# Morphological and Physiological Response of Different Lettuce Genotypes to Salt Stress

^{*}

## Abstract

**:**

## 1. Introduction

^{+}and Cl

^{−}) is critical for improving salt tolerance in crops.

## 2. Results

#### 2.1. Morphological Traits

#### 2.1.1. Leaf Number

#### 2.1.2. Fresh Mass (FM)

#### 2.1.3. Dry Mass (DM)

#### 2.2. Physiological Traits

#### 2.2.1. Chlorophyll Index

#### 2.2.2. Flavonoid Index

#### 2.2.3. Nitrogen Balance Index

#### 2.2.4. Anthocyanin Index

#### 2.2.5. Salt Tolerance Coefficient Index

#### 2.3. Principal Component Analysis

## 3. Discussion

#### 3.1. Response of Morphological Traits under Salt Stress

#### 3.2. Response of Physiological Traits under Salt Stress

## 4. Materials and Methods

#### 4.1. Growth Condition and Plant Materials

#### 4.2. Salt Treatment

#### 4.3. Morphological Traits Measurement

#### 4.4. Physiological Traits Measurement

#### 4.5. Salt Tolerance Evaluation

## 5. Data Analysis

## 6. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**Principal component analysis (PCA) of first two principal components (PC) scores, PC1 and PC2 describing the classification of different lettuce genotypes into different salt-tolerant groups (high tolerance, moderate tolerance, low tolerance, and highly sensitive) based on seven morpho-physiological parameters measured 42 days after sowing for all the genotypes (10 days after salt treatment).

**Figure 2.**Loading plot in principal component analysis (PCA) of first two principal components (PC) scores, PC1 and PC2 describing the classification of different lettuce genotypes into different salt-tolerant groups (high tolerance, moderate tolerance, low tolerance, and highly sensitive) based on seven morpho-physiological parameters measured 42 days after sowing for all the genotypes.

**Table 1.**Leaf number (LN) (mean ± standard deviation, n = 6) of butterhead- and crisphead-type lettuce cultivars grown under control and salt-stress conditions (100 mM) after 42 days of sowing (10 days after salt treatment).

Type | Variety | LN ^{1} | ||||||||
---|---|---|---|---|---|---|---|---|---|---|

Control | Salt | % ^{3} | ||||||||

Butterhead | Burpee Bibb | 12.2 | ± | 1.4 | a ^{2} | 10.9 | ± | 1.6 | a | −10.7 |

Buttercrunch-1 | 12.7 | ± | 1.8 | a | 12.4 | ± | 3.4 | a | −2.4 | |

Buttercrunch-2 | 12.9 | ± | 3.6 | a | 12.9 | ± | 2.1 | a | 0.0 | |

Hybrid Bibb | 15.5 | ± | 2 | a | 12.3 | ± | 0.3 | a | −20.6 | |

PI 273606 | 11.9 | ± | 1 | a | 11.3 | ± | 2.3 | a | −5.0 | |

PI 342448 | 10.8 | ± | 1 | ab | 11.6 | ± | 1.7 | a | 7.4 | |

PI 342515 | 14.7 | ± | 0.9 | a | 12.6 | ± | 0.8 | a | −14.3 | |

PI 358020 | 12.7 | ± | 1.5 | a | 12.2 | ± | 1.8 | a | −3.9 | |

PI 372908 | 12.8 | ± | 2 | a | 11.8 | ± | 2 | a | −7.8 | |

PI 615052 | 11.8 | ± | 2 | a | 11.5 | ± | 1.6 | a | −2.5 | |

PI 634671 | 12.1 | ± | 1.3 | a | 10.8 | ± | 2.3 | ab | −10.7 | |

Crisphead | Crispino | 8.4 | ± | 1.4 | hijk | 8.2 | ± | 2.7 | jk | −2.4 |

Iceberg | 10.3 | ± | 0.2 | abc | 10 | ± | 1.9 | abcde | −2.9 | |

Parris Island | 9.9 | ± | 0.9 | abcde | 10.1 | ± | 2.2 | abcd | 2.02 | |

PI 177423 | 10.2 | ± | 2 | abcd | 9.1 | ± | 1.2 | abcd | −11 | |

PI 536803 | 8.1 | ± | 1.8 | k | 7.6 | ± | 1.8 | k | −6.2 | |

PI 536822 | 8.5 | ± | 2 | ghijk | 8.3 | ± | 3.3 | jk | −2.4 | |

PI 593426 | 9.1 | ± | 1.6 | cdefgh | 7.7 | ± | 0.8 | k | −15 | |

PI 600773 | 9.4 | ± | 0.5 | bcdefg | 9.2 | ± | 1.2 | abcdef | −2.1 | |

PI 635075 | 9.1 | ± | 2.2 | cdefgh | 8.9 | ± | 1.2 | defghi | −2.2 | |

PI 635077 | 8.6 | ± | 2.9 | abcdef | 9.4 | ± | 1.4 | cdefg | 9.3 | |

Prizehead | 10.7 | ± | 2.2 | ab | 10.0 | ± | 1.2 | abcde | −6.5 |

^{1}Leaf number on per plant basis.

^{2}Means followed by different letters are significantly different from each other (p < 0.05) as assessed by Tukey’s test.

^{3}% indicates the percentage change in each variety due to change in treatment levels where negative sign (−) indicates a percentage decrease in LN.

**Table 2.**Leaf number (LN) (mean ± standard deviation, n = 6) of leaf- and romaine-type lettuce cultivars grown under control and salt-stress conditions (100 mM) after 42 days of sowing (10 days after salt treatment).

