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Published in Crop Sci. 43:2089-2098 (2003).
© 2003 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

CROP PHYSIOLOGY & METABOLISM

Photosynthesis and Productivity of Old and Modern Durum Wheats in a Mediterranean Environment

Müjde Koç*, Celaleddin Barutçular and Ibrahim Genç

Dep. of Field Crops, Faculty of Agriculture, Univ. of Çukurova, 01330 Adana, Turkey

* Corresponding author (mkoc{at}mail.cu.edu.tr).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Genetic variability of leaf net photosynthesis rate (An) and its relation to productivity in durum wheat (Triticum turgidum L. var. durum) is not well defined. Two field experiments were conducted at the experimental area of the Faculty of Agriculture, in the University of Çukurova, Turkey, to evaluate the differences between old and modern cultivars in An and productivity. Experiments with sowing dates in November 1996 and February 1997 were performed under rainfed and irrigated conditions, respectively. Measurements included leaf gas exchange and agronomic traits. Old cultivars (OC) were later flowering and taller than modern cultivars (MC) and their harvest indexes were lower. Before the onset of leaf senescence, most of the OC exhibited higher An values than most of the MC, but the extent of variability was not great. High An was correlated with high stomatal conductance (gs), but no overall correlation between An and the ratio of internal [CO2] to [CO2] in the air was observed. In spite of higher An, OC showed no superiority in grain yield. The results indicate that mesophyll conductance rather than gs has an effect on variation in An, and that preanthesis dry matter distribution and total flag leaf photosynthesis and its duration appear to be more relevant than An for grain yield, especially under drought.

Abbreviations: An, rate of net CO2 assimilation per unit leaf area • An/Ci, apparent leaf mesophyll conductance to CO2 • Ca, ambient CO2 concentration • Ci, leaf intercellular CO2 concentration • DAP, days after planting • DAS, days after sowing • gs, stomatal conductance to water vapor • LAI, leaf area index • MC, modern cultivars • OC, old cultivars


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
DURUM WHEAT is commonly grown in water-limited environments where water use efficiency and drought-resistance traits play major roles in the successful adaptation of new genotypes to stressful environments. As stated by Austin (1980), an increase in the rate of photosynthesis per unit leaf area may cause substantial increases in dry matter production for the same or even less amounts of water use, while leaf area remains unchanged. A high net photosynthesis rate is considered to be one of the most important breeding strategies for better adaptation to stressful environments (Austin, 1987, 1989). Thus, detecting and exploiting genetic variation in photosynthesis rate could lead to the improvement of durum wheat genotypes for stressful environments. Information about the extent of variability of leaf photosynthetic traits in durum wheat genotypes is lacking (Ecochard et al., 1988). As far as we know, only two investigations dealing with photosynthesis rates of a series of old and modern genotypes of durum wheat have been published (Rees et al., 1993; Rekika et al., 1998). As stated by Delgado et al. (1994) for bread wheat (Triticum aestivum L.), there have been relatively few published studies on durum wheat which examine the genotypic variability of photosynthesis under field conditions. Moreover, the relationship between photosynthesis rate and productivity has not been well defined for durum wheat.

Studies of genotypic differences in the photosynthesis rate have often been made on flag leaves. Genotypic differences in net assimilation rate during the early stages of development have also been found by many researchers in hexaploid wheat and in related species (Fischer et al., 1981; Johnson et al., 1987; Gent and Kiyomoto, 1985; LeCain et al., 1989; Morgan et al., 1990; Rees et al., 1993; Delgado et al., 1994). However, as far as we know there exists no published study that examines such genotypic differences in durum wheat under field conditions throughout development.

