Seasonal changes of whole root system conductance by a drought-tolerant grape root system

Abstract

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.

Fig. 1.

Seasonal root production (±1 SE) for root systems of V. berlandieri×V. rupestris cv. 1103P and V. riparia×V. rupestris cv. 101-14Mgt (season×root system interaction: P=0.002). Data represent total root length produced cm⁻² of observational window over three month periods for the years, 2003–2005. Each season corresponded to the following months: Autumn, September–November (significance of difference between 1103P and 101-14Mgt, P=0.328); winter, December–February (P=0.009), spring, March–May (P=0.230), and summer, June–August (P=0.032). (Bauerle et al., 2008b). Reprinted with the permission of The New Phytologist (Chesire England UK).

Fig. 2.

Shown for spring (June) and autumn (September) are square metres of leaf area, A_L (A) and fruit weight, FW (B) per vine for Vitis vinifera cv. Merlot grafted onto V. rupestris×V. riparia cv. 101-14Mgt (open bars) and V. berlandieri ×V. rupestris cv. 1103P (shaded bars) root systems. Each value is the mean of five vines of the same rootstock for each sampling date. Bars represent the standard error. Means that do not share the same lower case letter are significantly different (P ≤0.05).

Fig. 3.

Diurnal courses of leaf stomatal conductance (g_s, mmol m⁻² s⁻¹), net photosynthetic rate (A, μmol CO₂ m⁻² s⁻¹), canopy transpiration rate once scaled with the leaf area values (E_c, mmol vine⁻¹ s⁻¹), leaf to air vapour pressure deficit (VPD_leaf, kPa), leaf temperature (T_leaf, C) and leaf temperature-air temperature diference (T_air–T_leaf, C) on 21 June (A, B, C, D, E, F) and 19 September (G, H, I, J) for V. vinifera cv. Merlot grafted onto V. berlandieri×V. rupestris cv. 1103P (closed symbols) and, V. riparia×V. rupestris cv. 101-14Mgt (open symbols) rootstocks. Each value is the mean of observations from six vines. Vertical bars represent the standard error (P ≤0.05) and an asterisk denotes a significant difference (P ≤0.05) between rootstocks. Errors smaller than the symbols are hidden.

Fig. 4.

Shown are diurnal courses of air temperature (T, open symbols) and vapour pressure deficit (VPD, closed symbols) for 21 June (A) and 19 September 2006 (B) for a vineyard in Oakville California, Napa Valley USA.

Fig. 5.

Diurnal course of leaf water potential (Ψ_L) for V. vinifera cv. Merlot growing on V. berlandieri×V. rupestris cv. 1103P (closed symbols) and V. riparia×V. rupestris cv. 101-14Mgt (open symbols). Shown are the mean values for six vines of each rootstock and sampling date, June (A) and September (B). Vertical bars represent the standard error and an asterisk indicates significant differences between rootstocks (P ≤0.05). Errors smaller than the symbols are hidden.

Fig. 6.

(A) Root hydraulic conductance (k_r, kg H₂O MPa⁻¹ s⁻¹) and (B) canopy specific root hydraulic conductance (k_L, kg H₂O MPa⁻¹ s⁻¹ m⁻²) in spring (June) and autumn (September) for V. vinifera cv. Merlot grafted onto on to V. rupestris×V. riparia cv. 101-14Mgt (open bars) and V. berlandieri×V. rupestris cv. 1103P (shaded bars). (C) Hydraulic conductivity (K_r, kg H₂O m s⁻¹ MPa⁻¹) for individual root segments. Each value is the mean of five vines of each and sampling date. Vertical bars represent the standard error. Different lower case letters indicate statistically significant differences between rootstocks (P ≤0.05).

Fig. 7.

Frequency distribution of conduits (Px) in each of the 25, 2 cm length classes from a silicon injection point, representing the proximal end with respect to the vine trunk, up to 50 cm distal to the trunk for roots of Vitis vinifera cv. Merlot grafted onto V. rupestris×V. riparia cv. 101-14Mgt (open bars) and V. berlandieri×V. rupestris cv. 1103P (shaded bars).