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WHEAT

Evaluation of Grain Yield and Its Components in Durum Wheat under Mediterranean Conditions

An Ontogenic Approach

^a Dep. Fisiología Vegetal, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain
^b Centre UdL-IRTA, Area de Conreus Extensius, Rovira Roure, 191, 25198 Lleida, Spain

^* Corresponding author (lfgm{at}ugr.es)

Received for publication May 6, 2002.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Grain yield of durum wheat (Triticum turgidum L. var. durum) under Mediterranean conditions is frequently limited by both high temperature and drought during grain growth. Path coefficient analyses, based on an ontogenetic diagram, were conducted to study grain yield formation under different temperature and moisture regimes across Spain. Six ICARDA–CIMMYT inbred lines and four Spanish commercial cultivars were grown during 1998 and 1999 at two temperature regimes (cool and warm) and under both rainfed and irrigated conditions. Path coefficient analysis revealed that grain yield under cooler conditions was mostly determined by kernel weight, whereas the number of spikes per square meter predominantly influenced grain production in the warmer environments. The number of kernels per spike had a significant contribution to grain yield, especially under drought stress conditions. These associations do not clearly appear in the simple correlation analysis. Compensatory effects among yield components were almost absent in the cooler environments, probably due to the relative availability of water and N during the critical phases of plant development. Contrarily, under warmer conditions, negative effects of the number of spikes per square meter were registered on both the number of kernels per spike and kernel weight. Path analysis appears to be a useful tool for understanding grain yield formation and provides valuable additional information for improving grain yield via selection for its yield components.

Abbreviations: CIMMYT, International Maize and Wheat Improvement Center • GFP, grain-filling period • ICARDA, International Center for Agricultural Research in Dry Areas • VP, vegetative period

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
DEVELOPING CROP CULTIVARS with high grain yield has been the principal aim of durum wheat breeding programs worldwide. In the Mediterranean region, it is of special interest because of the low and erratic distribution of rainfall, which has explained as much as 75% of the variation in wheat yield (Blum and Pnuel, 1990). In this region, most rain falls during autumn and winter, and water deficit emerges in the spring, resulting in a moderate stress for rainfed wheat around anthesis, which increases in severity throughout grain filling (Edmeades et al., 1989). Grain yield in wheat can be analyzed in terms of three yield components (number of spikes per square meter, number of kernels per spike, and kernel weight) that appear sequentially with later-developing components under control of earlier-developing ones (Hamid and Grafius, 1978; García del Moral et al., 1991). Understanding the effect of water stress and temperature regimes on yield formation becomes an essential step in the development of higher-yielding and more stable cultivars.

Tiller proliferation is one of the first developmental processes, and it occurs during early growth and depends mainly on the availability of water and N (García del Moral et al., 1991; Simane et al., 1993). Development of floral primordia takes place during the phase of rapid vegetative growth; thus, competition for limiting resources between vegetative and floral organs may occur (Miralles et al., 2000). Later, grain filling is maintained by a high contribution from assimilation before and immediately after anthesis and remobilization of vegetative reserves during kernel growth (Bidinger et al., 1977; Royo et al., 1999). The growth period most sensitive to drought stress, with respect to grain yield, is from double ridge to anthesis due to its negative impact on spikelet number and kernels per spike (Shpiler and Blum, 1991). In the same way, water deficit around anthesis may lead to a loss in yield by reducing spike and spikelet number and the fertility of surviving spikelets (Giunta et al., 1993). In addition, drought stress from anthesis to maturity, especially if accompanied by high temperatures, hastens leaf senescence, reduces the duration and rate of grain filling, and hence reduces mean kernel weight (Royo et al., 2000). Under different drought treatments, Giunta et al. (1993) and Zhong-hu and Rajaram (1994) found that kernels per spike and spikes per square meter were the yield components most sensitive to drought while kernel weight remains relatively stable due to high remobilization of stored preanthesis assimilates.

Spanish environmental conditions are characterized by an adequate amount of rainfall during winter (December and January) and late spring (mid-April to mid-May) while few precipitation events are registered during February and March. The northern area of the country has a continental climate with lower temperatures during winter and spring in contrast to the southern part, which has a Mediterranean climate characterized by a mild winter and higher temperatures at the end of the crop cycle. Therefore, under these conditions, wheat plants generally suffer a midseason drought stress that reduces spike number and fertility while kernel weight may suffer a terminal stress caused by high temperatures that increase evaporation from the soil, particularly in the warmer southern region.

