Net Photosynthesis, Dry Matter Production, and Phenological Development of Apricot Trees (Prunus armeniaca L.) Cultivated in the Negev Highlands (Israel)

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Summary Growth rates, phenological development, and daily courses of net photosynthesis were measured on apricot trees ( Prunus armeniaca L.) together with environmental and physiological parameters in the Negev desert under runoff farming conditions. The investigations were conducted during an entire growing period with two experimental trees. One of them received water only from rain and runoff (non-irrigated tree). The other tree was additionally irrigated starting 11 months prior and during the experiments in order to eliminate water stress (irrigated tree). The irrigated tree opened flowers and leaf buds later, also elongation growth started later, but continued for a longer period of time (time of elongation growth months) than in the non-irrigated counterpart (months). Growth was terminated in both trees at a time when predawn water potential reached —7 to —8 bar and daily minimum water potential reached —35 to —40 bar. Trunk growth continued beyond this stage of water stress until the highest water potential in the soil profile reached —10 bar for the non-irrigated and —6 bar for the irrigated tree. The seasonal development of the ratio dry weight/surface area and the ratio chlorophyll/dry weight of the leaves and their anatomy was identical for both trees. The root system was studied intensively for the non-irrigated tree. It had 3 m long tap roots. Horizontal roots were mainly developed in the range of 2 to 3 m, but some lateral roots extended as far as 8 m. The root system exploited a soil volume of about 44—76 m 3 . The root/shoot ratio was 1.1. Because of earlier leaf development, the non-irrigated tree had higher rates of daily CO 2 uptake m spring than the irrigated counterpart, which, in contrast, had higher rates during the dry season. Per unit leaf dry weight the non-irrigated tree had a higher seasonal CO 2 uptake than, the irrigated one (29.5 versus 25.1 gCO 2 • gdw -1 • year -1 ). But because its larger leaf number the irrigated tree achieved an about 30% higher total CO 2 gain per year than its counterpart (about 75 kg C versus 56 kg). The allocation of carbon for leaf, wood, fruit and root growth showed main differences in root and fruit development. It was estimated, that the irrigated tree allocated about 16% (kg C) of its total carbon gain into fruits but only about 13% into roots, whereas the non-irrigated tree allocated% (kg C) of its carbon gain into fruits and about 52% into its below ground parts. The ecological implications of different strategies in growth, photosynthetic production and carbon allocation in trees of differing water stress are discussed.