The objective of this study was to determine whether there are genotypic differences in pre- and postanthesis leaf net photosynthesis rates (An) between old and modern durum wheat cultivars and, if so, whether these differences affect the productivity level. Old durum cultivars of southeastern Anatolia, one of the most important areas for the domestication and diversification of durum wheat (Harlan and Zohary, 1966), may show useful photosynthetic traits. Therefore, productivity, An, and associated gas exchange characteristics in six old durum wheat cultivars from southeastern Anatolia were compared under field conditions with modern cultivars commercially grown in this region.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Genotypes and Cultural Practices
Two field experiments were conducted at the experiment field of the Faculty of Agriculture of the University of Çukurova at Balcali, Adana (36°59' N and 35°18' E, 121 m above sea level), Turkey. The soil type is a coarse sandy clay, mixed montmorillonitic, thermic, Lithic Rhodoxeralf, low in organic matter and slightly alkaline (pH 7.1–7.6).

Experiment 1 was conducted during the 1996 to 1997 wheat growing season, with six old, well-known cultivars in southeastern Anatolia (‘Bagacak’, ‘Beyaziye’, ‘Devedisi’, ‘Hacihalil’, ‘Menceki’, and ‘Sorgül’) and six modern cultivars grown commercially in southern and southeastern Anatolia. The modern cultivars are either direct releases of CIMMYT (‘Dicle-74’, ‘Gediz-75’, ‘Balcali-85’, ‘Ege-88’) and CIMMYT/ICARDA (‘Cham-1’) advanced lines, or they were selected from crosses in which a CIMMYT line was used as a parent (‘Diyarbakir-81’). The experimental field was sown on 14 Nov. 1996, with a sowing density of 600 seeds m-2. This high seeding density was used because the realized emergence rate under experimental conditions is generally low. Plots consisted of eight rows, each 5.0 m long, with a row spacing of 0.15 m. Phosphorus (45 kg ha-1 P2O5) was applied before seeding in the form of triple super phosphate. Nitrogen was applied as ammonium nitrate in three split doses at Zadoks growth stages (ZGS) (Zadoks et al., 1974) 00, 20 and 30, at 60, 60, and 40 kg N ha-1, respectively.

In Exp. 2, eight cultivars were used comprising the same set of six old cultivars as in Exp. 1 plus two modern cultivars Cham-1 and Diyarbakir-81, which are often used as check cultivars in southern and southeastern Anatolia. Genotypes were transplanted on 13 Feb. 1997 into 1.0- by 1.2-m plots with a row spacing of 0.20 m, after the imbibed seeds were vernalized for 4 wk at 1.0 ± 0.5°C under controlled conditions. Planting density was 100 plants m-2. Four rows between plots were seeded with Cham 1. Phosphorus (80 kg ha-1 P2O5) was applied before planting. Nitrogen was top dressed at 80, 40 and 40 kg N ha-1 at ZGS'es 20, 30 and 55, respectively. Genotypes were grown under irrigated conditions. Irrigation was performed by flooding the plots. Plots received 407-mm seasonal irrigation plus 213 mm of rainfall.

Growing-season climatic conditions are presented in Fig. 1. Rainfall from 14 Nov. 1996 to the end of May 1997 was 408 mm (Fig. 1). This was 155 mm less than the long-term average. Only 18 mm of rain fell during the period from 5 March to 3 April. During this period the majority of the cultivars in Exp. 1 were in the stem elongation (ZGS:32) and early ear emergence (ZGS:50) stages, respectively. As noted above, Exp. 2 was performed under irrigated conditions. The experimental design for both experiments was a randomized complete block with four replications.



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  		Fig. 1. Climatic conditions during the 1996/1997 growing season at Adana, Turkey. Vertical arrows (bold and light in Exp. 1 and 2, respectively) indicate gas-exchange-measurement days.

 
Sampling Procedures
Crop development was scored by means of Zadoks Growth Scale (ZGS) (Zadoks et al., 1974) throughout the growing season. In Exp. 1, at ear emergence and at maturity, four rows, each 0.30 m long, were harvested from each plot by cutting the plants at soil level. Plant material was separated into different parts. Ear-bearing stems were counted. At ear emergence, the green area of the leaf blades, detached from the leaf sheath at the ligule, was measured.

In Exp. 2, area and dry weight of 10 detached main shoot flag leaves per plot were measured at ear emergence. At maturity, five plants per plot were removed by cutting at soil level. Plant material was separated into main shoot and tillers and then into different plant parts.

