Response of bulb onion (Allium cepa L.) to water stress: photosynthesis, stomatal conductance and osmotic adjustment

The photosynthetic response, stomatal conductance and osmotic adjustment in three genotypes of onion varying in bulb size viz., SM-11 (small size bulb), N-2–4-1 and Arka Niketan (medium to big size bulb) were studied under three water stress regimes (control, 50% stress and 100% stress) in pot cultured plants. Water stress was imposed 30 days after transplanting for a period of 15 days. Leaf area and bulb growth were considerably decreased during the 15 days stress. The cultivar SM-11 maintained higher photosynthetic rate (Pn) during water stress. The higher osmotic adjustment (0.7 Mpa) was found in SM-11. Though there was a recovery in photosynthetic rate and other physiological components after re-watering, it appears that there was long-term consequence of water stress as indicated by the reduction in final bulb yield in stressed plant. A reduction of 17–26% in yield was observed under 100% stress in Arka Niketan and N-2–4-1, while the minimum yield reduction (15%) was found in SM-11 under the same level of water stress. The study indicates that the genotype SM-11 which has smaller bulb size and narrow and upright leaf orientation has performed better than the other two genotypes which have relatively big bulb size and large leaves under different levels of moisture stress.