The effect of high temperatures on wheat growth and development has been studied both in field environments (Shpiler and Blum, 1986; Zhong-hu and Rajaram, 1994) and in controlled growth chamber conditions (Wardlaw et al., 1989). In previous growth chamber studies, reduced yields were attributed mostly to lower kernel weight and only slightly to lower kernel number (Sofield et al., 1977; Tashiro and Wardlaw, 1990). High temperatures during grain filling decreased wheat by reduction in kernel weight (Stone and Nicolas, 1994). Gibson and Paulsen (1999) found that increasing day and night temperatures from 20 and 20°C to 35 and 20°C, respectively, from 10 d after anthesis to maturity reduced grain yield by 78%, kernel number by 63%, and kernel weight by 29%.

A considerable number of studies in small-grain cereals include correlations between grain yield and its related characters. Although these are helpful in determining the principal components influencing final grain yield, they provide incomplete information on the relative importance of the direct and indirect effects on the individual factors involved. These considerations are practically observed in cereals where yield components occur successively and may, therefore, interact in compensatory patterns during plant development (García del Moral et al., 1991; Gibson and Paulsen, 1999). Thus, simple correlations may not provide a clear picture of the importance of each component in determining grain yield. Path coefficient analysis divides the correlation coefficients into direct and indirect effects. It allows, then, the separation of the direct influence of each yield component on grain yield from the indirect effects caused by the mutual relationships among yield components themselves. Although abundant literature is found on the use of path coefficient analysis to evaluate yield formation in cereals, little information exists on the use of this technique in durum wheat, and none exists for this crop under Spanish environmental conditions. Thus, the novelty of our contribution to the subject is that it (i) provides an overall view on grain yield formation under two temperature and moisture regimes in Spain by using an ontogenetic diagram and (ii) evaluates the usefulness of path coefficient analysis as a supplement for correlation analysis to elucidate the interrelationships among characters determining grain yield in durum wheat.

	MATERIALS AND METHODS

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Eight field trials were conducted during 1998 and 1999 in different temperature regimes in Spain (Ebro Valley in the North and eastern Andalusia in the South), under both irrigated and rainfed conditions. Sites were chosen to represent two contrasting environmental conditions within Spain (Table 1). The southern area has a Mediterranean climate, with mild winters and hot, dry summers. Average seasonal precipitation is 340 mm. The northern area has a more continental climate, with lower temperatures during winter and spring and an average seasonal precipitation of 310 mm, which is irregularly distributed. During the 2 yr of this study, precipitation was generally below the long-term average, particularly in the South (Table 1). Temperatures during the grain-filling period (GFP) were much greater than the long-term averages.

The length of vegetative period (VP) was calculated as days from sowing to anthesis (growth stage 65 according to Zadoks et al., 1974). Duration of GFP was considered to be the days from anthesis to physiological maturity (growth stage 91). The number of spikes per square meter was calculated by counting the spikes contained in 1 m of one of the central rows in each plot. The number of kernels per spike was determined by counting kernels on every spike from a subsample of 10 plants selected from 1 m of row taken completely at random in each plot before harvest. Mean kernel weight was calculated from the weight of three sets of 300 kernels plot^-1. Grain yield was determined on the basis of the harvested plot in all trials and corrected to a 120 g kg^-1 moisture basis.

Combined analyses of variance for grain yield and its related characters were performed over trials after verifying the homogeneity of trial variance errors using Bartlett's test. Year and block were considered random factors and the other effects fixed. Least significant difference (LSD) values were calculated at the 5% probability level. Correlation coefficients among all characters were computed from the mean values, over years and blocks, for each temperature–moisture regime combination. The SAS (SAS Inst., 1997) procedures and programs were used for these calculations. The characters used were

1. Duration of VP
   
2. Number of spikes per square meter
   
3. Duration of GFP
   
4. Number of kernels per spike
   
5. Kernel weight
   
6. Grain yield
   

Path coefficient analysis was performed to partition the correlation coefficient, r_ij, into direct and indirect effects. The following four sets of simultaneous equations were solved to determine the path coefficient, P_ij (with subscripts indicating the six characters):

In the equation r₁₃ = P₁₃ + r₁₂P₂₃, P₁₃ is the direct effect of Character 1 on 3 (the path coefficient) while r₁₂P₂₃ is the indirect effect of Character 1 on 3 via 2. Similar definitions apply to the other equations. The causal system assumed (as described in García del Moral et al., 1991) was based on the ontogeny of the cereal plant, and it is shown in Fig. 1 .

