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Physiological Performance of Synthetic Hexaploid Wheat-Derived Populations.

COMMON WHEAT is an allopolyploid species that originated from hybridization between wild diploids having the A, B, and D genomes. Evolution and artificial selection have modified morphological and physiological traits, leading to a substantially improved hexaploid wheat, which is further characterized by having the highest grain yield within the Triticum genus. Evolution, higher level of ploidy, and selection from the wild ancestors tO the modern wheat have resulted in increased grain and leaf size, longer grain filling duration (related to delayed senescence of upper leaves), and decreased net photosynthetic rate under saturating irradiance (Welbank et al., 1966, 1968; Evans and Dunstone, 1970; Khan and Tsunoda, 1970). Expanded grain and leaf dimensions were the consequence of increases in endosperm and mesophyll cell number and size. The D genome, which originated from Aegilops tauschii Coss. (commonly known as goatgrass), is the carrier of many baking quality traits. This genome also supports the wide adaptation that allows wheat cultivation even in semiarid areas (Zohary et al., 1969).

Synthetic hexaploid wheat is a relatively new germplasm obtained by artificially crossing durum wheat, Triticum turgidum L. ssp. durum (Desf.) Husn. (2n = 4x = 28, AABB) and Aegilops tauschii Coss. (2n = 14, DD), [Syn. Triticum tauschii (Coss.) Schmal]. This germplasm has proven to be very useful as a source of resistance to diseases and pests, as well as for tolerance to environmental stresses (Gorham, 1990; Limin and Fowler, 1993). Synthetic hexaploids are routinely crossed and backcrossed with common wheat (2n = 6x = 42, AABBDD) to achieve acceptable agronomic types.

Net assimilation of [CO.sub.2] through the process of photosynthesis is the initial step for biomass production. Some authors (Austin et al., 1989; Carver and Nevo, 1990) proposed the utilization of genes for higher photosynthetic rate, commonly present in wild relatives, to increase grain yield of wheat. Domestication and breeding of wheat over many years have resulted in lower photosynthetic rate (Evans and Dunstone, 1970). Wild relatives of wheat have been reported to have higher photosynthetic rates than modern cultivars (Evans and Dunstone, 1970; Khan and Tsunoda, 1970; Dunstone et al., 1973; Austin et al., 1982; Johnson et al., 1987; Carver et al., 1989). A higher photosynthetic rate may be beneficial when sink strength (kernel number, kernel weight) is increased, as seems to be the case of synthetic-derived wheats (del Blanco, 1999). Zelitch (1982) indicated that increasing the rates of net photosynthesis and translocation while enlarging the storage capacity may bring about large increases in grain yield, especially in [C.sub.3] species. Dunstone et al. (1973) found that the higher carbon exchange rate (CER) in wild genotypes was associated with reductions in stomatal and residual resistances and with increases in stomatal density. Dunstone and Evans (1974) observed that, from wild diploids to modern wheat cultivars, CER decreased as mesophyll cell size increased. Number of chloroplasts and content of Rubisco per cell increased almost threefold from diploid to hexaploid (Dean and Leech, 1982). A strong negative correlation between leaf size and CER has been reported for wheat species (Evans and Dunstone, 1970; Austin et al., 1982; Johnson et al., 1987). Planchon and Fesquet (1982) suggested that the D genome, besides being the carrier of baking quality and wide adaptation characteristics, also weakened the negative relation between CER and flag leaf area.

A variety of photoreceptor pigments, in the thylakoid membranes of the chloroplast, absorb physiologically useful radiation. Among them there are two groups of functionally cooperating chlorophyll molecules consisting of photochemically active chlorophyll a (reaction centers) and photochemically inactive chlorophyll b. Austin et al. (1987) observed that, under high light intensity, diploid wheats tend to have a higher ratio of chlorophyll a/b than hexaploid wheats. Chlorophyll a is directly involved in the conversion of light to chemical energy. Chlorophyll b and carotenoids absorb light at different wavelengths from those absorbed by chlorophyll a. They apparently can transfer the energy to chlorophyll a, extending the range of wavelengths available for photosynthesis. A higher chlorophyll a/b ratio indicates a higher concentration of photosystems per chlorophyll, i.e., a smaller photosynthetic unit size. This condition could be advantageous in high light intensity environments.