Leaf area was measured with an area meter (AAM-5 Hayashi-Denko, Tokyo). Plant material was dried for 48 h at 70°C and weighed. Specific flag leaf weight was calculated at ear emergence by dividing the dry weight of flag leaf by the area of the flag leaf. Above-ground biomass and grain-yield data were expressed on a per unit ground area and on a per shoot basis in Exp. 1 and on a per main shoot basis in Exp. 2.

Photosynthetic Gas Exchange Measurements
Photosynthetic measurements in Exp. 1 were made in the period between late tillering and the onset of flag leaf senescence. However, in Exp. 2, measurements were made on the flag leaf only during the grain filling period. Gas exchange characteristics of individual leaves were measured with a portable infrared gas-analyzer using an open system (LCA-3, Analytical Development Corp., Hoddeston, UK). Readings were taken with the leaf chamber (PLC-N) positioned relative to the sun to provide similar photosynthetic photon flux densities within each measurement date as illustrated in Table 1. Data were collected on clear days between 1000 and 1600 h. For each measurement, a leaf was placed across the leaf chamber, with the adaxial surface of the leaf facing up.


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  		Table 1. Gas-exchange measurement dates, growth stages and measurement conditions in Exp. 1 and 2.

 
At each measurement, CO2 assimilation (An) and transpiration rates per unit leaf area were measured from which stomatal conductance (gs) and internal CO2 concentration (Ci) were calculated. The ratio of internal leaf concentration of CO2 to the CO2 concentration of the air (Ci/Ca) and the apparent leaf mesophyll conductance (An/Ci) were also calculated. Because the ambient CO2 concentration during measurements changes, Ci/Ca ratios instead of Ci are presented in the tables. Apparent leaf mesophyll conductance (the conductance of CO2 from the stomatal cavity to the chloroplast) was calculated according to Fischer et al. (1998) to produce information on the relative importance of the mesophyll limitation to An. Measurement dates and conditions are given in Table 1. Measurements during vegetative development in Exp. 1 were made on the last fully expanded leaf at late tillering (7th leaf) on 29 Jan. 1997 and at stem elongation (penultimate leaf) on 8 Mar. 1997, 76 and 114 days after planting (DAS), respectively. Four (Exp. 1) and three (Exp. 2) measurements were made on the flag leaf after full leaf expansion and until leaf senescence (Table 1).

Data Analysis
Data collected as complete blocks within a sampling or measurement date were subject to separate or combined analysis of variance. If the cultivar effect was significant (P <= 0.05), a LSD (0.05) applicable across cultivar means was calculated. Significance of differences between the means for all old cultivars and all modern cultivars in Exp. 1 was computed. In Exp. 2, where the number of old and modern cultivars was different, t tests were applied to the data. Correlations of An with related leaf characteristics were calculated, and the significance was tested at the 0.05 and 0.01 levels of probability.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Development, Plant Height, Leaf Size, and Dry Matter Production
In Exp. 1, no statistically significant cultivar and cultivar group (old and modern) differences were observed in plant density (292 ± 34 plants m-2). In Exp. 2, plant spacing was the same (0.05 by 0.20 m) for all cultivars. In both experiments, the development of the genotypes showed small differences in early vegetative stages (data not shown). After the onset of stem elongation, the development of modern cultivars was faster than that of old genotypes (Table 2). Flag leaf expansion, ear emergence, and anthesis of modern cultivars occurred 5 to 17 d earlier than in the old cultivars.


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  		Table 2. Number of days from seeding (Exp. 1) and planting (Exp. 2) to flag leaf expansion, ear emergence and anthesis of durum wheat cultivars grown under field conditions at Balcali, Adana, Turkey.

