the more slowly degraded tissues. Easily degraded cells were degraded by nonadhering microorganisms. Results indicated that differences existed in the manner of attack on plant cell walls by rumen bacteria and that these differences were reflected in variation in the ease of digestion.
V.-The Alkaloid of Hyoscyamus muticus and of Datura Xtramonium grown in Egypt.
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