Weaker photosynthetic acclimation to fluctuating than to corresponding steady UVB radiation treatments in grapevines.

The effects of transient increases in UVB radiation on plants are not well known; whether cumulative damage dominates or, alternately, an increase in photoprotection and recovery periods ameliorates any negative effects. We investigated photosynthetic capacity and metabolite accumulation of grapevines (Vitis vinifera Cabernet Sauvignon) in response to UVB fluctuations under four treatments: fluctuating UVB (FUV) and steady UVB radiation (SUV) at similar total biologically effective UVB dose (2.12 and 2.23 kJ m-2 day-1), and their two respective no UVB controls. We found a greater decrease in stomatal conductance under SUV than FUV. There was no decrease in maximum yield of photosystem II (Fv/Fm) or its operational efficiency (ɸPSII) under the two UVB treatments, and Fv/Fm was higher under SUV than FUV. Photosynthetic capacity was enhanced under FUV in the light-limited region of rapid light-response curves but enhanced by SUV in the light-saturated region. Flavonol content was similarly increased by both UVB treatments. We conclude that, while both FUV and SUV effectively stimulate acclimation to UVB radiation at realistic doses, FUV confers weaker acclimation than SUV. This implies that recovery periods between transient increases in UVB radiation reduce UVB acclimation, compared to an equivalent dose of UVB provided continuously. Thus, caution is needed in interpreting the findings of experiments using steady UVB radiation treatments to infer effects in natural environments, as the stimulatory effect of steady UVB is greater than that of the equivalent fluctuating UVB.

Deficit irrigation and leaf removal modulate anthocyanin and proanthocyanidin repartitioning of Cabernet Sauvignon (Vitis vinifera L.) grape and resulting wine profile.

BACKGROUND: In monsoonal climates, grape anthocyanin and proanthocyanidin (PA) accumulations are unsatisfactory for producing optimal wine. Agronomical practices are often considered to be effective means for regulating fruit components. However, there is a lack of quantitative information on the effects of deficit irrigation (DI), basal leaf removal (LR) or their combination of deficit irrigation and leaf removal (DILR) on the characteristics of anthocyanin and PA compositions and their implications on the resulting wine quality. In this study, the dynamics of grape anthocyanin and PA accumulation were investigated in DI, LR and DILR during grape ripening, and the resulting wine profile was assessed. RESULTS: The contents of reducing sugar and total anthocyanins in Cabernet Sauvignon berries were significantly increased by DI, LR and DILR, while titratable acidity, total flavan-3-ols and tannins levels were generally decreased. Notably, the levels of 3'5'-substituted anthocyanins, such as malvidin and its derivatives significantly increased, and 3'-substituted anthocyanins decreased in both grape and wine under DI and DILR strategies. Skin PAs were sensitive to water deficits, whereas they were insensitive to LR. In resulting wine, PAs content and the proportion of 3'-hydroxylated PAs, such as (+)-catechin, (-)-epicatechin and (-)-epicatechin-3-O-gallate units were significantly decreased under DI and DILR, while molecular mass and the proportion of 3'5'-hydroxylated units of PAs were increased in response to DILR. CONCLUSION: The DILR was the most favorable for the repartitioning of anthocyanin and PA metabolites, and promoted the accumulation of tri-substituted forms contributing a higher color intensity, mouthfeel persistence, structure, and astringency of wine. This information provides an important strategy for modulating the anthocyanin and PA compositions by agricultural practices and achieving the desired quality of grapes and wines in monsoonal climates. © 2021 Society of Chemical Industry.

1-Aminocyclopropane-1-Carboxylate Deaminase-Producing Plant Growth-Promoting Rhizobacteria Improve Drought Stress Tolerance in Grapevine (Vitis vinifera L.).

Plant growth-promoting rhizobacteria (PGPRs) that produce 1-aminocyclopropane-1-carboxylate (ACC) deaminase are capable of reducing limits to plant growth due to water-deficient conditions. Here, seven PGPR strains that can produce ACC deaminase were successfully obtained from the rhizosphere soil of grapevine (Vitis vinifera L.) in arid regions of China. The strains belonged to three different genera: Pseudomonas, Enterobacter, and Achromobacter, according to their 16S rDNA sequencing analysis. A drought tolerance experiment revealed two PGPR strains (DR3 and DR6) with exceptionally high phosphate solubilization, nitrogen fixation, indoleacetic acid (IAA), and exopolysaccharides secretion potential. Both strains were selected for use in a pot experiment to evaluate their growth-promoting effects on grapevines under drought conditions. Each of these two PGPRs and their mixed inoculation into grapevines were expected to alleviate the comprehensive growth inhibition of grapevines caused by drought stress. The mixed inoculation was hypothesized to elicit the best growth-promoting effects. Inoculation with the PGPRs not only enhanced the root-adhering soil/root tissue ratios and soil aggregate stability, but it also increased the nitrogen and phosphorus levels in the soil and plant leaves. Further, inoculation with PGPRs significantly altered the plant height, biomass of shoot and root organs, relative water contents, and net photosynthetic rate of leaves, enabling grapevines to better cope with drought. Moreover, the contents of IAA, abscisic acid, and malondialdehyde in these grapevines under drought stress were significantly changed by PGPRs. They indirectly affected biochemical and physiological properties of grapevines to alleviate their drought stress. Taken together, these results demonstrate that the DR3 and DR6 PGPRs might be useful for effectively weakening the growth inhibition caused by drought in grapevines. The strains might also be applied as effective bioinoculants to maintain the quality of wine grapes.