View larger version (17K): [in this window] [in a new window]   Fig. 1. Path coefficient diagrams showing the interrelationships among (1) duration of vegetative period (VP), (2) number of spikes per square meter (S), (3) duration of grain-filling period (GFP), (4) number of kernels per spike (KS), (5) kernel weight (KW), and (6) grain yield (GY). The single-headed arrows indicate path coefficients, and the double-headed arrows indicate simple correlation coefficients.

	RESULTS

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Direct effects obtained in the path analysis showed that spikes per square meter had a significant influence on grain yield, mainly under rainfed conditions at both temperature regimes (Table 6). Particularly in the warmer environment of the South, negative indirect effects of the number of spikes per square meter on grain yield, via kernel weight in the irrigated trials and via kernels per spike in the rainfed sites, reduced the final values of the correlations. The number of kernels per spike had a moderate to high positive effect on grain yield at both temperature regimes, mainly under rainfed conditions, although in the warmer experiments, this effect was masked in the correlation analysis because of the negative indirect effect of kernels per spike on grain yield via spikes per square meter (Table 6). Kernel weight had a greater direct effect on grain yield under the cooler conditions of the North and was less important compared with spikes per square meter and kernels per spike under the warmer conditions of the South (Table 6). The number of spikes per square meter did not influence kernel weight, except under warmer irrigated conditions where it exerted a moderate negative effect on kernel weight (Table 7). The number of kernels per spike had, in general, a slight negative influence on kernel weight (Table 7). Grain-filling period showed a significant and positive direct effect on kernel weight, mostly under irrigated conditions at both temperature regimes (Table 7), whereas it did not influence kernels per spike in the South under rainfed conditions, showing only a small negative direct effect in the cooler sites determined by its negative indirect effect via VP (Table 8). The length of VP exerted a positive direct effect on kernels per spike, predominantly under rainfed conditions at both temperatures regimes (Table 8). The number of spikes per square meter had a negligible direct effect on the number of kernels per spike in the North, whereas under southern conditions, it had a negative influence, particularly in the rainfed treatment (Table 8). The direct effect of VP on GFP was variable throughout temperature–moisture regime combinations. Thus, it was strongly negative under the cooler rainfed conditions of the North and the warmer irrigated conditions of the South and was positive in the remainder of experiments (Table 9).

	DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

Variations in grain yield between moisture regimes were predominantly associated with variations in spikes per square meter and kernels per spike. This result agrees with previous reports under water-limited conditions (Fischer and Maurer, 1978; García del Moral et al., 1991; Simane et al., 1993). Midseason drought, experienced under rainfed conditions, reduced mainly the number of spikes per square meter and kernels per spike, traits that develop during the period most sensitive to drought stress, i.e., from double ridge to anthesis (Shpiler and Blum, 1991). Giunta et al. (1993) also reported that severe water deficit around anthesis produces serious effects on wheat yield, reducing the number of spikes and spikelets and therefore causing a decrease in plant fertility. In the present study, kernel weight was not affected by water shortage under rainfed conditions, appearing as a very stable trait, possibly due to the high proportion of translocated preanthesis reserves for grain filling when photosynthetic source is limited by stress (Bidinger et al., 1977; Blum, 1983; Blum et al., 1994).

Genotype x Environment Interaction
Genotype x environment interaction is differential genotypic expression across environments and largely arises from the diverse response of genotypes to climatic variables (mainly temperature and rainfall) and soil characteristics during plant growth and development. In Mediterranean climates, variation in heading is usually an important cause of genotype x environment interaction for yield. Earlier genotypes generally perform better than latter ones in low-yielding environments because of higher water availability at the end of the crop season, but this advantage tends to disappear under high-yielding irrigated conditions. In this study, the most important interaction for grain yield and related traits was genotype x temperature regime. Overall response to temperature includes variation in heading date (earliness) of genotypes when exposed to contrasting environments. The cooler conditions of the North compared with the South delayed anthesis date for 15 d in the rainfed experiments but only for 3 d in the irrigated ones, in spite of the different sowing dates across environments and years. This effect could be due to a multiple interaction of genotype x temperature x moisture regimes, as was verified in the analysis of variance, causing a more intense effect of temperature on phenological development under water shortage conditions.

