Claudia Negron

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

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 (AL:AR). Seasonal root proliferation in a directed manner could increase the water supply function of roots independent of total root area (AR) and represents a mechanism whereby water supply to demand could be increased. To address this issue, seasonal root proliferation, stomatal conductance (gs) and whole root system hydraulic conductance (kr) 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 (ψL) 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, gs 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 (kr) increased from 0.004 to 0.009 kg MPa−1 s−1 during that same time period. The changes in kr 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.

How different pruning severities alter shoot structure: a modelling approach in young ‘Nonpareil’ almond trees

Axillary meristem fate patterns along shoots, also referred to as shoot structure, appear to be fairly consistent among trees within a genotype growing under similar conditions. Less is known about shoot structural plasticity following external manipulations, such as pruning. The aim of this study on almond (Prunus dulcis (Mill.)) shoots was to investigate how pruning severity affects the structure of 1-year-old shoots that grew after pruning (regrowth shoots), the 2-year-old portion of shoots that remained from the previous years growth after pruning (pruned shoots), and whether regrowth shoots reiterate the structure of the original 1-year-old shoots before pruning. Three pruning severities were imposed and the structures along the different shoots were assessed by building hidden semi-Markov models of axillary meristem fates. The structures of regrowth and pruned shoots depended on pruning severity, but maintained some of the original shoot characteristics. Regrowth shoots developed more complex structures with severe pruning, but had simpler structures than original shoots indicating progressive simplification with tree age. Pruned shoot structures were affected by the severity of pruning, by the structure when the shoots were 1 year old, and probably by local competition among buds. Changes in structure due to pruning can be modelled and be predictable.