Physiological characteristics of several cultivars of European plum (Prunus domestica) in the conditions of northern Oltenia - Romania

Several physiological parameters (photosynthetically active radiation on leaf surface, photosynthesis rate, transpiration rate and stomatal conductance of CO₂) were determined for a number of 9 cultivars of European plum (3 introduced and 6 Romanian cultivars) using a Portable Photosynthesis System LCI with automatic recording. Total leaf chlorophyll content was determined using SPAD 502DL Plus Chlorophyll Meter. Measurements were carried out in July, in a temperature range from 32.8 to 38.6°C and atmospheric pressure of 982 to 984 mbar. The leaf chlorophyll content ranged in July between 39.6 SPAD units ('Vânăt românesc cl. 4') and 52.6 SPAD units ('Andreea'). In September it decreased from 1.4 to 12.1 SPAD units depending on the cultivar. The coefficients of variation (s%) of these determinations are low (8.1 to 12.1%), revealing a very uniform content of chlorophyll in the leaves. The photosynthetically active radiation on leaf surface (Qleaf) and photosynthesis rate (A) varies from one cultivar to another (587-886.7 µmol m^-2 s^-1, respectively from 3.9 to 13.9 µmol m^-2 s^-1) but also from time of the day. The photosynthetically active radiation on leaf surface (Qleaf) is maximum from 1:00 to 2:00 pm (1124.2 µmol m^-2 s^-1) as the rate of photosynthesis (8.1 µmol m^-2 s^-1). Transpiration rate (E) ranges between 1.5 and 5.4 mmol m^-2 s^-1 and stomatal conductance of CO₂ (gs) between 0.051 and 0.298 mol m^-2 s^-1. The physiological factors varied depending of cultivars and moment of data recording (manifested more intensely from 1:00 to 2:00 pm).