The general objective of this study was to explore some physiological features of this relatively new, synthetic-derived, germplasm. Specific objectives were (i) to measure maximum photosynthetic rate, or maximum [CO.sub.2] assimilation ([A.sub.m], expressed as [micro]mol [CO.sub.2][m.sup.-2][s.sup.-1]), of synthetic-derived lines and their respective recurrent parents; (ii) to estimate concentration of photosynthetic pigments spectrophotometrically, as well as chlorophyll a/b ratio; (iii) to assess leaf greenness with a hand-held chlorophyll meter and to relate this score to the total concentration of chlorophyll estimated spectrophotometrically; and (iv) to determine associations among physiological and agronomic traits.


Plant Material, Experimental Design, and Growing Conditions

Data were collected on 21 [BC.sub.2][F.sub.2:6] synthetic hexaploid-derived lines and the recurrent parent from three populations: Altar 84/A. tauschii (219)//2*Esmeralda (Population 1), Altar 84/A. tauschii (223)//2*Flycatcher (Population 2), Duergand 2/,4. tauschii (214)//2*Seri (Population 3).

The populations were evaluated in three adjacent, randomized complete block designs with three replications during the 1996-1997 crop season at the Agricultural Research Center for the Northwest (INIFAP) Experimental Station, near Ciudad Obregon, Sonora, Mexico. This location is 40 m above sea level, and about 27 [degrees] N and 109 [degrees] W. The climate in this region is semiarid with an average annual rainfall of 266 mm, distributed mainly during summer. The soil is coarse sandy clay, mixed montmorillonitic, typic Calciorthid (USDA-Soil Taxonomy, 1975), low in organic matter and pH of 7.7.

Seed for the experiment was sown on 28 Nov. 1996, within the optimum seeding period for the Yaqui Valley, and harvested on 9 May 1997. Seeding rate was approximately 90 kg [ha.sup.-1]. Plots consisted of six rows, 3 m long and 0.20 m between rows. The plots were fertilized with 150 kg [ha.sup.-1] N and 40 kg [ha.sup.-1] P before planting. Six irrigations, from late November to early April, ensured adequate water availability. Preventive chemical control of weeds, diseases, and insects was applied as required. Pesticides used were clodinafop {(R)-2-[4-(5-chloro-3-fluoro-2-pyridyloxy)phenoxy] propionic acid); bromoxinil (3,5-dibromo-4-hydroxybenzonitrile); and fluroxypyr {[(4-amino-3,5-dichloro-6-fluoro-2-pyridinyl)oxy] acetic acid} to control weeds; tebuconazole {[Alpha]-[2-(4 chlorophenyl)-ethyl][Alpha]-(1,1-dimethylethyl)-1H-1,2,4-triazole-l-ethanol}; and propiconazole {1-[[2-(2,4-dichlorophenyl) -4-propyl-1,3-dioxolan-2-yl]methyl]-1H, 1,2,4-triazole} twice during the crop cycle to prevent diseases; methamidophos (O,S-dimethyl phosphoramidothioate); and chlorpyrifos [O,O-diethyl O-(3,5,6-trichloro-2-pyridinyl) phosphorothioate] to control insects. All pesticides were applied at recommended rates. Neither biotic nor abiotic factors had observable effects on plant growth and development.

Photosynthesis Measurements

Maximum rate of photosynthesis per unit area ([A.sub.m]) was measured with a CIRAS-1 Portable Photosynthesis System (PP Systems, Hitchin, England) further referred to as PP-system. Stomatal conductance ([g.sub.s]), air temperature, leaf temperature, and internal carbon dioxide concentration (Ci) were simultaneously recorded by the PP-system. Measurements were taken on three flag leaf blades per plot, during the grain filling period (Feekes scale 11). Data were collected on clear days in each replication, between 1100 and 1530 h. The three replications were measured on consecutive days, completing a replication per day. Leaf temperature depression ([Delta]T) was calculated as the difference in [degrees] C between air temperature and leaf temperature. The apparent mesophyll conductance (mc) was calculated from the values of intracellular [CO.sub.2] concentration (Ci) given by the PP-system and [A.sub.m] with the common relationship (Fischer et al., 1998):

mc = [A.sub.m]/Ci

This equation assumes that the carbon dioxide concentration is zero at the active site of rubisco. Reference [CO.sub.2] of the PP-system was set approximately between 380 to 400 mg [kg.sup.-1], and it was stable. The reference relative humidity was set at 70%. Photosynthetically active radiation (PAR) was between 1200 to 1600 [micro]mol [m.sup.-2] [s.sup.-1], with most [reading.sub.s] between 1400 and 1600 [micro]mol [m.sup.-2] [s.sup.-1].

