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Naschitz S.,Hebrew University of Jerusalem | Naor A.,The Golan Research Institute | Wolf S.,Hebrew University of Jerusalem | Goldschmidt E.E.,Hebrew University of Jerusalem
Trees - Structure and Function

Key message: Autumnal senescence of apple in a warm climate corresponds to accumulated degree-days beneath 22 °C. Summer drought delays senescence and enables replenishment of carbohydrate reserves. Recovery of the root system plays a key role. Autumnal senescence of apple (Malus domestica Borkh.), a deciduous, temperate climate species, is triggered by a rather abrupt temperature drop, down to the lower teens. Under the warmer, east Mediterranean climate of northern Israel, the temperature drop is gradual and much more moderate. Another characteristic of this climate is the complete lack of precipitation during summer. The aim of the present study was to elucidate the effects of summer drought on seasonal leaf senescence in a warm autumn. We hypothesized that summer drought delays senescence due to an increased demand for carbohydrates during autumn. The advent of autumnal senescence was followed for 3 years (2009-2011) on trees exposed to various levels of drought. Total canopy green area (effective leaf area, ELA) and hue angle were estimated periodically by means of image analysis, as a measure of leaf drop and autumnal color change. Photosynthesis, midday stem water potential, and roots' non-structural carbohydrate contents were measured on several occasions. The time course of leaf drop followed the decline in air and soil temperatures. The rate of decline in ELA closely corresponded to accumulated degree-days beneath 22 °C in the soil, a much higher temperature threshold than previously reported for apple. Drought stress during the summer delayed leaf senescence even further, when compared with well-irrigated trees. Leaves maintained their photosynthetic functionality throughout autumn, until late December. The delayed senescence enabled replenishment of root carbohydrate reserves, which is critical for next year's growth and fruiting. The eco-physiological significance of the findings is discussed. © 2014 Springer-Verlag Berlin Heidelberg. Source

Silber A.,Institute of Soil, Water and Environmental Sciences | Israeli Y.,Northern R and D | Levi M.,Northern R and D | Keinan A.,Northern R and D | And 8 more authors.
Agricultural Water Management

The effects of drip irrigation frequency on 'Hass' avocado trees grown in lysimeters were examined. The experimental design comprised three irrigation frequencies: (a) pulsed irrigation (10-20min every 30min) throughout the day (Irg1); (b) one daily irrigation event beginning at night and terminated in the morning every day (Irg2); and (c) one irrigation event every two days (Irg3). Irrigation treatments induced significant differences in water availability in the root zone and in plant water uptake. The effects of the fruit sink on gas-exchange properties and water uptake were assessed by comparing the performance of fruiting and defruited avocado trees. Despite the higher vegetative growth of defruited trees, their daily water uptake was 40% lower than that of fruiting trees and therefore, crop load should play an important role on irrigation scheduling. Measurements of stomatal conductance (gs) and photosynthesis per unit leaf area (A) during two vegetative years were not in accordance with irrigation treatments or with diurnal changes in atmospheric conditions. Similar pattern was observed for the defruited trees. Leaf-carbohydrate concentrations in trees with and without fruits were lowest before sunrise, and increased during the day in different patterns. In defruited trees the carbohydrate concentrations increased steeply to a maximum around 09:00, while in fruiting trees, it increased monotonically until midday. Our findings may indicate that leaf-carbohydrate plausibly play a role in the complex framework of stomata aperture. © 2012 Elsevier B.V. Source

Naschitz S.,Hebrew University of Jerusalem | Naor A.,The Golan Research Institute | Sax Y.,The Golan Research Institute | Shahak Y.,Israel Agricultural Research Organization | Rabinowitch H.D.,Hebrew University of Jerusalem
Scientia Horticulturae

Sunscald of apple (Malus×domestica Borkh.) fruit is a major cause for economical losses. It is widely accepted that exposure to high temperature and light increases the incidence and severity of this fruit injury, but the separate contribution of each factor and the mechanisms involved in the injury development in apples are not well understood. The present study aimed at the quantification of both temperature and light required for photo-oxidative sunscald development and for tolerance acquisition, under controlled conditions. In mid-summer, immature green 'Smoothie' apples from the inner (shaded) part of the canopy were picked biweekly and placed in the dark for 48h at 25°C for decondidting. Fruit were then exposed to a variety of temeprature and light regimes. Sunscald injury developed only when peel temperature exceeded 40°C and was light-dependent. Maximum tolerance to sunscald was acquired in fruit pre-exposed to 38°C for 24h. This temporary tolerance gradually diminished with storage time at 25°C. Development of brown stains within the bleached peel areas increased with temperature. Fv/Fm was negatively correlated with PFD and the duration of exposure to temperatures above 40°C. Threshold values of Fv/Fm required for bleaching and browning of the peel were 0.11 and 0.072, respectively. A 'photo-destructive quantum' (PDQ) was characterized as the minimum absorbed energy required for peel bleaching. The calculated PDQ diminished with the rise in temperature. Pre-treatment with 5 and 10mgL-1 methyl viologen increased fruit susceptibility to sunscald compared to controls, whereas 1mgL-1 increased fruit tolerance to the combined injurious effect of light and heat. We conclude that sunscald is an expression of photo-oxidative stress in the fruit peel which is facilitated by peel temperatures above 40°C. Short-term field tolerance is acquired by insolation at sub-injurious temperatures, but this tolerance is rather weak and cannot accommodate the harsh atmospheric conditions which prevail in the Israeli summer. © 2015 Elsevier B.V. Source

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