Type | Variety | LN ^{1} | ||||||||
---|---|---|---|---|---|---|---|---|---|---|

Control | Salt | % ^{3} | ||||||||

Leaf | PI 171675 | 11.4 | ± | 1.8 | a ^{2} | 8 | ± | 0.7 | k | −30 |

PI 171676 | 8 | ± | 2.1 | k | 7.8 | ± | 1.2 | k | −2.5 | |

PI 175737 | 9.6 | ± | 0.6 | abcde | 8 | ± | 1 | k | −17 | |

PI 212099 | 9.5 | ± | 2.4 | abcde | 9.6 | ± | 1.9 | abcde | 1.05 | |

PI 358018 | 13.3 | ± | 2.6 | a | 10.9 | ± | 1.4 | ab | −18 | |

PI 601339 | 10.6 | ± | 1.1 | ab | 9.4 | ± | 1.7 | abcde | −11 | |

PI 601488 | 10.3 | ± | 3 | abcd | 9.3 | ± | 2.3 | bcdef | −9.7 | |

Ruby | 9.4 | ± | 1.4 | bcdef | 7.6 | ± | 2.1 | k | −19 | |

Romaine | Burgundy Delight | 8.8 | ± | 2.3 | efghij | 9.4 | ± | 0.3 | abcde | 6.82 |

Green Forest | 10.9 | ± | 0.8 | a | 9.5 | ± | 1.5 | abcde | −13 | |

PI 278066 | 10.2 | ± | 2.4 | abcd | 9.7 | ± | 1.8 | abcde | −4.9 | |

PI 289023 | 11.6 | ± | 0.6 | a | 12.4 | ± | 1.6 | a | 6.9 | |

PI 536708 | 11.0 | ± | 1.4 | a | 12.2 | ± | 1.2 | a | 10.9 | |

PI 612664 | 9.8 | ± | 0.8 | abcde | 11.3 | ± | 1.0 | a | 15.3 | |

PI 613577 | 11.3 | ± | 2 | a | 8.3 | ± | 1.3 | ijk | −27 | |

PI 274366 | 10.3 | ± | 1.6 | abc | 10.2 | ± | 0.9 | abcd | −1 |

^{1}Leaf number on per plant basis.

^{2}Means followed by different letters are significantly different (p < 0.05) as assessed by Tukey’s test.

^{3}% indicates the percentage change in each variety due to change in treatment levels where negative sign (−) indicates a percentage decrease in LN.

**Table 3.**Fresh mass (FM) (mean ± standard deviation, n = 6) of butterhead- and crisphead-type lettuce cultivars grown under control and salt-stress conditions (100 mM) after 42 days of sowing (10 days after salt treatment).

Type | Variety | FM ^{1} | ||||||||
---|---|---|---|---|---|---|---|---|---|---|

Control | Salt | % ^{3} | ||||||||

Butterhead | Burpee Bibb | 58.8 | ± | 15 | abcde ^{2} | 54.3 | ± | 20 | abcde | −7.7 |

Buttercrunch-1 | 39.9 | ± | 21 | efg | 63.8 | ± | 31 | abcd | 59.9 | |

Buttercrunch-2 | 49.5 | ± | 34 | abcde | 68 | ± | 13 | abc | 37.4 | |

Hybrid Bibb | 61.4 | ± | 38 | abc | 45.5 | ± | 12 | cde | −26 | |

PI 273606 | 43.6 | ± | 25 | cde | 33.2 | ± | 22 | def | −24 | |

PI 342448 | 12.6 | ± | 19 | fg | 14.3 | ± | 24 | efd | 13.5 | |

PI 342515 | 41.9 | ± | 24 | cde | 47 | ± | 23 | bcde | 12.2 | |

PI 358020 | 33.5 | ± | 30 | abcde | 45.3 | ± | 32 | abcde | 35.2 | |

PI 372908 | 44.4 | ± | 19 | abcde | 37.8 | ± | 21 | abcde | −15 | |

PI 615052 | 19.3 | ± | 21 | cde | 14.6 | ± | 49 | cdef | −24 | |

PI 634671 | 25.1 | ± | 10 | bcde | 30.2 | ± | 25 | bcde | 20.3 | |

Crisphead | Crispino | 53.4 | ± | 5.5 | ab | 56.7 | ± | 9.3 | abc | 6.18 |

Iceberg | 45.5 | ± | 15 | abcd | 43.2 | ± | 20 | abcde | −5.1 | |

Parris Island | 33.2 | ± | 26 | bcd | 55.4 | ± | 56 | abcde | 66.9 | |

PI 177423 | 33.8 | ± | 52 | abcde | 40.3 | ± | 13 | abcd | 19.2 | |

PI 536803 | 47.1 | ± | 22 | abcd | 42.5 | ± | 23 | abcde | −9.8 | |

PI 536822 | 47.2 | ± | 16 | abcd | 33.3 | ± | 26 | abcde | −29 | |

PI 593426 | 65.3 | ± | 19 | ab | 60.1 | ± | 14 | ab | −8 | |

PI 600773 | 52.9 | ± | 18 | abc | 63.1 | ± | 40 | abcd | 19.3 | |

PI 635075 | 54.1 | ± | 20 | abcd | 47.3 | ± | 9.9 | bcde | −13 | |

PI 635077 | 54.6 | ± | 9.6 | abcd | 74.9 | ± | 11 | a | 37.2 | |

Prizehead | 44.1 | ± | 5.3 | bcd | 47.0 | ± | 27 | abcde | 6.58 |

^{1}Fresh mass (in gram) on per plant basis.

^{2}Means followed by different letters are significantly different from each other (p < 0.05) as assessed by Tukey’s test.

^{3}% indicates the percentage change in each variety due to change in treatment levels (Control vs. Salt), where negative sign (−) indicates a percentage decrease in FM.

**Table 4.**Fresh mass (FM) (mean ± standard deviation, n = 6) of leaf- and romaine-type lettuce cultivars grown under control and salt-stress conditions (100 mM) after 42 days of sowing (10 days after salt treatment).

