Stronger seasonal adjustment in leaf turgor loss point in lianas than trees in an Amazonian forest

Pan-tropically, liana density increases with decreasing rainfall and increasing seasonality. This pattern has led to the hypothesis that lianas display a growth advantage over trees under dry conditions. However, the physiological mechanisms underpinning this hypothesis remain elusive. A key trait influencing leaf and plant drought tolerance is the leaf water potential at turgor loss point ( π tlp ). π tlp adjusts under drier conditions and this contributes to improved leaf drought tolerance. For co-occurring Amazonian tree ( n = 247) and liana ( n = 57) individuals measured during the dry and the wet seasons, lianas showed a stronger osmotic adjustment than trees. Liana leaves were less drought-tolerant than trees in the wet season, but reached similar drought tolerances during the dry season. Stronger osmotic adjustment in lianas would contribute to turgor maintenance, a critical prerequisite for carbon uptake and growth, and to the success of lianas relative to trees in growth under drier conditions.

RpoE2 of Sinorhizobium meliloti is necessary for trehalose synthesis and growth in hyperosmotic media

Adaptation to osmotic stress can be achieved by the accumulation of compatible solutes that aid in turgor maintenance and macromolecule stabilization. The genetic regulation of solute accumulation is poorly understood, and has been described well at the molecular level only in enterobacteria. In this study, we show the importance of the alternative sigma factor RpoE2 in Sinorhizobium meliloti osmoadaptation. Construction and characterization of an S. meliloti rpoE2 mutant revealed compromised growth in hyperosmotic media. This defect was due to the lack of trehalose, a minor carbohydrate osmolyte normally produced in the initial stages of growth and in stationary phase. We demonstrate here that all three trehalose synthesis pathways are RpoE2 dependent, but only the OtsA pathway is important for osmoinducible trehalose synthesis. Furthermore, we confirm that the absence of RpoE2-dependent induction of otsA is the cause of the osmotic phenotype of the rpoE2 mutant. In conclusion, we have highlighted that, despite its low level, trehalose is a crucial compatible solute in S. meliloti, and the OtsA pathway induced by RpoE2 is needed for its accumulation under hyperosmotic conditions.

Genotypic variation for drought stress response traits in soybean. II. Inter-relations between epidermal conductance, osmotic potential, relative water content, and plant survival

As part of a project exploring the potential for using leaf physiological traits to improve drought tolerance in soybean, studies were conducted to explore whether epidermal conductance (ge), osmotic potential (π), and relative water content (RWC) influenced turgor maintenance and ultimately the survival of droughted plants. In a glasshouse study, plants of 8 soybean genotypes that showed different expression of the traits were grown in well watered soil-filled beds for 21 days and then exposed to terminal water deficit stress. The trends in each trait were then monitored periodically until plant death. Genotypic differences were observed in the rate of decline in RWC as the soil dried, in the temporal patterns of change in ge and π, in the duration of survival after watering ceased, and in the critical relative water content (RWCC) at which plants died. In general, ge became smaller and π became more negative as RWC declined and plants acclimated to the increasing stress. Genotypic differences in ge remained broadly consistent as RWC declined. In contrast, the genotypic rankings for π in stressed plants were poorly correlated with those for well watered plants, indicating differential genotypic capacity for osmotic adjustment (OA) in response to stress. Survival times among genotypes after stress commenced ranged from 27 to 41 days, while RWCC ranged from 49% down to 41%. The differences in survival time among the genotypes were able to be explained by genotypic differences in the rate of decline in RWC and in the RWCC, using a multiple linear regression relationship (R 2 = 0.94**). In turn, genotypic differences in the rate of decline in RWC were positively correlated (r = 0.75*) with ge at 70% RWC, and with OA over the drying period (r = 0.98**). In a second study in a controlled environment facility, leaf area retention at 90% soil water extraction was greatest in the one genotype that combined low ge, high OA, and low RWCC. Overall, the responses from the two studies were consistent with the hypothesis that turgor maintenance and ultimately leaf and plant survival of different genotypes during advanced stages of drought stress are enhanced by low ge, high OA capacity, and low RWCC.

Evaluation of the differential osmotic adjustments between roots and leaves of maize seedlings with single or combined NPK-nutrient supply

Many physiological mechanisms associated with nutrient supply have been implicated as improving plant growth under drought conditions. However, benefits to plant growth under drought might derive from an increased recovery of soil water through osmotic adjustment in the shoots and especially in the roots. Thus, experiments were carried out to investigate the effects of the nutrients N, P and K applied singly or in combination, on the osmotic adjustment and turgor maintenance in the roots and leaves of maize seedlings. The seedlings were harvested between 18 and 37 days after sowing according to the soil matric threshold potentials. Soil matric potentials and shoot and root biomass were determined at harvest. Turgor pressure and osmotic adjustment of the leaves and roots were estimated by measurements of their water and osmotic potentials. Results showed that plants with either of the combined fertilisation treatments NPK or NP grew faster at a given level of drought stress than those with no fertilisation, N, P or K applied individually or the combined nutrient treatments PK and NK. Among the fertiliser applications with either a single or two combined nutrients, plants treated with any of N, P or NP grew faster than those with either K or NK. The association between the interactive effects of nutrients and drought stress on the osmotic adjustment and turgor maintenance in roots may partially explain the role of nutrients in drought tolerance of maize seedlings. In particular, the roots exhibited a higher osmotic adjustment than the leaves for all nutrient treatments, suggesting that shoot growth shows a higher sensitivity to water deficit compared to root growth. We conclude that the maintained turgor of roots under drought stress obtained with an optimal nutrient supply results in better root growth and apparently promotes overall plant growth, suggesting that osmotic adjustment is an adaptation not only for surviving stress, but also for growth under such conditions.