Quantifying the Stress Responses of Brassica Rapa Genotypes, With Experimental Drought in Two Nitrogen Treatments

Abstract:

In a greenhouse study designed to quantify and compare stress responses of four genotypes of Brassica rapa, broccolette (bro), cabbage (cab), turnip (tur), and oil, leaf water potential and net CO₂ assimilations were measured. Individuals from each genotype, grown either with high or low nitrogen, were exposed to experimental drought of the same duration. One hypothesis was that the genotypes would differ significantly in their responses to periodic drought. The other hypothesis was that the nitrogen treatment versus no nitrogen treatment would play a significant role in the stress responses during drought. It would be expected that the nitrogen treated would have greater dry leaf mass. A LI-6400 XT portable photosynthesis system was used to obtain A/Ci curves (net CO₂ assimilation rate versus substomatal CO₂) for each treatment group. Predawn and midday water potentials were obtained throughout the hydrated and drought periods using a Model 670 pressure chamber. The dry leaf mass was significantly greater among the high nitrogen group versus the low nitrogen group for each genotype. Nitrogen and genotype were both determinants in variation of water potentials and net CO₂ assimilation. Bro and cab genotypes with high nitrogen showed the highest net CO₂ assimilation rates during hydration, but the assimilation rates dropped to the lowest during droughts. The water potentials for bro and cab were lower than values for tur and oil. Nitrogen treated genotypes had lower water potentials, but higher net CO₂ assimilation rates. Bayesian ecophysiological modeling with the TREES model showed significant differences in trait expression, quantified in terms of differences in model parameter posteriors, among the four genotypes.