PHYSIOLOGICAL RESPONSES OF CANOLA (Brassica napus L.) GENOTYPES TO WATER DEFICIT

Abstract

Pakistan is primarily an agricultural country, with its economy heavily dependent on agricultural outputs. However, in the case of oilseed crops, the country spends a significant amount of foreign exchange on the import of edible oil. This situation highlights the urgent need to enhance domestic oilseed production through focused research efforts. To address this issue, research was conducted in the the Department of Crop Physiology, University of Agriculture, Faisalabad, Pakistan. The main objective of this research was to evaluate the genotypic response of canola (Brassica napus L.) to moisture stress, which could be valuable in breeding programs aimed at developing drought-tolerant accessions. In this study, four canola accessions were evaluated under varying levels of moisture stress. The experiments were laid out in a completely randomized design with three replications. Three moisture stress treatments were applied: T₁ (100% field capacity), T₂ (75% field capacity), and T₃ (50% field capacity). For each accession in every replication, three plants were uprooted to record data. Physiological parameters including water potential, osmotic potential, turgor pressure, relative water content percentage, photosynthetic rate, transpiration rate, stomatal conductance, and sub-stomatal carbon dioxide (CO₂) conductance were measured. The analysis of variance revealed significant differences among the accessions for all traits except water potential, osmotic potential, and turgor pressure. The treatments showed significant effects on all measured traits, while the interaction between accessions and treatments was non-significant for most traits, indicating the presence of genetic variability in the breeding material.

Water potential values among accessions ranged from -17,416 to -19,575 MPa across all treatments. Under T₁, values ranged from -11,810 to -19,333 MPa; under T₂, from -1,176 to -24,569 MPa; and under T₃, from -14,822 to -28,678 MPa. Osmotic potential values ranged from -697 to -740 MPa across treatments, with T₁ ranging from -468 to -631 MPa, T₂ from -601 to -743.67 MPa, and T₃ from -846 to -1,075 MPa. Turgor pressure ranged from 16,701 to 18,835 MPa overall, with treatment-specific ranges of 11,342 to 18,702 MPa in T₁ and 11,160 to 23,820 MPa in T₂. Relative water content among accessions ranged from 37.89% to 53.07%. The photosynthetic rate ranged from 16.31 to 25.85 µmol/m²/sec. The transpiration rate varied from 2.13 to 4.61 µmol/m²/sec in T₁, 1.81 to 3.45 µmol/m²/sec in T₂, and 0.85 to 3.06 µmol/m²/sec in T₃. Stomatal conductance ranged from 0.05 to 0.22 µmol/m²/sec across treatments, with T₁ ranging from 0.06 to 0.16 µmol/m²/sec, T₂ from 0.07 to 0.22 µmol/m²/sec, and T₃ from 0.02 to 0.19 µmol/m²/sec. Sub-stomatal CO₂ conductance ranged from 136.92 to 332.5 µmol/m²/sec across treatments, with T₁ ranging from 157.44 to 368 µmol/m²/sec, T₂ from 133.78 to 343.11 µmol/m²/sec, and T₃ from 119.55 to 286.44 µmol/m²/sec.

The study concluded that the values of all physiological traits declined with increasing moisture stress. Among the tested accessions, Dunkeld and Zn-R-7 exhibited superior performance in terms of water potential, osmotic potential, photosynthetic rate, transpiration rate, stomatal conductance, and sub-stomatal CO₂ conductance. In contrast, Zn-N-6 and Zn-M-9 showed poor performance across these parameters. Based on these findings, it is suggested that the accessions Dunkeld and Zn-R-7 can be effectively utilized in breeding programs aimed at developing drought-tolerant canola varieties or hybrids.