Type | Variety | FM ^{1} | ||||||||
---|---|---|---|---|---|---|---|---|---|---|

Control | Salt | % ^{3} | ||||||||

Leaf | PI 171675 | 71.6 | ± | 28 | ab ^{2} | 20.2 | ± | 5.5 | f | −72 |

PI 171676 | 15 | ± | 23 | def | 30.6 | ± | 21 | cdef | 104 | |

PI 175737 | 28.9 | ± | 13 | cdef | 30 | ± | 2.3 | cde | 3.81 | |

PI 212099 | 26 | ± | 29 | cdef | 77.1 | ± | 23 | a | 197 | |

PI 358018 | 39.6 | ± | 33 | abcd | 33.9 | ± | 11 | cde | −14 | |

PI 601339 | 37.3 | ± | 15 | de | 34.7 | ± | 6.5 | cde | −7 | |

PI 601488 | 51.9 | ± | 25 | abcde | 31.7 | ± | 14 | cde | −39 | |

Ruby | 67.4 | ± | 6.9 | a | 43.7 | ± | 17 | bcde | −35 | |

Romaine | Burgundy Delight | 29.9 | ± | 31 | bcdef | 44.2 | ± | 26 | abcde | 47.8 |

Green Forest | 62.6 | ± | 33 | abcde | 44 | ± | 22 | abcde | −30 | |

PI 278066 | 54.5 | ± | 24 | abcde | 36.5 | ± | 16 | cdef | −33 | |

PI 289023 | 40.6 | ± | 5.5 | bcde | 39.8 | ± | 9.1 | cdef | −2 | |

PI 536708 | 62.8 | ± | 11 | abc | 65.7 | ± | 3.6 | a | 4.62 | |

PI 612664 | 30.1 | ± | 9.5 | de | 34 | ± | 11 | cdef | 13 | |

PI 613577 | 45.5 | ± | 18 | cde | 38.6 | ± | 13 | cdef | −15 | |

PI 274366 | 32.6 | ± | 18 | cdef | 25.3 | ± | 6.4 | ef | −22 |

^{1}Fresh mass (in gram) on per plant basis.

^{2}Means followed by different letters are significantly different from each other (p < 0.05) as assessed by Tukey’s test.

^{3}% indicates the percentage change in each variety due to change in treatment levels (Control vs. Salt), where negative sign (−) indicates a percentage decrease in FM.

**Table 5.**Dry mass (DM) (mean ± standard deviation, n = 6) of butterhead- and crisphead-type lettuce cultivars grown under control and salt-stress conditions (100 mM) after 42 days of sowing (10 days after salt treatment).

Type | Variety | DM ^{1} | ||||||||
---|---|---|---|---|---|---|---|---|---|---|

Control | Salt | % ^{3} | ||||||||

Butterhead | Burpee Bibb | 7.5 | ± | 2.8 | abc ^{2} | 4.9 | ± | 1.4 | cdef | −35 |

Buttercrunch-1 | 6.4 | ± | 1.8 | bcd | 8.6 | ± | 2.9 | abc | 34.4 | |

Buttercrunch-2 | 7.2 | ± | 4.5 | abcde | 7.8 | ± | 1 | bcde | 8.33 | |

Hybrid Bibb | 9.5 | ± | 2.8 | A | 6.6 | ± | 1.1 | cd | −31 | |

PI 273606 | 5.2 | ± | 3.1 | de | 3.8 | ± | 2.8 | ef | −27 | |

PI 342448 | 1.6 | ± | 1.8 | F | 1.5 | ± | 2.5 | f | −6.3 | |

PI 342515 | 5.2 | ± | 2.9 | def | 4.2 | ± | 3.4 | def | −19 | |

PI 358020 | 4.5 | ± | 3.4 | def | 6 | ± | 4.5 | abcde | 33.3 | |

PI 372908 | 5.4 | ± | 2.2 | def | 3.9 | ± | 2.6 | def | −28 | |

PI 615052 | 2.3 | ± | 2.1 | ef | 3.5 | ± | 3.7 | def | 52.2 | |

PI 634671 | 2.8 | ± | 1.4 | ef | 2.9 | ± | 4.3 | def | 3.57 | |

Crisphead | Crispino | 7.5 | ± | 0.8 | ab | 7.7 | ± | 1.1 | ab | 2.67 |

Iceberg | 6.6 | ± | 1.7 | abc | 6.1 | ± | 1.3 | bc | −7.6 | |

Parris Island | 6.1 | ± | 2.6 | abc | 8 | ± | 3.7 | abcd | 31.1 | |

PI 177423 | 5.1 | ± | 5.1 | abcde | 4.6 | ± | 1.0 | cde | −9.8 | |

PI 536803 | 7.3 | ± | 1.9 | bcde | 5.3 | ± | 1.6 | def | −27 | |

PI 536822 | 5.1 | ± | 2.7 | bcdef | 3.5 | ± | 3.0 | cdef | −31 | |

PI 593426 | 8.4 | ± | 1.8 | abcde | 7.1 | ± | 1.5 | abcd | −15 | |

PI 600773 | 7.8 | ± | 1.7 | abcd | 7.1 | ± | 2.0 | abcde | −9 | |

PI 635075 | 7.1 | ± | 2.4 | abcde | 7 | ± | 0.5 | bcde | −1.4 | |

PI 635077 | 7.4 | ± | 0.8 | bcd | 7.9 | ± | 0.8 | abc | 6.76 | |

Prizehead | 5.9 | ± | 0.6 | cd | 5.5 | ± | 2.0 | cd | −6.8 |

^{1}Dry mass (in gram) on per plant basis.

^{2}Means followed by different letters are significantly different from each other (p < 0.05) as assessed by Tukey’s test.

^{3}% indicates the percentage change in each variety due to change in treatment levels (Control vs. Salt), where negative sign (−) indicates a percentage decrease in DM.