 
In both experiments, old cultivars were taller than modern cultivars (Table 3). In Exp. 1, the leaf area index (LAI) at ear emergence varied between 2.20 (Bagacak) and 3.77 (Diyarbakir-81) (Table 3). With respect to LAI, however, there were no clear genotypic trends between old and modern cultivars. Flag leaves were generally smaller and narrower in old cultivars than modern cultivars. In Exp. 2, differences in flag leaves between old and modern cultivars were not obvious. Thus, these results provide only limited evidence that flag leaves were smaller in old cultivars. Failure to find consistent differences in individual leaf area indicates that the expression of genetic differences in leaf area may depend on the diversity of the cultivars compared, on developmental stage, or on environmental conditions. Specific flag leaf weight of most of the old cultivars tended to have higher values, but cultivar differences were evident only in Exp. 2.


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  		Table 3. Plant height, leaf area index at ear emergence and flag leaf width, area and specific weight of durum wheat cultivars grown under field conditions in Exp. 1 and 2 at Balcali, Adana, Turkey.

 
In Exp. 1, no significant genotypic differences were observed in biomass per unit ground area (Table 4). On a per shoot basis, however, genotypic differences in biomass were significant in both experiments with no clear trend for both groups of cultivars. In Exp. 1, modern cultivars were higher yielding on both per area and per ear bases than old cultivars. The greater yield of modern cultivars was associated with their higher harvest index, which is a result of greater number of grains and larger individual grain weight relative to the stover (Table 4). The higher number of grains per unit area of modern cultivars was the result of higher number of grains per ear. Ear density showed no statistically significant differences between cultivars and cultivar groups. Mean ear density was 431 ± 35 ears m-2. In Exp. 2, no clear differences were observed between old and modern cultivars for main-shoot ear yield, components of main-shoot ear yield, and plant biomass (data not shown), while harvest index (on a main shoot weight basis) was higher in modern cultivars than in old cultivars (Table 4).


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  		Table 4. Above-ground biomass, grain yield, harvest index and yield components (grain number and grain weight) of investigated durum wheat cultivars grown under field conditions in Exp. 1 and 2 at Balcali, Adana, Turkey.§

 
Cultivar Differences in An and An Related Leaf Gas Exchange Characteristics
Preanthesis Gas Exchange
Leaf gas exchange characteristics before ear emergence were only measured in Exp. 1. Cultivar differences in photosynthesis rates were already observed at the first measurement on the seventh leaf at late tillering on 29 January (Table 5). Most of the old cultivars tended to cluster in higher ranks than modern cultivars. The largest range among cultivars occurred for the penultimate leaf. For this leaf, with the exception of Sorgül, An was higher in the old cultivars. The old cultivar Sorgül, the one with the lowest An among the old cultivars, exhibited rates similar to the best modern cultivars.


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  		Table 5. Gas exchange characteristics of 7th and penultimate leaves of durum wheat cultivars grown under field conditions in Exp. 1 at Balcali, Adana, Turkey.

 
Significant cultivar differences in gs were observed for both the seventh and the penultimate leaves (Table 5). Stomatal conductance was higher in most of the old cultivars than in the modern cultivars. Because the ambient CO2 concentration during measurements changed, Ci/Ca ratios instead of Ci are presented in Table 5. There was no significant difference in Ci/Ca among cultivars. In most of the old cultivars, the apparent leaf mesophyll conductance (An/Ci: the conductance of CO2 from the stomatal cavity to the chloroplast) tended to be higher. However, cultivar differences were significant only for the penultimate leaf.

Gas Exchange of Flag Leaves
As presented in Table 1, several gas exchange measurements were made on flag leaves. At first, data for flag leaf An of each measurement were separately subjected to an analysis of variance. Significant cultivar differences were observed at each measurement in both experiments. In Exp. 2, the effect of measurement time (morning and afternoon) and cultivar x measurement time interaction at 95 DAP were not significant. Due to different measurement conditions, the magnitude of the cultivar differences for flag leaf An differed throughout development. However, when data were pooled across measurement dates before the onset of leaf senescence (measurements at 131, 148, and 159 DAS in Exp. 1; at 85 and 95 DAP in Exp. 2), there was no significant cultivar and measurement-date interaction observed for An. Thus, cultivar means for An and related gas exchange characteristics, averaged across the measurements before the onset of leaf senescence, are presented in Table 6 for nonsenescent flag leaves.