Leaf Characteristics

Five healthy and intact flag leaf blades were sampled at random from the central rows of each plot. Sampling was performed during the grain filling stage, coincident with photosynthesis measurements. Leaf area was measured with a Li-Cor Area Meter (Li-3100, LI-COR Inc., Lincoln, NE).

Leaf greenness was measured on sampled fresh leaves using a hand-held leaf greenness meter (SPAD-502, Chlorophyll Meter, Minolta Camera Co., Ltd., Tokyo, Japan), further referred to as SPAD. The average of three readings (top, center, and base of the leaf blade) per leaf was used as a greenness score.

Staygreen Determination

It is crucial for optimum grain filling that flag leaves should remain photosynthetically active until physiological maturity. Staygreen trait (Stg) was determined as the difference between senescence and physiological maturity (staygreen = days to senescence -- days to physiological maturity). Days to senescence were estimated as the number of days from sowing to the date when visually 50% of the flag leaves turned yellow (50% of chlorophyll remaining) in each plot. Days to maturity were the number of days from sowing to the date when visually 50% of the peduncles turned yellow, which indicated the cessation of assimilate translocation to the spike. A duration of two or fewer days between senescence and maturity is considered "good" (Stg = -2 to 0), a duration of more than 6 d is considered "poor" (Stg = -6).

Determination of Grain Yield and Biomass

Grain yield (kg [ha.sup.-1]) was calculated from the weight of the grain harvested in a 4.8-[m.sup.2] plot. A grain sample of approximately 100g was taken from each plot, weighed, and oven-dried at 70 [degrees] C for 48 h to determine grain moisture content. Grain yields were adjusted to 0% moisture level.

Above ground biomass or biological yield was derived from a random subsample of 50 tillers from each plot. Tillers were sampled at physiological maturity and oven dried for 48 h at 70 [degrees] C. Biomass was estimated by solving the following equation:

Biomass (g [m.sup.-2]) = [(TGW + 50 GW)/HA]/HI

where TGW = total grain weight from the 4.8 [m.sup.2] plot combined-harvested and adjusted to 0% moisture; 50 GW = grain weight of the 50 sampled tillers adjusted to 0% moisture; HA: harvested area (4.8 [m.sup.2]); HI = harvest index estimated from: 50 GW / 50 SW (where 50 SW is the dry weight of the 50 sampled tillers).

Determination of Photosynthetic Pigments

Chlorophyll and carotenoid concentrations, and chlorophyll a/b ratios were determined spectrophotometrically according to the methods of MacKinney (1941) and Arnon (1949). Equations used were based on Lichtenthaler (1987).

Statistical Analysis

Separate analysis of variance for all measured traits and three populations were performed using the Statistical Analysis System Software (SAS Institute Inc., 1993). Separation of means was determined by least significant differences (LSD).

Associations among traits were calculated by the "CORR" procedure that estimates phenotypic (Pearson) correlations.

Simple linear regression of maximum photosynthetic rate on leaf area, as well as on leaf conductances, was performed to assess the variation in [CO.sub.2] assimilation associated with independent variables. Simple linear regression of leaf greenness on chlorophyll concentration was also estimated to ascertain whether the SPAD could be a practical substitute to spectrophotometric chlorophyll determination.


Observed mean-square values from the analysis of variance for maximum photosynthetic rate per unit area ([A.sub.m]) are presented in Table 1. Differences (P [is less than] 0.01) in [CO.sub.2] assimilation rate were detected among genotypes for Population 1. Suggestive differences (P [is less than] 0.07) were found for Populations 2 and 3. Coefficients of variation ranged between 11.6 and 13.1%. The coefficient of determination ([R.sup.2]) was higher for Population 1 (0.80) than for Populations 2 and 3 (0.48 and 0.57 respectively).