**Table 6.**Dry mass (DM) (mean ± standard deviation, n = 6) of leaf-and romaine-type lettuce cultivars grown under control and salt-stress conditions (100 mM) after 42 days of sowing (10 days after salt treatment).

Type | Variety | DM ^{1} | ||||||||
---|---|---|---|---|---|---|---|---|---|---|

Control | Salt | % ^{3} | ||||||||

Leaf | PI 171675 | 7.9 | ± | 3 | abc ^{2} | 1.8 | ± | 0.5 | f | −77 |

PI 171676 | 1.9 | ± | 2 | f | 3 | ± | 4 | bcde | 57.9 | |

PI 175737 | 3.2 | ± | 1.5 | ef | 3.4 | ± | 0.1 | ef | 6.25 | |

PI 212099 | 3 | ± | 2.8 | cdef | 4.9 | ± | 1.9 | bcde | 63.3 | |

PI 358018 | 5.8 | ± | 3.6 | abcde | 4.6 | ± | 0.9 | cde | −21 | |

PI 601339 | 5.7 | ± | 1.5 | cdef | 4.8 | ± | 1.4 | cd | −16 | |

PI 601488 | 8.3 | ± | 2.8 | abcde | 5.3 | ± | 0.4 | cd | −36 | |

Ruby | 7.6 | ± | 1.8 | abcd | 5.5 | ± | 2.7 | abcd | −28 | |

Romaine | Burgundy Delight | 4.3 | ± | 3.2 | cdef | 5.6 | ± | 2.6 | bcde | 30.2 |

Green Forest | 10.1 | ± | 3.8 | a | 7 | ± | 2.4 | abc | −31 | |

PI 278066 | 7.4 | ± | 2.5 | abcd | 5.7 | ± | 1.1 | cde | −23 | |

PI 289023 | 5.7 | ± | 0.7 | bcde | 4.9 | ± | 1.0 | cde | −14 | |

PI 536708 | 8.6 | ± | 2.6 | abcd | 8.2 | ± | abc | −4.7 | ||

PI 612664 | 3.6 | ± | 1.2 | de | 3.7 | ± | 3.5 | cdef | 2.78 | |

PI 613577 | 5.1 | ± | 1.5 | cde | 4 | ± | 1.7 | def | −22 | |

PI 274366 | 4.0 | ± | 1.8 | def | 3.6 | ± | 0.7 | def | −10 |

^{1}Dry mass (in gram) on per plant basis.

^{2}Means followed by different letters are significantly different from each other (p < 0.05) as assessed by Tukey’s test.

^{3}% indicates the percentage change in each variety due to change in treatment levels (Control vs. Salt), where negative sign (−) indicates a percentage decrease in DM.

**Table 7.**Chlorophyll index (CI) (mean ± standard deviation, n = 6) of butterhead- and crisphead-type lettuce cultivars grown under control and salt-stress conditions (100 mM) after 42 days of sowing (10 days after salt treatment).

Type | Variety | CI ^{1} | ||||||||
---|---|---|---|---|---|---|---|---|---|---|

Control | Salt | % ^{3} | ||||||||

Butterhead | Burpee Bibb | 14 | ± | 5.1 | abcd ^{2} | 11.3 | ± | 2.8 | e | −18 |

Buttercrunch-1 | 9.8 | ± | 2.5 | e | 11.7 | ± | 1.1 | e | 19.4 | |

Buttercrunch-2 | 13 | ± | 1.5 | de | 10.9 | ± | 1.4 | e | −17 | |

Hybrid Bibb | 14 | ± | 4.4 | bcde | 12.4 | ± | 3.7 | de | −8.1 | |

PI 273606 | 7 | ± | 3.4 | efgh | 5.6 | ± | 1.8 | fghi | −20 | |

PI 342448 | 6.3 | ± | 2.1 | fghi | 7.5 | ± | 1.4 | fghi | 19 | |

PI 342515 | 6.6 | ± | 1.2 | fghi | 6.5 | ± | 1.2 | fghi | −1.5 | |

PI 358020 | 5.8 | ± | 2.7 | ghi | 4.1 | ± | 3.1 | i | −29 | |

PI 372908 | 8.2 | ± | 3.7 | e | 7 | ± | 0.9 | efgh | −15 | |

PI 615052 | 7.5 | ± | 1.2 | efgh | 8.2 | ± | 3.5 | efgh | 9.33 | |

PI 634671 | 3.7 | ± | 3.3 | i | 4.5 | ± | 3.6 | ghi | 21.6 | |

Crisphead | Crispino | 7.3 | ± | 1.1 | efg | 7.3 | ± | 1.9 | fghi | 0 |

Iceberg | 7.2 | ± | 2.1 | efgh | 6.5 | ± | 5.3 | fghi | −9.7 | |

Parris Island | 13 | ± | 3 | 6 | 14.3 | ± | 2.9 | abcd | 14.4 | |

PI 177423 | 9.5 | ± | 1.6 | e | 7.4 | ± | 3.5 | ef | −22 | |

PI 536803 | 6.6 | ± | 2.7 | efgh | 8.3 | ± | 1.4 | e | 25.8 | |

PI 536822 | 5.6 | ± | 2.3 | fghi | 3.7 | ± | 3.2 | i | −34 | |

PI 593426 | 5.2 | ± | 1.5 | ghi | 7.5 | ± | 1.8 | efgh | 44.2 | |

PI 600773 | 15 | ± | 1.1 | abcd | 14.4 | ± | 0.6 | abcd | −0.7 | |

PI 635075 | 14 | ± | 3 | abcd | 11.4 | ± | 3 | de | −16 | |

PI 635077 | 11 | ± | 4.1 | e | 13.8 | ± | 3.7 | bcde | 25.5 | |

Prizehead | 6.8 | ± | 3.2 | efgh | 5.4 | ± | 1 | fghi | −21 |

^{1}Chlorophyll index on per plant basis.