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  		Table 6. Gas exchange characteristics of nonsenescent flag leaves of durum wheat cultivars grown under field conditions in Exp. 1 and 2 at Balcali, Adana, Turkey.

 
Nonsenescent flag leaves of most old cultivars exhibited higher An values than most of the modern cultivars (Table 6). The extent of variability was not high. In Exp. 1, no clear differences between old and modern cultivars were observed in nonsenescent flag leaf gs. In Exp. 2, however, most of the old cultivars tended to have lower gs. Leaf intercellular CO2 concentration, expressed as Ci/Ca ratio, was generally lower in old cultivars than in modern cultivars. There was a significant cultivar x measurement date interaction for An/Ci in Exp. 2, but cultivars differed when tested against this interaction. The ranking of An/Ci followed very closely those of An rates. As with the An values, with the exception of Sorgül, An/Ci of old cultivars ranked higher than the modern cultivars indicating greater mesophyll capacity (i.e., greater An at a given Ci) of these leaves as well. Therefore, leaf mesophyll conductance may be more important than stomatal conductance in causing the observed cultivar differences in An.

After the onset of flag leaf senescence, most of the old cultivars still showed higher An and An/Ci in Exp. 2 under well watered conditions. However in Exp. 1 under drought conditions no significant cultivar differences in An were observed, although the flag leaves of old cultivars were younger (Table 7).


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  		Table 7. Gas exchange characteristics of senescent flag leaves of durum wheat cultivars grown under field conditions in Exp. 1 and 2 at Balcali, Adana, Turkey.

 
The Relationships between An and An Related Leaf Characteristics
Consistent with previous studies (Johnson et al., 1987; Morgan et al., 1990; Morgan and LeCain, 1991; Del Blanco et al., 2000), correlation coefficients between An and gs were positive (Table 8), but this relationship was significant only for low numbered leaves of old cultivars. Contrary to gs, An was not correlated with Ci/Ca over the course of the development.


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  		Table 8. Correlation coefficients between An and An related leaf traits of durum wheat grown under field conditions in Exp. 1 and 2 at Balcali, Adana, Turkey.

 
Correlation coefficients between An and the leaf width were generally negative, but correlation was significant only for the penultimate leaf of old cultivars. Correlations of flag leaf An with leaf area and specific leaf weight were weak and inconsistent.

Seasonal Changes in An of the Old and Modern Cultivars
Although the magnitude of cultivar differences for leaf An was not great, a consistent trend in old and modern cultivars was observable. To visualize this trend, changes in An means of both groups as a function of time, in Exp. 1 and 2, are presented in Fig. 2. Because of the limited number of measurement dates, these data should not be considered as representing the daily time course of An. However, curves have been traced through the measurement dates to make it easier to distinguish seasonal trends of old and modern cultivars.



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  		Fig. 2. Seasonal changes in leaf net photosynthesis rate (An) of old and modern cultivars grown under field conditions in Exp. 1 and 2 at Balcali, Adana, Turkey. Bars represent standard errors. Dashed and solid arrows indicate mean date of ear emergence of old and modern cultivars, respectively.

 
As shown in Fig. 2, the seasonal pattern of An was fairly similar in both groups. Old cultivars tended to have higher values of An than modern cultivars. The extent of differences in flag leaf An between groups was not as great as it was at early stages of development, which was measured only in Exp. 1. The greater flag leaf An of old cultivars may be related to differences in the age of flag leaves between groups. The mean date of flag leaf expansion of modern cultivars was about 1 wk earlier than in the old cultivars. This suggests a confounding influence of the relative phenology of the groups on the flag leaf An values. However, repeated measurements before the onset of flag leaf senescence showed that old cultivars had consistently high An. In Exp. 1, flag leaf An began to decline at about the same time in both groups, although the flag leaves of old cultivars were younger.