Table 1. Observed mean squares, F-test level of probability, (P [is greater than] F), coefficient of variation and coefficient of determination for maximum photosynthetic rate ([A.sub.m]) in three synthetic-derived populations grown near Ciudad Obregon, Sonora, in crop season 1996-1997.
Source of Variation       df   Population 1   Population 2
                                ([dagger])     ([dagger])

Replications               2      71.07          35.21
Genotypes                  7      96.36          18.06
Genotypes x replication   14      15.17           7.16
Error                     48       5.38           6.77
C.V.%                             11.65          13.11
P > F (Genotypes)                 0.002          0.067
[R.sup.2]                          0.80           0.48

Source of Variation       Population 3

Replications                121.16
Genotypes                    14.99
Genotypes x replication       6.02
Error                         6.81
C.V.%                        12.20
P > F (Genotypes)            0.069
[R.sup.2]                     0.57

([dagger]) Population 1: Altar 84/A. tauschii(219)//2*Esmeralda. Population 2: Altar 84/A. tauschii(223)//2*Flycatcher. Population 3: Duergand 2/A. tauschii(214)//2*Seri.

Mean separation by LSD for Am and other physiological and agronomic traits is presented in Table 2. Synthetic-derived lines from Populations 1 and 2 had higher or equal [A.sub.m] than their respective recurrent parent. Synthetic-derived lines from Population 3 were within one LSD of their recurrent parent. No line had lower [A.sub.m] than their respective recurrent parent. Means for stomatal and mesophyll conductances followed the same pattern. The Stg of the genotypes was not substantially different for the synthetic-derived lines than for their recurrent parents. Only two genotypes in Population 1 had lesser Stg than their recurrent parent (Seri). Entry 16, which had the highest [A.sub.m], had a very poor Stg period (Stg = -10). This entry had a very long phenological cycle, and undoubtedly this, together with the inferior Stg period, affected biomass production, grain filling, and yield.

This entry was considered an extreme outlier, with atypical vegetative cycle and non-adapted to the experimental location. Therefore, it was removed from further analysis (regression and correlation). Entry 30 also had a less than desirable Stg period (Stg = -4).

Table 2. Maximum photosynthetic rate per unit of area ((A.sub.m]), stomatal conductance (gs), mesophyll conductance (mc), leaf temperature depression ([Delta]T), grain yield (Yield), biomass, maturity (Matur.), senescence (Senesc.), staygreen (Stg), and average flag leaf area (Leaf area) of advanced lines and their recurrent parents from three synthetic hexaploid-derived populations.
Genotype                     [A.sub.m]            gs    mc

                         [micro]mol [CO.sub.2]   [m.sup.-2]
                         [m.sup.-2] [s.sup.-1]   [s.sup.-1]

Population 1([dagger])

Entry 16                       27.2              1003   96
Entry 13                       21.4               653   75
Entry 44                       20.1               667   69
Entry 21                       19.1               726   64
Entry 30                       18.5               687   61
Entry 17                       18.4               641   62
Entry 31                       17.3               563   59
Esmeralda                      17.2               630   58
LSD (0.05)                      2.8               143   16

Population 2([dagger])

Entry 76                       22.0               711   76
Entry 59                       21.2               652   74
Entry 81                       20.8               538   75
Entry 77                       20.0               624   69
Entry 66                       19.6               672   66
Entry 83                       18.8               580   64
Entry 63                       18.2               564   63
Ocoroni                        18.2               600   61
LSD (0.05)                      2.5                ns   11

Population 3([dagger])

Entry 116                      23.3               822   79
Entry 113                      22.8               746   78
Entry 141                      22.3               851   75
Seri                           21.3               772   70
Entry 123                      20.7               760   70
Entry 127                      20.7               757   69
Entry 106                      20.0               781   65
Entry 108                      19.9               851   65
LSD (0.05)                      2.6                ns   ns

Genotypes                 [Delta]T     Yield   Biomass

                         [degrees] C   kg [ha.sup.-1]

Population 1([dagger])

Entry 16                  - 1.06       2545     8 431
Entry 13                  - 0.90       7519    17 090
Entry 44                  - 0.97       7592    17 221
Entry 21                  - 0.87       6438    15 519
Entry 30                  - 0.82       5649    13 649
Entry 17                  - 0.46       7347    19 413
Entry 31                  - 0.67       7234    17 263
Esmeralda                 - 0.77       6921    16 958
LSD (0.05)                  0.25        546     1 740

Population 2([dagger])

Entry 76                  - 1.00       4969    10 787
Entry 59                  - 0.74       6842    15 912
Entry 81                  - 0.38       4574    11 760
Entry 77                  - 0.31       6719    15 474
Entry 66                  - 0.73       7043    15 576
Entry 83                  - 0.78       6692    14 543
Entry 63                  - 0.36       6474    17 000
Ocoroni                   - 0.67       6516    15 362
LSD (0.05)                  0.40        560     1 590

