Water transport from soils to the atmosphere is critical for plant growth and survival. However, we have a limited understanding about many portions of the whole-tree hydraulic pathway, because the vast majority of published information is on terminal branches. Our understanding of mature tree trunk hydraulic physiology, in particular, is limited. The hydraulic vulnerability segmentation hypothesis (HVSH) stipulates that distal portions of the plant (leaves, branches and roots) should be more vulnerable to embolism than trunks, which are nonredundant organs that require a massive carbon investment. In the current study, we compared vulnerability to loss of hydraulic function, leaf and xylem water potentials and the resulting hydraulic safety margins (in relation to the water potential causing 50% loss of hydraulic conductivity) in leaves, branches, trunks and roots of four angiosperms and four conifer tree species. Across all species, our results supported strongly the HVSH as leaves and roots were less resistant to embolism than branches or trunks. However, branches were consistently more resistant to embolism than any other portion of the plant, including trunks. Also, calculated whole-tree vulnerability to hydraulic dysfunction was much greater than vulnerability in branches. This was due to hydraulic dysfunction in roots and leaves at less negative water potentials than those causing branch or trunk dysfunction. Leaves and roots had narrow or negative hydraulic safety margins, but trunks and branches maintained positive safety margins. By using branch-based hydraulic information as a proxy for entire plants, much research has potentially overestimated embolism resistance, and possibly drought tolerance, for many species. This study highlights the necessity to reconsider past conclusions made about plant resistance to drought based on branch xylem only. This study also highlights the necessity for more research of whole-plant hydraulic physiology to better understand strategies of plant drought tolerance and the critical control points within the hydraulic pathway.

The procedure for applying phosphorus (P) fertilizer to soil can be divided into three consecutive steps: (i) Measurement of soil‐P availability, (ii) calibration of the soil‐P fertility level and (iii) estimation of the recommended P dose. Information on each of these steps was obtained for 18 European countries and regions with the aim of comparing P fertilizer recommendation systems at the European scale. We collected information on P fertilizer recommendations through conventional or grey literature, and personal contacts with researchers, laboratories and advisory services. We found much variation between countries for each of the three steps: There are more than 10 soil‐P tests currently in use, apparent contradictions in the interpretation of soil‐P test values and more than 3‐fold differences in the P fertilizer recommendations for similar soil‐crop situations. This last result was confirmed by conducting a simple experimental inter‐laboratory comparison. Moreover, soil properties (pH, clay content) and crop species characteristics (P responsiveness) are used in some countries in the calibration and recommendation steps, but in different ways. However, there are also common characteristics: soil‐P availability is determined in all countries by extraction with chemical reagents and the calibration of the soil‐P test values, and the fertilizer recommendations are based on the results from empirical field trials. Moreover, the fertilizer recommendations are nearly all based on the amount of P exported in the crops. As long as rational scientific and theoretical backgrounds are lacking, there is no point in trying to synchronize the different chemical methods used. We therefore call for a mechanistic approach in which the processes involved in plant P nutrition are truly reproduced by a single standard method or simulated by sorption‐desorption models.

Although vast areas in tropical regions have weathered soils with low potassium (K) levels, little is known about the effects of K supply on the photosynthetic physiology of trees. This study assessed the effects of K and sodium (Na) supply on the diffusional and biochemical limitations to photosynthesis in Eucalyptus grandis leaves. A field experiment comparing treatments receiving K (+K) or Na (+Na) with a control treatment (C) was set up in a K-deficient soil. The net CO2 assimilation rates were twice as high in +K and 1.6 times higher in +Na than in the C as a result of lower stomatal and mesophyll resistance to CO2 diffusion and higher photosynthetic capacity. The starch content was higher and soluble sugar was lower in +K than in C and +Na, suggesting that K starvation disturbed carbon storage and transport. The specific leaf area, leaf thickness, parenchyma thickness, stomatal size and intercellular air spaces increased in +K and +Na compared to C. Nitrogen and chlorophyll concentrations were also higher in +K and +Na than in C. These results suggest a strong relationship between the K and Na supply to E. grandis trees and the functional and structural limitations to CO2 assimilation rates.

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