Effect of plant density on yield and physiological characteristics of six canola cultivars

An experiment was performed in the Seed and Plant Improvement Institute, Karaj, Iran to study the influence of plant densities on physiological traits of six canola cultivars. Treatment conditions included three different plant densities (40, 60, and 80 plants m-2) for triplicates of six canola cultivars, namely, Ahamadi, Opera, Okapi, L72, Karaj1, and Sw102. Results indicated that L72 cultivar exhibited the highest yield at the lowest plant density (40 plants m-2). For all cultivars, both plant densities of 60 and 80 plants m-2 resulted in lower relative water content than 40 plants m-2. Proline and carbohydrate content significantly increased with increasing plant densities. The highest proline content was obtained from L72 under the highest plant density, whereas the lowest was also detected in this cultivar at the lowest plant density. All cultivars grown at the lowest density showed higher amounts of photosynthetic pigments chlorophylls a and b than those at the highest density. Glucosinolate increased with increasing plant densities, with L72 yielding the highest quantity when grown at the highest density. In conclusion, a density of 40 plants m-2 is recommended for growing


Introduction
Canola is the second edible oil resource in the world with high seed oil and lowest saturated fatty acid among oilseed crops (Raymer, 2002;Shahsavari et al., 2014).Canola oil has higher nutrient value than other oilseeds due to its high unsaturated fatty acid content (Naseri et al., 2012;Shahsavari and Dadrasnia, 2016).
Using suitable cultivars that match the climate conditions and choosing ideal plant density with minimum competition among plants are methods of increasing seed yield.
Previous studies showed that plant density plays an important role in canola yield and a uniform distribution is required for yield stability (Diepenbrock, 2000).Plant density had significant effect on number of pod, secondary branch, and seeds per plant (Salehian et al., 2002;Ozone Davaji et al., 2008).The same researcher reported that the maximum number of pod and seeds per plant was observed at 50 plants m

Materials and methods
Outdoor experiments were conducted in 2014 and repeated in 2015 under identical conditions in the Seed and Plant Improvement Institute (35°59′N, 51°6′E; 300 m), Karaj, Iran.The study was designed as a factorial experiment based on a complete block design to evaluate effects of different plant densities on physiological traits of six canola cultivars, namely, Ahamadi, Opera, Okapi, L72, Karaj1, and Sw102.Treatment conditions comprised three plant densities (40, 60, and 80 plants m-2) for triplicates of six canola cultivars.
Each plot consisted of four lines (5 m), and each plot measured 6 m2.Soil texture was clay-loam with electrical conductivity, pH, organic carbon percentage, total N, total P, Fe, Zn, and Cu of 2.22, 7.24, 0.58%, 0.06, 12.6, 5.02, 0.32, and 1.47, respectively.Seeds were obtained from the Oil Seed Research Department of Seed and Plant Improvement Institute (Karaj, Iran) and were planted manually.Urea (46% N) was applied thrice at 350 kg h -1 (1/3 planting time, 2/3 before flowering), and triple superphosphate (46% P2O5) was applied at 50 kg h -1 .Irrigation was performed regularly.At the end of plant maturity, plants were harvested to measure seed yield, seed oil, and fatty acid composition.
Fatty acids composition of was determined by gas liquid chromatography (GC).The chlorophyll (chl) concentration in leaf was determined according to the method of Hiscox and Israeltem (1979).
Relative water content (RWC) was calculated according to the method of Ritchie and Nguyen (1990).
The study was conducted thrice as a factorial split-plot experiment based on a complete block design.Data set was analyzed using SAS software.All data were subjected to two-way analysis of variance (ANOVA), and means were compared for significance using the least significant difference test at p < 0.05.