Population 3([dagger])

Entry 116                 - 0.88       7108    16 758
Entry 113                 - 0.81       6200    14 237
Entry 141                 - 0.99       6734    14 357
Seri                      - 0.81       7122    15 246
Entry 123                 - 0.72       6505    14 227
Entry 127                 - 1.08       5872    17 102
Entry 106                 - 0.90       6514    15 848
Entry 108                 - 1.00       6081    16 030
LSD (0.05)                    ns        505     1 560

Genotypes                Matur.   Senesc.    Stg   Leaf area

                                   days            [cm.sup.2]

Population 1([dagger])

Entry 16                  160      150      - 10     75.2
Entry 13                  132      134         2     38.4
Entry 44                  130      130         0     47.0
Entry 21                  129      131         2     43.2
Entry 30                  140      136       - 4     37.5
Entry 17                  129      130         1     44.3
Entry 31                  128      129         1     46.7
Esmeralda                 133      131       - 2     48.1
LSD (0.05)                  4        4

Population 2([dagger])

Entry 76                  133      131       - 2     36.1
Entry 59                  130      129       - 1     41.6
Entry 81                  131      131         0     32.8
Entry 77                  125      127         2     41.6
Entry 66                  135      132       - 3     43.0
Entry 83                  126      126         0     31.5
Entry 63                  124      126         2     31.3
Ocoroni                   128      127       - 1     39.5
LSD (0.05)                  3        3

Population 3([dagger])

Entry 116                 134      133       - 1     40.6
Entry 113                 126      126         0     36.7
Entry 141                 127      126       - 1     30.4
Seri                      133      131       - 2     36.2
Entry 123                 126      126         0     30.4
Entry 127                 136      136         0     43.2
Entry 106                 126      126         0     41.2
Entry 108                 127      129         2     33.4
LSD (0.05)                  2        3

([dagger]) Population 1: Altar 84/A. tauschii(219)//2*Esmeralda. Population 2: Altar 84/A. tauschii(223)//2*Flycatcher. Population 3: Duergand 2/A. tauschii(214)//2*Seri.

Maximum Photosynthesis and Leaf Characteristics

Simple linear regression of [A.sub.m] on leaf area is shown in Fig. 1. There was a linear and negative response of [A.sub.m] on leaf area (P = 0.11). This suggests that lines with smaller leaves tended to have higher [A.sub.m].


The negative association between [A.sub.m] and leaf area has been found consistently between primitive relatives and modern wheats (Evans and Dunstone, 1970; Austin et al., 1982) and also among modern wheat cultivars (Gale et al., 1974; Planchon, 1979).

Photosynthetic Pigments

Observed mean squares for concentration of photosynthetic pigments and greenness rating score are presented in Table 3. There were no significant differences among genotypes for chlorophyll a, a/b ratio, and total chlorophyll. Populations 2 and 3 had significant differences for chlorophyll b. Significant differences among genotypes in carotenoid content were also observed in Population 2. There were also differences among genotypes in leaf greenness rating for the three populations. According to these results, differences in [CO.sub.2] assimilation among genotypes cannot be explained either by the ratio of chlorophyll a to chlorophyll b, or by the total chlorophyll content.

Table 3. Observed mean squares and coefficient of variations for chlorophyll a ([Chl.sub.a]), chlorophyll b ([Chl.sub.b]), total chlorophyll ([Chl.sub.ab]), carotenoids (carot.), ratio chlorophyll a/b (a/b ratio), and greenness.
Source of variation      df   [Chl.sub.a]   [Chl.sub.b]

Population 1([dagger])

Replications              2    1.07          0.95(*)
Genotypes                 7    1.36          0.34
Error                    14    0.90          0.17
C.V. %                         7.2           8.2

Population 2([dagger])

Replications              2    3.90(**)      1.35(**)
Geuotypes                 7    0.81          0.10(*)
Error                    14    0.43          0.03
C.V. %                         4.7           3.5

Population 3([dagger])

Replications              2    2.28          0.95(**)
Genotypes                 7    2.39          0.43(*)
Error                    14    0.98          0.13
C.V. %                         8.2           8.3

Source of variation      [Chl.sub.ab]   a/b ratio   Carot.