^{2}Means followed by different letters are significantly different from each other (p < 0.05) as assessed by Tukey’s test.

^{3}% indicates the percentage change in each variety due to change in treatment levels (Control vs. Salt), where negative sign (−) indicates a percentage decrease in CI.

**Table 8.**Chlorophyll index (CI) (mean ± standard deviation, n = 6) of leaf- and romaine-type lettuce cultivars grown under control and salt-stress conditions (100 mM) after 42 days of sowing (10 days after salt treatment).

Type | Variety | CI ^{1} | ||||||||
---|---|---|---|---|---|---|---|---|---|---|

Control | Salt | % ^{3} | ||||||||

Leaf | PI 171675 | 5.5 | ± | 4.1 | fghi ^{2} | 4 | ± | 2.5 | i | −27 |

PI 171676 | 7.1 | ± | 5.3 | efgh | 5.4 | ± | 1 | ghi | −24 | |

PI 175737 | 4.2 | ± | 5.3 | ghi | 4.8 | ± | 4.3 | ghi | 14.3 | |

PI 212099 | 4.1 | ± | 2.1 | i | 4.9 | ± | 7.2 | ghi | 19.5 | |

PI 358018 | 7.1 | ± | 0.6 | fghi | 6.4 | ± | 2.4 | fghi | −9.9 | |

PI 601339 | 9.5 | ± | 3 | e | 6.6 | ± | 2.4 | efgh | −31 | |

PI 601488 | 11.4 | ± | 2.7 | e | 9.6 | ± | 2.6 | e | −16 | |

Ruby | 6.4 | ± | 2.2 | fghi | 4.3 | ± | 0.9 | hi | −33 | |

Romaine | Burgundy Delight | 13.4 | ± | 1.5 | abcd | 15.3 | ± | 1.4 | ab | 14.2 |

Green Forest | 15.4 | ± | 2.2 | a | 11.3 | ± | 1.6 | e | −27 | |

PI 278066 | 8.5 | ± | 1.4 | e | 5.8 | ± | 2.6 | efgh | −32 | |

PI 289023 | 8 | ± | 3.7 | e | 5.6 | ± | 3.1 | efgh | −30 | |

PI 536708 | 14.9 | ± | 0.8 | abc | 13.6 | ± | 4.7 | abcd | −8.7 | |

PI 612664 | 9 | ± | 4.7 | e | 11.5 | ± | 4.6 | e | 27.8 | |

PI 613577 | 9.6 | ± | 3.9 | e | 9.4 | ± | 0.5 | e | −2.1 | |

PI 274366 | 4.1 | ± | 1.2 | i | 2.6 | ± | 1.8 | i | −37 |

^{1}Chlorophyll index on per plant basis.

^{2}Means followed by different letters are significantly different from each other (p < 0.05) as assessed by Tukey’s test.

^{3}% indicates the percentage change in each variety due to change in treatment levels (Control vs. Salt), where negative sign (−) indicates a percentage decrease in CI.

**Table 9.**Flavonoid index (FI) (mean ± standard deviation, n = 6) of butterhead- and crisphead-type lettuce cultivars grown under control and salt-stress conditions (100 mM) after 42 days of sowing (10 days after salt treatment).

Type | Variety | FI ^{1} | ||||||||
---|---|---|---|---|---|---|---|---|---|---|

Control | Salt | % ^{3} | ||||||||

Butterhead | Burpee Bibb | 0.9 | ± | 0.1 | fg ^{2} | 0.9 | ± | 0.3 | fg | 0 |

Buttercrunch-1 | 1.3 | ± | 0.1 | abcd | 1.2 | ± | 0.2 | abcde | −7.7 | |

Buttercrunch-2 | 1.3 | ± | 0.2 | abcd | 1.2 | ± | 0 | abcde | −7.7 | |

Hybrid Bibb | 1.6 | ± | 0.2 | abcd | 1.5 | ± | 0.1 | a | −6.3 | |

PI 273606 | 1.1 | ± | 0.3 | defg | 1 | ± | 0.2 | efg | −9.1 | |

PI 342448 | 1.1 | ± | 0.2 | cdef | 1.3 | ± | 0.3 | abcde | 18.2 | |

PI 342515 | 1.2 | ± | 0.1 | abcde | 1.1 | ± | 0 | cdef | −8.3 | |

PI 358020 | 1.3 | ± | 0.1 | abcde | 1.3 | ± | 0.3 | abcde | 0 | |

PI 372908 | 1.2 | ± | 0.1 | abcde | 1.1 | ± | 0.2 | defg | −8.3 | |

PI 615052 | 1.1 | ± | 0.1 | defg | 1.2 | ± | 0.4 | abcde | 9.09 | |

PI 634671 | 1.2 | ± | 0.2 | defg | 1.1 | ± | 0.2 | efg | −8.3 | |

Crisphead | Crispino | 1 | ± | 0.1 | g | 0.8 | ± | 0.1 | g | −20 |

Iceberg | 1.4 | ± | 0.2 | abc | 1.4 | ± | 0 | abc | 0 | |

Parris Island | 1.5 | ± | 0 | a | 1.4 | ± | 0.1 | a | −6.7 | |

PI 177423 | 1 | ± | 0.2 | efg | 1.1 | ± | 0.1 | cdef | 10 | |

PI 536803 | 1 | ± | 0.1 | efg | 1 | ± | 0 | fg | 0 | |

PI 536822 | 1.1 | ± | 0.1 | efg | 1.2 | ± | 0.2 | abcde | 9.09 | |

PI 593426 | 0.8 | ± | 0.1 | g | 0.8 | ± | 0.1 | g | 0 | |

PI 600773 | 1 | ± | 0.2 | fg | 1 | ± | 0.1 | efg | 0 | |

PI 635075 | 1 | ± | 0.1 | fg | 1 | ± | 0.2 | efg | 0 | |

PI 635077 | 1 | ± | 0.4 | fg | 0.9 | ± | 0.2 | fg | −10 | |

Prizehead | 1.4 | ± | 0.1 | abc | 1.4 | ± | 0.2 | abc | 0 |

^{1}Flavonoid index on per plant basis.

