This paper provides a mini-review of evidence for negative turgor pressure in leaf cells starting with experimental evidence in the late 1950s and ending with biomechanical models published in 2014. In the present study, biomechanical models were used to predict how negative turgor pressure might be manifested in dead tissue, and experiments were conducted to test the predictions. The main findings were as follows: (i) Tissues killed by heating to 60 or 80 °C or by freezing in liquid nitrogen all became equally leaky to cell sap solutes and all seemed to pass freely through the cell walls. (ii) Once cell sap solutes could freely pass the cell walls, the shape of pressure-volume curves was dramatically altered between living and dead cells. (iii) Pressure-volume curves of dead tissue seem to measure negative turgor defined as negative when inside minus outside pressure is negative. (iv) Robinia pseudoacacia leaves with small palisade cells had more negative turgor than Metasequoia glyptostroboides with large cells. (v) The absolute difference in negative turgor between R. pseudoacacia and M. glyptostroboides approached as much as 1.0 MPa in some cases. The differences in the manifestation of negative turgor in living versus dead tissue are discussed.
Pressure-volume (PV) curve analysis is the most common and accurate way of estimating all components of the water relationships in leaves (water potential isotherms) as summarized in the Höfler diagram. PV curve analysis yields values of osmotic pressure, turgor pressure, and elastic modulus of cell walls as a function of relative water content. It allows the computation of symplasmic/apoplastic water content partitioning. For about 20 years, cavitation in xylem has been postulated as a possible source of error when estimating the above parameters, but, to the best of the authors' knowledge, no one has ever previously quantified its influence. Results in this paper provide independent estimates of osmotic pressure by PV curve analysis and by thermocouple psychrometer measurement. An anatomical evaluation was also used for the first time to compare apoplastic water fraction estimates from PV analysis with anatomical values. Conclusions include: (i) PV curve values of osmotic pressure are underestimated prior to correcting osmotic pressure for water loss by cavitation in Metasequoia glyptostroboides; (ii) psychrometer estimates of osmotic pressure obtained in tissues killed by freezing or heating agreed with PV values before correction for apoplastic water dilution; (iii) after correction for dilution effects, a solute concentration enhancement (0.27MPa or 0.11 osmolal) was revealed. The possible sources of solute enhancement were starch hydrolysis and release of ions from the Donnan free space of needle cell walls.
Aims The variations in leaf size result from the integrated effects of many factors. Study of the mechanism to reach the optimum leaf size could help us better understand plant adaption and evolution, and plant life history strategies. Here we aim to test the hypothesis that leaf size is affected by the biomass allocation strategy within a leaf. Methods The relationships between leaf size and different biomass partitioning patterns within a leaf were studied for 19 evergreen and 30 deciduous broadleaved woody species from Qingliang Mountain, Zhejiang, China. The standardized major axis estimation method was used to examine the scaling relationship between lamina size and petiole size within a leaf. The relationship between leaf size and support investment ratio within a leaf was estimated by the Model Type I regression analysis. Important findings Biomass allocation in petiole increased with leaf size similarly in both evergreen and deciduous leaves, which resulted from the significant allometric scaling relationship between petiole mass and lamina mass (and area) with slopes significantly larger than 1.0, independent of leaf habit. However, evergreen species were found to have a greater petiole mass at a given lamina mass or area than deciduous species, which may be due to their higher demand for mechanic support and resistance to freezing-induced embolism in petioles. Results suggest that leaf size could be affected by the fraction of support investment within a leaf.
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