Path Analysis versus Correlation Analysis
With respect to the associations among characters and with grain yield, path analyses gave a different picture than simple correlations. Thus, coefficients of correlation provide the misleading impression that the number of spikes per square meter did not affect grain yield in the warmer environments, whereas path analysis showed the high dependency of grain yield on spikes per square meter, particularly under rainfed conditions. The relevant importance of spikes per square meter in determining grain yield in the Mediterranean region, as demonstrated in our study, confirms previous reports in barley (Hordeum vulgare L.) (Ramos et al., 1982; García del Moral et al., 1991; Dofing and Knight, 1992) and wheat (Simane et al., 1993; Del Blanco et al., 2001; Donaldson et al., 2001). Moreover, correlation analysis showed no influence of kernels per spike on grain yield under warmer rainfed conditions, whereas path analysis reveals a high and significant relationship between the two traits in accordance with previous studies (Simane et al., 1993; Del Blanco et al., 2001). Such relationship seems to derive from the fact that grain yield in wheat is frequently sink limited (Fischer, 1985; Slafer and Andrade, 1991), and for this reason, the number of kernels per spike has been reported as a promising trait in increasing grain yield in wheat, especially under drought stress conditions (Simane et al., 1993; Slafer and Andrade, 1991; Deni et al., 2000). Correlation analysis only indicated significant influences of kernel weight on grain yield in the cooler irrigated site. However, significant direct effects were revealed by the path coefficient analysis in all environments. Nevertheless, correlation and path coefficients analysis showed that kernel weight had the major effect on grain yield under cooler irrigated conditions, as found in previous works in the region (Royo, 1997; Royo and Tribó, 1997).

Yield Components Compensation
The interrelationships among yield components were, in general, of low magnitude; they were not significant in the cooler environments, probably due to a sufficient amount of water and N, accompanied by moderate temperature during the growing season. In contrast, spikes per square meter had a negative direct effect on kernels per spike and kernel weight in the irrigated and rainfed sites under the warmer conditions of the South. This agrees with results of Simane et al. (1993) in durum wheat and may indicate a compensatory effect between tiller production, apical development, and grain growth in wheat, presumably deriving from the negative allometry between these traits during plant development (Hamid and Grafius, 1978). Moreover, yield component compensation in cereals, arising from the fact that these components develop sequentially with later-developing components under control of earlier-developing ones (García del Moral et al., 1991), is particularly important under water stress (Fischer, 1985). Grain-filling period had a significant positive direct effect on kernel weight, especially under irrigated conditions. This probably resulted from increased photosynthesis when grain filling occurs under the mild stress experienced in the irrigated experiments at both temperature regimes, in accordance with previous reports (Blum, 1983; Van Oosterom and Acevedo, 1992).

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Durum wheat yield in the cooler environments of northern Spain appears to be most determined by mean kernel weight while under the warmer conditions of the South, the number of spikes per square meter seems to be the most important factor in determining grain yield both under irrigated and rainfed conditions. Selection for these traits may contribute to important increases in grain yield, particularly in drought-prone environments at both temperature regimes. The virtual absence of compensatory effects among yield components in favorable environments and the important negative compensatory effects of the number of spikes per square meter on both the number of kernels per spike and kernel weight registered in the warmer environments may explain the restricted success in durum wheat improvement observed in water-limited environments of the Mediterranean region. Path analyses were very useful in clarifying the effects of yield components and phenology on grain yield formation, which were not accurately reflected in simple correlation analyses, thus providing helpful information for durum wheat breeders.

	ACKNOWLEDGMENTS

 
The authors acknowledge Dr. J. Marinetto, L.F. Roca, and A. Cabello for management of field trials at Granada; Dr. N. Aparicio and the staff of the ACE of the Centre UdL-IRTA for their skilled technical assistance; and Dr. M.M. Nachit of the CIMMYT–ICARDA durum breeding program for providing genetic materials. Authors also thank anonymous reviewers for their valuable comments and criticism.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Funding for this study was provided by the Spanish government throughout INIA Project SC97-039-C2 and CICYT Project AGF99-0611-CO3.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

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