Population 1([dagger])

Replications               3.75          0.11       0.13
Genotypes                  2.95          0.015      0.09
Error                      1.83          0.006      0.05
C.V. %                     7.4           3.1        7.1

Population 2([dagger])

Replications               9.7(**)       0.06(**)   0.08(*)
Geuotypes                  1.4           0.01       0.08(*)
Error                      0.64          0.008      0.02
C.V. %                     4.1           3.3        4.1

Population 3([dagger])

Replications               6.21          0.06(**)   0.50(*)
Genotypes                  4.75          0.02       0.05
Error                      1.78          0.01       0.08
C.V. %                     8.1           3.4        9.1

Source of variation      Greenness

Population 1([dagger])

Replications              2.13
Genotypes                10.77(**)
Error                     2.01
C.V. %                    2.8

Population 2([dagger])

Replications              2.09
Geuotypes                 3.54(*)
Error                     1.22
C.V. %                    2.2

Population 3([dagger])

Replications              4.14
Genotypes                14.80(**)
Error                     1.5
C.V. %                    2.6

(*), (**), Significant at the 0.05 and 0.01 probability level.

([dagger]) Population 1: Altar 84/A. tauschii(219)//2*Esmeralda. Population 2: Altar 84/A. tauschti(223)//2*Flycatcher. Population 3: Duergand 2/A. tauschii(214)//2*Seri.

Association among Physiological and Agronomic Traits

Pearson correlations among several physiological and agronomic traits are presented in Table 4. There was a strong association (P [is less than] 0.01) of [A.sub.m] with stomatal and mesophyll conductances, and with [Delta]T. The highest coefficient value was for the correlation between [A.sub.m] and mc. Stomatal conductance to water vapor is normally related to photosynthesis because the diffusion pathways for [CO.sub.2] and [H.sub.2]O are similar.

Table 4. Pearson phenotypic correlations for physiological and agronomic traits of [BC.sub.2][F.sub.2:6] lines in three synthetic-derived populations grown near Ciudad Obregon, Sonora, Mexico in crop season 1996 to 1997.
Character         gs        Ci          mc       [Chl.sub.ab]


[A.sub.m]      0.50(**)   0.27(*)     0.85(**)     -0.07
gs                        0.43(**)    0.26(*)      -0.25(*)
Ci                                   -0.27(*)       0.24(*)
mc                                                 -0.19
a/b ratio

Character      a/b ratio   [Delta]T    Yield   Biomass


[A.sub.m]      -0.06       -0.51(**)   -0.05   -0.16
gs             -0.06       -0.64(**)    0.05    0.03
Ci             -0.58(**)   -0.46(**)    0.15    0.17
mc              0.24(*)    -0.26(*)    -0.13   -0.25(*)
[chl.sub.ab]   -0.29(*)     0.01       -0.14   -0.17
a/b ratio                   0.07       -0.06   -0.06
[Delta]T                               -0.06   -0.02

Character      Grains [m.sup.-2]   Maturity   Staygreen


[A.sub.m]           -0.16             0.11       0.16
gs                  -0.00             0.18      -0.05
Ci                   0.12             0.10       0.08
mc                  -0.22             0.07       0.11
[chl.sub.ab]         0.14             0.01      -0.15
a/b ratio           -0.06            -0.08      -0.10
[Delta]T            -0.04            -0.23       0.02

(*), (**) Significant at the 0.05 and 0.01 levels of probability (n = 69).

In general, physiological traits were not associated with agronomic traits. The unique exception was the negative correlation between mc and biomass. This was perhaps related to an increased mc in smaller leaves.

Regression of Maximum Photosynthesis on Stomatal and Mesophyll Conductances

There was strong evidence (P [is less than] 0.01) that increases in [A.sub.m] were associated with increments in [g.sub.s] (Fig. 2). The [R.sup.2] suggests that 26% of the variation in [A.sub.m] were explained by differences in [g.sub.s]. This conclusion is supported by the positive association between [A.sub.m] and Ci (Table 4).


There also was strong evidence (P [is less than] 0.01) that increases in [A.sub.m] were associated with increments in mc (Fig. 3). The [R.sup.2] suggests that 85% of the variation in [A.sub.m] can be explained by differences in mc. The [R.sup.2] for the multiple regression of [A.sub.m] on [g.sub.s] and mc (not shown) was 89%, indicating that differences in the diffusion pathway of [CO.sub.2] were the main reasons for the variation in [A.sub.m].