^{2}Means followed by different letters are significantly different from each other (p < 0.05) as assessed by Tukey’s test.

^{3}% indicates the percentage change in each variety due to change in treatment levels (Control vs. Salt), where negative sign (−) indicates a percentage decrease in FI.

**Table 10.**Flavonoid index (FI) (mean ± standard deviation, n = 6) of leaf- and romaine-type lettuce cultivars grown under control and salt-stress conditions (100 mM) after 42 days of sowing (10 days after salt treatment).

Type | Variety | Fl ^{1} | ||||||||
---|---|---|---|---|---|---|---|---|---|---|

Control | Salt | % ^{3} | ||||||||

Leaf | PI 171675 | 1 | ± | 0.2 | abcde ^{2} | 1.1 | ± | 0.2 | cdef | 10 |

PI 171676 | 1.4 | ± | 0.2 | abc | 1.1 | ± | 0 | bcdef | −21 | |

PI 175737 | 1.2 | ± | 0.1 | abcde | 1 | ± | 0.1 | efg | −17 | |

PI 212099 | 1.1 | ± | 0.2 | efg | 1 | ± | 0.2 | efg | −9.1 | |

PI 358018 | 1.2 | ± | 0.2 | bcdef | 1.3 | ± | 0.2 | abcd | 8.33 | |

PI 601339 | 1.4 | ± | 0.4 | a | 1.3 | ± | 0.3 | abcd | −7.1 | |

PI 601488 | 0.8 | ± | 0.1 | g | 1 | ± | 0.2 | fg | 25 | |

Ruby | 1.1 | ± | 0.2 | cdefg | 1.3 | ± | 0.1 | abcde | 18.2 | |

Romaine | Burgundy Delight | 1.6 | ± | 0.1 | a | 1.5 | ± | 0.2 | a | −6.3 |

Green Forest | 1.3 | ± | 0.1 | abc | 1.4 | ± | 0 | ab | 7.69 | |

PI 278066 | 1.4 | ± | 0.3 | abc | 1.9 | ± | 0.1 | a | 35.7 | |

PI 289023 | 1.2 | ± | 0.2 | cdefg | 1.3 | ± | 0.8 | abcde | 8.33 | |

PI 536708 | 1.3 | ± | 0.1 | abcde | 1.2 | ± | 0.2 | abcde | −7.7 | |

PI 612664 | 1.2 | ± | 0.2 | abcde | 1.2 | ± | 0.2 | abcde | 0 | |

PI 613577 | 1.5 | ± | 0.3 | a | 1.4 | ± | 0.2 | a | −6.7 | |

PI 274366 | 1.1 | ± | 0.2 | abc | 1.4 | ± | 0.2 | a | 27.3 |

^{1}Flavonoid index on per plant basis.

^{2}Means followed by different letters are significantly different from each other (p < 0.05) as assessed by Tukey’s test.

^{3}% indicates the percentage change in each variety due to change in treatment levels (Control vs. Salt), where negative sign (−) indicates a percentage decrease in FI.

**Table 11.**Nitrogen balance index (NB (mean ± standard deviation, n = 4) of butterhead- and crisphead-type lettuce cultivars grown under control and salt-stress conditions (100 mM) after 42 days of sowing (10 days after salt treatment).

Type | Variety | NB ^{1} | ||||||||
---|---|---|---|---|---|---|---|---|---|---|

Control | Salt | % ^{3} | ||||||||

Butterhead | Burpee Bibb | 15 | ± | 8.6 | ab ^{2} | 12.8 | ± | 5.6 | bc | −16 |

Buttercrunch-1 | 7.2 | ± | 1.8 | de | 10.7 | ± | 3.2 | bcde | 48.6 | |

Buttercrunch-2 | 11 | ± | 2.1 | cde | 9.7 | ± | 1.1 | cde | −12 | |

Hybrid Bibb | 7.6 | ± | 5.4 | de | 9.1 | ± | 4 | de | 19.7 | |

PI 273606 | 6.8 | ± | 10 | de | 6.5 | ± | 3.8 | de | −4.4 | |

PI 342448 | 5.3 | ± | 1.4 | de | 5.3 | ± | 3.6 | de | 0 | |

PI 342515 | 6.3 | ± | 0.3 | de | 7.2 | ± | 0.8 | de | 14.3 | |

PI 358020 | 5.2 | ± | 1.4 | de | 3.2 | ± | 4.5 | e | −38 | |

PI 372908 | 7.2 | ± | 3.6 | de | 8.9 | ± | 3.9 | de | 23.6 | |

PI 615052 | 9.7 | ± | 1.7 | de | 6.9 | ± | 8.3 | de | −29 | |

PI 634671 | 3.3 | ± | 5.5 | e | 4.5 | ± | 5.4 | e | 36.4 | |

Crisphead | Crispino | 7.9 | ± | 1.3 | cde | 8.4 | ± | 1.2 | de | 6.33 |

Iceberg | 5.2 | ± | 5 | e | 4.6 | ± | 6.3 | e | −12 | |

Parris Island | 9.6 | ± | 1.7 | de | 10.9 | ± | 1.4 | bcde | 13.5 | |

PI 177423 | 8.6 | ± | 1.6 | de | 6.6 | ± | 4.7 | de | −23 | |

PI 536803 | 6.5 | ± | 1.9 | de | 9.5 | ± | 2.5 | de | 46.2 | |

PI 536822 | 4.8 | ± | 2.7 | de | 3.4 | ± | 7.7 | e | −29 | |

PI 593426 | 6.5 | ± | 2.7 | de | 9.1 | ± | 2.4 | de | 40 | |

PI 600773 | 15 | ± | 0.2 | ab | 15.8 | ± | 1.0 | ab | 3.27 | |

PI 635075 | 18 | ± | 2.6 | a | 11.3 | ± | 2.7 | bcd | −37 | |

PI 635077 | 13 | ± | 11 | bc | 13.7 | ± | 3.7 | b | 7.03 | |

Prizehead | 4.9 | ± | 3.7 | e | 4.1 | ± | 2.7 | e | −16 |

^{1}Nitrogen balance index on per plant basis.