Results
ANOVA showed that simple and interaction effects of plant density and cultivars significantly affected seed yield, stomata resistance, canopy temperature, proline and glucosinolate content, RWC, and chlorophylls a and b at P = 0.01 (Table 1).Note: ns: Non-significant, * Significant at 5%, and ** Significant at 1%; D: Density, C: Cultivar Seed yield significantly decreased in both 60 and 80 plant/m2 in comparison with that in 40 plants m-2 (Table 2).Stomata resistance, canopy temperature, proline, and glucosinolate improved with increasing plant densities.RWC and chlorophylls a and b significantly decreased at 60 and 80 plants m-2 (Table 2).As shown in Table 3, the Ahmadi cultivar featured the highest seed yield, whereas Karaj1 and Opera cultivars exhibited the lowest.
Ahmadi, L72, and SW102 cultivars showed the highest RWC, whereas Opera and Karaj1 presented the highest stomata resistance and canopy temperature.Glucosinolate and proline significantly increased in Okapi, Opera, and Karaj1 cultivars.The highest carbohydrate yield was detected in Opera and Karaj1 and the lowest in Ahmadi.Highest level of chlorophyll a was obtained in Ahmadi, whereas the lowest was present in Opera and Karaj1 cultivars.Ahmadi, L72, and SW102 cultivars yielded the highest levels of chlorophyll b (Table 3).
Seed yield significantly decreased with increasing plant density, and L72 featured the highest and lowest seed yield under the lowest and highest densities, respectively (Table 4).4).

Discussion
Guerfel et al. ( 2010) showed that RWC of olive increased when plant densities decreased from 156 trees ha -1 to 51 trees ha -1 .Our study showed that RWC in all samples of canola cultivars significantly decreased with increasing plant densities, and this result may be a consequence of competition of plants for absorbed water in soil.Rafiee (2012) showed that plant density posed no significant effect on chlorophyll and proline levels of corn.Results showed increased chlorophyll a levels at high density and decreased quantities of chlorophyll b and proline.Low penetration of light at high plant density may cause decrease in chlorophyll.
Similar to our results, proline content in sorghum leaves increase under increasing plant densities (Alderfasi et al., 2016).In our study, proline content of canola leaves increased with increasing plant density (80 plants m -2 ).This phenomenon may result from tight competition of plants for water, light, and nutrient under high plant density, thus increasing the amount of proline.
Reta-Sanches and Fowler (2002) suggested that changing plant densities and increasing light penetration in lower parts of the canopy can improve plant yield.In the present study, seed yield decreased with increasing plant densities (60 plants m -2 ) for all canola cultivars, which possibly resulted from reduction of light penetration under high plant densities.Naseri et al. (2012) indicated that sunlight absorption and photosynthetic production decreased under thicker plant density (80 plants m -2 ) due to competition among plants.In our study, reduction in photosynthetic pigments, such as chlorophylls a and b, lowered seed yield when plant density was increased to 60 plants m -2 .Similar to our results, Larry et al. ( 2002) observed that increasing plant density decreased plant seed yield.In contrast to our results, James and Anderson (1994) reported that seed yield improved from increasing plant densities through raising pod m -2 .This difference may be attributed to variations between cultivars and densities in both studies.

Conclusion
The present study indicated that plant density significantly affects all physiological traits and poses an advantageous effect on seed yield by elevating photosynthetic pigment levels and RWC with increasing sunlight absorption.Different cultivars yielded different responses to plant densities, and canola L72 cultivar featured the highest yield because of its highest levels of chlorophylls a and b and RWC and its lowest proline content, stomata resistance, and canopy temperature.Among canola cultivars, L72 cultivar exhibited the lowest glucosinolate content under the lowest plant density.Based on these results, growing canola L72 cultivar at a plant density of 40 plants m -2 can be employed to improve canola yield, especially in regions similar to Karaj, Iran.

Table 1 .
Summary of combined F significance from analysis of variance.

Table 2 .
Mean comparisons of plant density on yield and physiological characteristics.
Mean values with different letter (s) across treatments are significantly different at p < 0.05

Table 3 .
Mean comparisons of canola cultivars on yield and physiological characteristics.Mean values with different letter (s) across treatments are significantly different at p < 0.05.

Table 4 .
Interaction effects of plant density and cultivar on yield and physiological characteristics.
Mean values with different letter (s) across treatments are significantly different at p < 0.05.