This result agrees with the explanation given by Evans and Dunstone (1970) for the decline in photosynthetic rate at high light intensities during wheat evolution. They proposed the decrease in photosynthesis from primitive to modern wheat was likely due to the reduction in surface area-volume ratio of mesophyll cells, i.e., the larger mesophyll cells of more advanced wheats offer a higher resistance to [CO.sub.2] exchange due to their reduced surface area/volume ratio. Wilson and Cooper (1969) found that differences in photosynthetic rate at high light intensity among lines of Lolium perenne L. were associated with differences in mesophyll cell size. Wilson and Cooper (1970) proposed the use of mesophyll cell cross-sectional area as a selection criterion for rate of photosynthesis in relatively high light intensity. Furthermore, they indicated that it was possible to select genotypes with small mesophyll cells without reducing leaf size.

The higher rate of photosynthesis mainly due to increased mc to [CO.sub.2] exchange might prove very convenient even at low levels of water supply (Austin, 1980), since [CO.sub.2] fixation can be increased without increasing stomatal opening, and therefore [g.sub.s] and transpiration, which means that increases in photosynthesis will not necessarily result in more water use.

Regression of Leaf Greenness on Chlorophyll Content

To assess the accuracy and precision of the SPAD to ascertain chlorophyll concentration (chlorophyll a + b) among different genotypes, the association of leaf greenness readings and extractable chlorophyll measured spectrophotometrically was estimated by linear regression (Fig. 4). The response of greenness on total chlorophyll is linear and positive; hence, increases in greenness of the leaves were associated with increments in chlorophyll content (P [is less than] 0.01). Nevertheless, the distribution of the data around the regression line ([R.sup.2] = 0.35) suggests that the variation in greenness is not completely explained by changes in chlorophyll concentration.



Genetic variability in [A.sub.m] was detected among genotypes. Synthetic-derived lines having higher [A.sub.m] than their respective recurrent parents were observed in two populations. No synthetic-derived line had inferior [A.sub.m] compared with its recurrent parent, indicating that synthetic hexaploids may constitute a valuable source to enhance wheat photosynthesis. Staygreen of the synthetic-derived lines, with two exceptions, was not substantially different from that of the recurrent parents, suggesting that higher photosynthetic rates are not necessarily associated with shorter leaf duration. Maximum photosynthetic rate was negatively associated with leaf area; nevertheless, this association was not strong enough to present an obstacle for future selection of genotypes having both desirable traits. Differences among genotypes were not detected for chlorophyll concentration and chlorophyll a/b ratio, indicating that these two traits did not explain differences in [A.sub.m]. Maximum [CO.sub.2] assimilation was associated with [g.sub.s], [Delta]T, and mc. Differences in mc explained most of the variation in [A.sub.m], suggesting that this germplasm could be useful to improve wheat resistance to stress conditions.

Abbreviations: [A.sub.m], maximum photosynthetic rate; CER, carbon exchange rate; Ci, intracellular [CO.sub.2] concentration; [Delta]T, leaf temperature depression; [g.sub.s], stomatal conductance; mc, mesophyll conductance; PAR, photosynthetically active radiation; Stg, staygreen.


The authors thank the Wheat Program at CIMMYT for all the field operations, Dr. R. Villareal for kindly providing the populations for this study, and Dr. T. Amaro for his guidance during physiological data collection. The comments and suggestions of all reviewers are greatly appreciated.


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I.A. del Blanco,(*) S. Rajaram, W.E. Kronstad, and M. P. Reynolds

I.A. del Blanco, Dep. of Plant Sciences, North Dakota State Univ., Fargo, ND 58105; W.E. Kronstad, Crop and Soil Sci. Dep., Oregon State Univ., Corvallis, OR 97331; S. Rajaram and M.P. Reynolds, CIMMYT, Apdo. Postal 6-641. 06600 Mexico DF, Mexico. Technical Paper no. 11560 of the Oregon State Univ. Agric. Exp. Stn. This paper is part of a dissertation submitted by I.A. del Blanco in partial fulfillement of the requirements for the Ph.D. degree at Oregon State Univ., Corvallis, OR. Received 23 Aug. 1999. (*) Corresponding author (Isabel_delblanco@ndsu.nodak.cdu).

Published in Crop Sci. 40:1257-1263 (2000).
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Author:del Blanco, I. A.; Rajaram, S.; Kronstad, W. E.; Reynolds, M. P.
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Date:Sep 1, 2000
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