^{2}Means followed by different letters are significantly different from each other (p < 0.05) as assessed by Tukey’s test.

^{3}% indicates the percentage change in each variety due to change in treatment levels (Control vs. Salt), where negative sign (−) indicates a percentage decrease in NB.

**Table 12.**Nitrogen balance index (NB (mean ± standard deviation, n = 4) of leaf- and romaine-type lettuce cultivars grown under control and salt-stress conditions (100 mM) after 42 days of sowing (10 days after salt treatment).

Type | Variety | NB ^{1} | ||||||||
---|---|---|---|---|---|---|---|---|---|---|

Control | Salt | % ^{3} | ||||||||

Leaf | PI 171675 | 6 | ± | 4.8 | de ^{2} | 5.9 | ± | 3.8 | de | −1.7 |

PI 171676 | 4.9 | ± | 5.5 | e | 6 | ± | 1.2 | de | 22.4 | |

PI 175737 | 4 | ± | 4.2 | e | 5.5 | ± | 2.8 | de | 37.5 | |

PI 212099 | 3.9 | ± | 4.8 | e | 6.1 | ± | 8.5 | de | 56.4 | |

PI 358018 | 6.9 | ± | 1.3 | de | 5.6 | ± | 3 | de | −19 | |

PI 601339 | 6.7 | ± | 4.1 | de | 5.9 | ± | 4.6 | de | −12 | |

PI 601488 | 13.9 | ± | 2.9 | b | 10 | ± | 6 | cde | −28 | |

Ruby | 5.9 | ± | 2.1 | de | 3.6 | ± | 1.9 | e | −39 | |

Romaine | Burgundy Delight | 8.1 | ± | 1.8 | de | 9.4 | ± | 1.8 | de | 16 |

Green Forest | 12.4 | ± | 1.4 | bcd | 9.5 | ± | 0.8 | de | −23 | |

PI 278066 | 7.2 | ± | 3.4 | de | 4 | ± | 2.3 | e | −44 | |

PI 289023 | 7.3 | ± | 4.4 | de | 5 | ± | 2.3 | de | −32 | |

PI 536708 | 12.5 | ± | 3.3 | bcd | 11.6 | ± | 6.4 | bcd | −7.2 | |

PI 612664 | 8 | ± | 6.2 | de | 11 | ± | 5.4 | bcde | 37.5 | |

PI 613577 | 6.5 | ± | 3.8 | de | 6.3 | ± | 0.7 | de | −3.1 | |

PI 274366 | 3.5 | ± | 1.9 | e | 2.9 | ± | 2.5 | e | −17 |

^{1}Nitrogen balance index on per plant basis.

^{2}Means followed by different letters are significantly different from each other (p < 0.05) as assessed by Tukey’s test.

^{3}% indicates the percentage change in each variety due to change in treatment levels (Control vs. Salt), where negative sign (−) indicates a percentage decrease in NB.

**Table 13.**Anthocyanin Index (AI) (mean ± standard deviation, n = 6) of butterhead- and crisphead-type lettuce cultivars grown under control and salt-stress conditions (100 mM) after 42 days of sowing (10 days after salt treatment).

Type | Variety | AI ^{1} | ||||||||
---|---|---|---|---|---|---|---|---|---|---|

Control | Salt | % ^{3} | ||||||||

Butterhead | Burpee Bibb | 0.1 | ± | 0.1 | klm ^{2} | 0.2 | ± | 0.1 | jk | 100 |

Buttercrunch-1 | 0.2 | ± | 0.1 | lm | 0.2 | ± | 0 | m | 0 | |

Buttercrunch-2 | 0.2 | ± | 0.2 | lm | 0.2 | ± | 0 | lm | 0 | |

Hybrid Bibb | 0.2 | ± | 0 | m | 0.2 | ± | 0 | m | 0 | |

PI 273606 | 0.2 | ± | 0 | klm | 0.2 | ± | 0 | klm | 0 | |

PI 342448 | 0.4 | ± | 0 | hij | 0.3 | ± | 0.1 | j | −25 | |

PI 342515 | 0.2 | ± | 0.2 | klm | 0.3 | ± | 0.2 | jk | 50 | |

PI 358020 | 0.2 | ± | 0 | jk | 0.2 | ± | 0 | jk | 0 | |

PI 372908 | 0.3 | ± | 0 | jk | 0.2 | ± | 0 | jk | −33.3 | |

PI 615052 | 0.2 | ± | 0.1 | j | 0.2 | ± | 0.1 | j | 0 | |

PI 634671 | 0.2 | ± | 0.1 | jkl | 0.2 | ± | 0.1 | klm | 0 | |

Crisphead | Crispino | 0.2 | ± | 0.1 | klm | 0.2 | ± | 0.1 | klm | 0 |

Iceberg | 0.4 | ± | 0 | ij | 0.3 | ± | 0.1 | j | −25 | |

Parris Island | 0.2 | ± | 0.1 | klm | 0.1 | ± | 0.1 | m | −50 | |

PI 177423 | 0.3 | ± | 0 | jk | 0.2 | ± | 0 | jk | −33 | |

PI 536803 | 0.2 | ± | 0.1 | klm | 0.2 | ± | 0.1 | klm | 0 | |

PI 536822 | 0.4 | ± | 0 | hi | 0.4 | ± | 0 | ij | 0 | |

PI 593426 | 0.2 | ± | 0 | klm | 0.2 | ± | 0 | klm | 0 | |

PI 600773 | 0.1 | ± | 0.1 | m | 0.2 | ± | 0 | m | 100 | |

PI 635075 | 0.1 | ± | 0 | m | 0.2 | ± | 0.1 | lm | 100 | |

PI 635077 | 0.2 | ± | 0.1 | m | 0.2 | ± | 0.1 | klm | 0 | |

Prizehead | 0.6 | ± | 0 | e | 0.5 | ± | 0 | g | −17 |

^{1}Anthocyanin index on per plant basis.

