Proline-mediated changes in antioxidant enzymatic activities and the physiology of sugar beet under drought stress

Drought stress (DS) is a major concern in the agricultural sector and, in particular, for sugar beet production and sugar content. As such several agricultural practices have been used to minimize yield losses from DS, and foliar application of proline is considered one such approach to improve drought tolerance in growing plants. Hence, the current study examined the proline-related improvements to induce drought tolerance in sugar beet plants. A field experiment was conducted at two locations (Shahrekord and Shalamzar) in Chaharmahal-Bakhtiari province, Iran. Experimental treatments comprised of three DS levels (well water: 100%; mild stress: 75%; severe stress: 50% water requirement of plant), and three proline applications (control: 0; low: 5 mM; high: 10 mM). DS caused a significant up-regulation in leaf proline content, malondialdehyde (MDA) content, hydrogen peroxide (H2O2) content, ascorbate peroxidase, catalase, and peroxidase enzymatic activities. This increase was more pronounced under proline application with concomitant down-regulation of MDA and H2O2 contents. DS also caused a decrease in leaf photosynthetic pigments, leaf relative water contents, membrane stability index and sugar beet root production; however, proline application mitigated these adverse DS effects. The study results suggest beneficial effects of proline applications, which is crucial to mitigation of the detrimental effects of DS in sugar beet by enhancing antioxidant enzymatic activities with concomitant reduction in MDA and H2O2 contents.