   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Agronomic Traits
The differences in investigated agronomic traits among old and modern cultivars that were observed in this study are very similar to those reported earlier for the same set of cultivars by Genç et al. (1993) and Koç et al. (2000). Thus, these traits appear to be robust across several environments. As reported for hexaploid wheat (Austin et al., 1980; Cox et al., 1988; Siddique et al., 1989) and for durum wheat (Pacetti and Annicchiarico, 1998), old cultivars were later and taller than modern cultivars and their harvest indexes were lower. The biomass of both groups was similar. Lower harvest index of old cultivars was a result of lower number of grains and lower individual grain weight. Differences in grain weight were pronounced under drought conditions. This suggests that the distribution of dry matter before anthesis is not effective in old cultivars and in addition to this, the supply of assimilate for grain growth is limited under drought conditions.

Photosynthesis and Related Traits
By repeated measurements during development, it was possible to show genotypic differences in An between old and modern cultivars, even if the extent of variability was not high. Old cultivars tended to have higher values of An than modern cultivars. Cultivar differences in An were already observed at early growth stages. The extent of differences in flag leaf An between groups was not as great as it was at early stages of development. There was a weak but significant correlation between the flag leaf An and the An of seventh (r = 0.60, P < 0.05) and penultimate (r = 0.61, P < 0.05) leaves, which implies the possibility of an early-stage prediction for a higher flag leaf photosynthesis. However, this association should be interpreted with caution because the correlation was weak.

In this study, although higher An was associated with higher gs, no overall correlation was observed between An and Ci/Ca. Farquhar and Sharkey (1982) theorized that a positive correlation between An and Ci/Ca is expected if CO2 supply is the dominant factor causing differences in An. Thus, it is suggested that mesophyll conductance was the dominant factor in the expression of high or low An. In earlier studies also, mesophyll conductance was found to be the dominant factor for the expression of genotypic differences in An (LeCain et al., 1989; Fischer et al., 1998; Del Blanco et al., 2000).

The maximum values of An for flag leaves measured in Exp. 1 were lower than those observed in Exp. 2 under irrigated conditions. These values were also lower than those usually obtained for durum wheat under more favorable conditions (Massacci et al., 1986; Ecochard et al., 1988; Di Marco et al., 1988; Araus et al., 1989; Rees et al., 1993). Levels of gs and Ci/Ca were also low in Exp. 1. The maximum average gs value, which can usually reach more than 500 mmol m-2 s-1 for durum wheat under irrigated conditions (Rees et al., 1993), was only 376 mmol m-2 s-1. In addition, only 53 to 62% of the CO2 of ambient air was found in the substomatal cavity. Thus, low values of flag leaf An in Exp. 1 were most probably a result of water stress during the growing season, which was more pronounced after ear emergence. The accelerated leaf senescence in old cultivars in Exp. 1 seems also to be related in some degree to abruptly increased water stress. Premature leaf senescence as a mechanism associated with genotypic differences in drought sensitivity has been reported for wheat (Ritchie et al., 1990). But, we must be cautious here, because the age of the leaves could also cause this sensitivity. During the severe drought period, flag leaves of the old cultivars were younger than those of the modern cultivars.

Under drought conditions (Exp. 1), An of nonsenescent flag leaves of the old cultivars was greater than the modern cultivars, although stomatal conductance was similar for both groups. This indicates greater water use efficiency in old cultivars. As can be seen in Table 6 and as was reported by Barutçular et al. (2000), the photosynthetic water use efficiency (An/gs) of old cultivars was also higher under irrigated conditions (Exp. 2). These results show that old cultivars use water more efficiently if the flag leaves are young. Under drought conditions, however, they are unable to maintain their photosynthetic activity as long as under irrigated conditions.

Photosynthesis and Productivity
The question that arises is to what extent these differences in leaf An affect productivity. If An is the only dominant factor causing differences in productivity (dry-matter gain), higher productivity of old cultivars would be expected before and after anthesis.