^{2}Means followed by different letters are significantly different from each other (p < 0.05) as assessed by Tukey’s test.

^{3}% indicates the percentage change in each variety due to change in treatment levels (Control vs. Salt), where negative sign (−) indicates a percentage decrease in AI.

**Table 14.**Anthocyanin Index (AI) (mean ± standard deviation, n = 6) of leaf- and romaine-type lettuce cultivars grown under control and salt-stress conditions (100 mM) after 42 days of sowing (10 days after salt treatment).

Type | Variety | AI ^{1} | ||||||||
---|---|---|---|---|---|---|---|---|---|---|

Control | Salt | % ^{3} | ||||||||

Leaf | PI 171675 | 0.2 | ± | 0.1 | klm ^{2} | 0.3 | ± | 0 | jk | 50 |

PI 171676 | 0.2 | ± | 0.3 | klm | 0.2 | ± | 0 | klm | 0 | |

PI 175737 | 0.2 | ± | 0 | klm | 0.3 | ± | 0 | jk | 50 | |

PI 212099 | 0.2 | ± | 0 | klm | 0.2 | ± | 0.1 | jk | 0 | |

PI 358018 | 0.2 | ± | 0 | klm | 0.2 | ± | 0 | jk | 0 | |

PI 601339 | 0.5 | ± | 0 | fg | 0.4 | ± | 0 | hi | −20 | |

PI 601488 | 0.2 | ± | 0 | m | 0.2 | ± | 0 | klm | 0 | |

Ruby | 0.5 | ± | 0 | fg | 0.6 | ± | 0 | ef | 20 | |

Romaine | Burgundy Delight | 1.1 | ± | 0 | a | 1 | ± | 0 | b | −9.1 |

Green Forest | 0.1 | ± | 0.1 | m | 0.2 | ± | 0.1 | klm | 100 | |

PI 278066 | 0.2 | ± | 0 | klm | 0.2 | ± | 0 | jk | 0 | |

PI 289023 | 0.2 | ± | 0 | klm | 0.2 | ± | 0 | klm | 0 | |

PI 536708 | 0.1 | ± | 0 | m | 0.1 | ± | 0 | m | 0 | |

PI 612664 | 0.3 | ± | 0 | j | 0.3 | ± | 0 | jk | 0 | |

PI 613577 | 0.8 | ± | 0.1 | c | 0.7 | ± | 0 | d | −13 | |

PI 274366 | 0.3 | ± | 0 | jk | 0.3 | ± | 0 | jk | 0 |

^{1}Anthocyanin index on per plant basis.

^{2}Means followed by different letters are significantly different from each other (p < 0.05) as assessed by Tukey’s test.

^{3}% indicates the percentage change in each variety due to change in treatment levels (Control vs. Salt), where negative sign (−) indicates a percentage decrease in AI.

**Table 15.**Tolerance classification (TC) of lettuce cultivars into various salt-tolerance groups (highly salt-tolerant, moderately salt-tolerant, low salt-tolerant and highly salt-sensitive) based on the Total salt tolerance coefficient index (STCI). Each STCI value is the sum of individual salt tolerance coefficient index values of three morphological and four biological parameters.

Tolerance Classification | Salt Tolerance Coefficient Index Value | Cultivars |
---|---|---|

Highly salt-sensitive | (≤0.89) | PI 171675, PI 536822, Ruby, PI 601339, Green Forest, PI 601488, PI 613577, PI 278066, Hybrid Bibb, PI 273606, Burpee Bibb |

Low salt tolerance | (0.90–1.03) | PI 372908, PI 358018, PI 274366, Prizehead, PI 177423, PI 289023, Iceberg, PI 635075, PI 342515, PI 358020, PI 536708, Buttercrunch-2, Crispino, PI 600773, PI 536803 |

Moderate salt tolerance | (1.04–1.17) | PI 342448, PI 615052, PI 175737, PI 593426, PI 634671, PI 612664, Parris Island, Burgundy Delight, PI 635077 |

High salt tolerance | (≥1.18) | PI 171676, Buttercrunch-1, PI 212099 |

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## Share and Cite

**MDPI and ACS Style**

Adhikari, B.; Olorunwa, O.J.; Wilson, J.C.; Barickman, T.C.
Morphological and Physiological Response of Different Lettuce Genotypes to Salt Stress. *Stresses* **2021**, *1*, 285-304.
https://doi.org/10.3390/stresses1040021

**AMA Style**

Adhikari B, Olorunwa OJ, Wilson JC, Barickman TC.
Morphological and Physiological Response of Different Lettuce Genotypes to Salt Stress. *Stresses*. 2021; 1(4):285-304.
https://doi.org/10.3390/stresses1040021

**Chicago/Turabian Style**

Adhikari, Bikash, Omolayo J. Olorunwa, Jeff C. Wilson, and T. Casey Barickman.
2021. "Morphological and Physiological Response of Different Lettuce Genotypes to Salt Stress" *Stresses* 1, no. 4: 285-304.
https://doi.org/10.3390/stresses1040021