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The role of root systems in drought tolerance is a subject of very limited information compared with above-ground responses. Adjustments to the ability of roots to supply water relative to shoot transpiration demand is proposed as a major means for woody perennial plants to tolerate drought, and is often expressed as changes in the ratios of leaf to root area (A(L):A(R)). Seasonal root proliferation in a directed manner could increase the water supply function of roots independent of total root area (A(R)) and represents a mechanism whereby water supply to demand could be increased. To address this issue, seasonal root proliferation, stomatal conductance (g(s)) and whole root system hydraulic conductance (k(r)) were investigated for a drought-tolerant grape root system (Vitis berlandieri×V. rupestris cv. 1103P) and a non-drought-tolerant root system (Vitis riparia×V. rupestris cv. 101-14Mgt), upon which had been grafted the same drought-sensitive clone of Vitis vinifera cv. Merlot. Leaf water potentials (ψ(L)) for Merlot grafted onto the 1103P root system (-0.91±0.02 MPa) were +0.15 MPa higher than Merlot on 101-14Mgt (-1.06±0.03 MPa) during spring, but dropped by approximately -0.4 MPa from spring to autumn, and were significantly lower by -0.15 MPa (-1.43±0.02 MPa) than for Merlot on 101-14Mgt (at -1.28±0.02 MPa). Surprisingly, g(s) of Merlot on the drought-tolerant root system (1103P) was less down-regulated and canopies maintained evaporative fluxes ranging from 35-20 mmol vine(-1) s(-1) during the diurnal peak from spring to autumn, respectively, three times greater than those measured for Merlot on the drought-sensitive rootstock 101-14Mgt. The drought-tolerant root system grew more roots at depth during the warm summer dry period, and the whole root system conductance (k(r)) increased from 0.004 to 0.009 kg MPa(-1) s(-1) during that same time period. The changes in k(r) could not be explained by xylem anatomy or conductivity changes of individual root segments. Thus, the manner in which drought tolerance was conveyed to the drought-sensitive clone appeared to arise from deep root proliferation during the hottest and driest part of the season, rather than through changes in xylem structure, xylem density or stomatal regulation. This information can be useful to growers on a site-specific basis in selecting rootstocks for grape clonal material (scions) grafted to them.

Abstract: Four walnut tree species, of value for their wood production, were evaluated for autumn cold resistance: Juglans regia L., Juglans nigra L., Juglans major Ton. A. Heller, and Juglans Xintermedia. Early frost events damage and reduce wood quality and lengthen timber harvesting rotation. The frost vulnerability of these species could be an even more important problem in the future, depending on which Mediterranean climatic scenario develops. The work is based on the use of the chlorophyll fluorescence parameter Fv/Fm- measured in cortical bark chiorenchyma-as a reliable estimator of freezing injuries in Juglans. Two methods traditionally used to evaluate the effects of frost on deciduous trees were also analyzed: visual damage and relative electrolyte leakage. The study was carried out using annual woody shoots collected in November 2009, which were subjected to low temperatures (from 5° C to -16.5° C) in a controlled cold chamber. The relationship between leaf fall stages and autumn frost damage has also been discussed. The results obtained using the different methods studied were comparable. No injuries were detected in any species until the temperature dropped below - 8.5° C. In the study, black walnut clones were the most resistant to subzero temperatures, J. regia were the least, and J. X intermedia showed an intermediate behavior with respect to its genitors. The assessment of cold damage using the FvIFm parameter was easy and quick. It seems that this would be particularly useful for screening a large number of samples. FOR. SCI. 59(5):559-565. [ABSTRACT FROM AUTHOR]