As shown before, biomass of both groups was similar. The harvest index of old cultivars was consistently lower than that of modern cultivars. This indicates that there was higher dry weight of nongrain plant parts of old cultivars, which were produced mainly before the onset of grain growth. No leaf area measurements were done before ear emergence. Differences in leaf area index (measured only in Exp. 1) at ear emergence showed no clear genotypic trends between old and modern cultivars, although flag leaves of old cultivars were smaller. Therefore high biomass level before grain growth of the old cultivars may be caused by their higher leaf An values. Fischer et al. (1981) reported a highly significant positive correlation between early crop growth rate and An measured at early stages in spring wheat. Gent (1995) showed that before ear emergence the biomass of tall isolines of winter wheat was greater than that of dwarf isolines, while the biomass at maturity was not consistently higher in these lines. The results of Gent (1995) also showed that the greater biomass of taller isolines was correlated with increased canopy photosynthesis during stem elongation.

The relationship between An and grain yield has not been well defined for durum wheat. The results of a study of Rees et al. (1993), under warm nonstress conditions, showed no association of An with the grain yield of different durum wheat cultivars. Waddington et al. (1987) showed that durum wheat yield improvement between 1960 and 1984 was associated with greater biomass, which implies a higher photosynthesis rate unless there are respiration rate differences. In this study it was found that, in both groups of cultivars, grain yield was not related to An (data not shown). Moreover, old cultivars showed no superiority in grain yield, although they exhibited higher An. Under drought-stressed conditions (Exp. 1), grain yield of old cultivars on both per area and per ear (ear density was similar for both groups) bases was considerably lower than that of modern cultivars. Aside from An and grain yield, a major difference between old and modern cultivars was that old cultivars had smaller flag leaves. Also, the flag leaves of old cultivars expanded later than the flag leaves of modern cultivars and they senesced at the same time as modern cultivars. As a result, flag leaves of old cultivars had a shorter leaf area duration. In modern cultivars, senescence was observed to occur after significant grain growth, while in old cultivars senescence was seen before substantial grain growth. Under irrigated conditions (Exp. 2), where differences in area and duration of flag leaves between groups were not pronounced, the differences in ear grain yield were also not great. Thus, for grain yield, in addition to an efficient distribution of dry matter before anthesis, whole flag leaf photosynthesis and cumulative flag leaf photosynthesis, integrated over the life of the leaf seem to be more important than An of nonsenescent leaves. Rawson et al. (1983) found that among six wheat cultivars and their 120 progeny, ear yield was unrelated to An but correlated closely with cumulative carbon production by the flag leaf. Delgado et al. (1994) and Reynolds et al. (2000) found that wheat yield was associated with changes in An, as well as with the duration of photosynthetic activity. Our results support the conclusions of Al-Khatib and Paulsen (1990) and Delgado et al. (1994) showing that genetic diversity in whole flag leaf photosynthesis and its duration may be an important factor in determining durum wheat yield especially in stressful environments.

In summary, these results show that differences in agronomic traits between old and modern cultivars of durum wheat are generally similar to differences observed in hexaploid wheat. Old cultivars with narrow leaves were late, tall, and inefficient in dry matter distribution to the grain. The leaves of most of the old cultivars exhibited greater An values, while senescence was more rapid under drought conditions. Although net leaf photosynthesis rates before the onset of leaf senescence were greater in old cultivars than modern cultivars, they showed no superiority in grain yield. These observations suggest that, in addition to an efficient distribution of dry matter before anthesis, total flag leaf photosynthesis and its duration may be causal factors in determining grain yield in stressful environments. Genotypes possessing an efficient preanthesis distribution of dry matter and a high photosynthesis rate without a significant reduction in flag leaf area and duration could lead to crop improvement in stressful environments.


   ACKNOWLEDGMENTS
 
The financial support of the State Planning Organization of the Republic of Turkey and the Research Fund of Çukurova University is gratefully acknowledged.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Supported by the State Planning Organization of the Republic of Turkey Project no. 91K120590 and by the Research Fund of Çukurova Univ. Project no. ZF9519. A part of the data has been presented at the Seminar on Durum Wheat Improvement in the Mediterranean Region: New Challenges, held at Zaragoza, Spain, 12–14 April 2000, organized by CIHEAM, Center UdL-IRTA, CIMMYT, ICARDA.

Received for publication May 28, 2002.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 


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Right arrow Wheat

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The SCI Journals Agronomy Journal Vadose Zone Journal
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome