Fulvio Pernice

Comparison of growth potentials of epicormic shoots of nectarine trees grown on size-controlling and vigorous rootstocks

Summary The vegetative performance of four nectarine tree scion/rootstock combinations, with varying growth capacities, were analysed in the Spring after the canopy:root ratio was dramatically reduced by severe pruning in the dormant season. We anticipated that severe pruning and reduction of the shoot:root ratio would mitigate known root water supply limitations related to the various rootstocks and determine if other factors associated with the rootstocks would control the rates of epicormic shoot growth. The trees used in this field experiment were 3 years-old, with ‘May Fire’ nectarine grafted onto four different rootstocks: ‘Nemaguard’ (a vigorous seed-propagated control, P. persica P. davidiana hybrid) and three size-controlling rootstocks, ‘K146-43’, ‘P30-135’ (P. salicina P. persica hybrids) and ‘K-119-50’ (P. salicina P. dulcis hybrid). At the beginning of the trial, there were clear differences in plant dimensions, confirming the higher vigour induced by ‘Nemaguard’ compared to the other rootstocks. Reducing the canopy:root ratio was able to annul potential differences in weekly shoot-extension growth rates and cumulative extension growth of individual shoots during the Spring flush of re-growth after pruning. This supported the idea that there were no rootstock-specific signals (hormonal or physical) that caused large differences in extension growth rates of individual epicormic shoots on trees. However, there were clear differences in total epicormic shoot re-growth per tree that were associated with initial tree size at the time of severe pruning, indicating the importance of trunk and root storage in influencing the amount of growth stimulation after dormant pruning.

CO2 fluxes of Opuntia ficus-indica Mill. trees in relation to water status

Gas exchange pattern in O. ficus-indica (OFI), refers to the Crassulacean Acid Metabolism (CAM); trees have nocturnal stomata opening, so net CO2 uptake and water loss occur during the cooler part of the 24-hour cycle. Succulent cladodes skip severe periods of drought through their water storer tissue (parenchyma). To study carbon fluxes in stress and no stress conditions, an experiment was carried out on 3-year-old irrigated and non-irrigated OFI potted trees; whole tree gas exchange was measured continuously with a balloon system made up by a portable Infrared Gas Analyzer. Continuous measurements (nighttime) during the summer season were useful to assess differences in carbon uptake under stress and no stress conditions. There was a gradual increment (5 μmol m s in June, 7 μmol m s in July and 8.8 μmol m s in August) in terms of CO2 uptake in irrigated trees from June to August 2010. The uptake was lower in stressed trees than in irrigated ones in each measurements date. Measurements carried out on non-irrigated trees showed carbon gain even 60 days after irrigation was stopped, with less than 2% of soil water content, far below the wilting point. Considering an average of 6.9 μmol CO2 m s, for well watered trees, from June to August, and a stem area index (SAI) of 2, a daily amount of 21.8 kg ha d of CO2 was accumulated in irrigated trees in that period, corresponding to a carbon assimilation of 0.54 T ha. INTRODUCTION Opuntia ficus-indica (L.) Mill. is a CAM species (Crassulacean Acid Metabolism) belonging to Cactaceae family. This group of species shows nocturnal stomata opening and CO2 uptake. During the nighttime, through phosphoenolpyruvate (PEP) carboxylation, malate is formed and stored in vacuoles, while, during the daytime, stomata are closed, malate is decarboxylated and CO2 is fixed by Rubisco. In the early 1900s CO2 uptake was obtained through indirectly measurements; indeed, Richards (1915), monitored gas exchange analyzing day/night fluctuation of acidity in stem tissues sampled from dusk to dawn. Direct measurements of gas exchange in O. ficus-indica, however, began in the early 1980s, when Nobel and Hartsock (1984) measured CO2 uptake on single cladodes grown in growth chambers. From then onward, cladodes gas exchange measurements have been done on single portions of a cladode, at specific intervals (2-4 h) during the night, using a portable system with leaf chambers, adapted to cladode morphology. This system is affected by a large variability of spot measurements, and much depends on operator’s skill. Under optimal conditions (25/15°C day/night) and light saturation, OFI may assimilate 3.44 g m d CO2; three weeks of drought conditions are required for halving net CO2 uptake over 24-h periods (Nobel, 1988), and net CO2 uptake over 24-h period is around zero after 50 days of drought (Acevedo et al., 1983). Instantaneous values of net CO2 uptake of 1-year-old cladodes range from 4.5 to 15 μmol m s (Inglese et al., 1994; Pimienta-Barrios et al., 2005). Response of cladodes to drought also depends on newly developing cladodes; indeed, as the number of current-year cladodes increased, total daily