Photosyntesis Intensity andVegetative Growth of Pedunculate Oak (Quercus roburL.) in Common-Garden Experiment

U radu je analiziran utjecaj mikrostaništa na fiziološke značajke i vegetativni rast hrasta lužnjaka. Istraživanje je provedeno na dvije pokusne plohe koje se odlikuju različitim kemijskim značajkama, sadržajem vlage i temperaturom tla. Cilj rada bio je utvrditi utjecaj različitih kemijskih značajki tla na vegetativni rast i intenzitet fotosinteze hrasta lužnjaka, kao i odnos koncentracije klorofila i intenziteta fotosinteze. Biljke uzrasle na tlu s većom koncentracijom humusa, dušika, fosfora i kalija imaju veći intenzitet fotosinteze, veću koncentaraciju klorofila te bolji vegetativni rast. Zbog povoljnog odnosa volumetrijskog sadržaja vlage u tlu i vodnog potencijala istraživanih biljaka, njihov značajniji utjecaj na fiziološke i vegetativne značajke hrasta lužnjaka nije utvrđen. Rezultati ukazuju na visoku pozitivnu korelaciju između indeksa ukupnih klorofila u proljetnim listovima i većine fizioloških te vegetativnih značajki istraživanih biljaka.The effect of various types of environmental stress factors on forest trees is most often manifested through the reduced absorption of mineral nutriments. This results in lowered efficiency of photosynthetic pigments on the leaf’s cellular level and production of insufficient amounts of carbohydrates necessary for normal vegetative growth. Due to sudden increase in concentration of carotenoids in relation to chlorophyll, which is susceptible to a sudden destruction under the effect of stress factors, a change takes place in the relative amounts of chlorophyll and carotenoids. Because of this, the leaves in the crown of a tree temporarily lose their green color and turn yellow, which is indicative of plant’s lack of nitrogen nutrition, i.e. the nitrogen deficiency in the soil. The role of nitrogen as a plant nutriment is connected with numerous physiological processes responsible for successful growth and development of plants. For example, nitrogen is an essential element responsible for an uninterrupted continuation of photosynthetic process and vegetative plant growth, primarily because of its role in the synthesis of chlorophyll and certain proteins, such as ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco), which is responsible for CO2assimilation. In the previous studies of forest trees, significant differences were determined in the intensity of photosynthesis and vegetative development regarding the diverse conditions in the habitats. These variations are usually due to the lengths of summer droughts and high temperatures, the intensity of illumination, and the differences between dry and wet years. The aim of this study was to, (1) determine the effect of various chemical characteristics of the soil on the intensity of photosynthesis and the vegetative growth of pedunculate oak, (2) establish a connection between content index of the total amount of chlorophyll in the leaf, and the intensity of photosynthesis and the vegetative growth of pedunculate oak. The study was conducted during the vegetative period in 2010, on 4-yearold plants, planted with 2 – 2,5 meters of space between plants. Based on visual differences in the leaf color, two plots have been set aside – the chlorotic and the control plots (without chlorosis). Each field contained 8 plants, i.e. 16 in total. The average height of the plants under study before the start amounted to 1.96 ± 0.44 meters, and the diameter of the trunk 2.5 centimeters, measured 30 centimeters above ground. In order to determine the differences in the soil’s chemical characteristics between the two studied plots, the samples were taken on the depth of 0–30 centimeters. The soil reaction was determined with a potentiometer in the suspension of soil and water, i.e. soil and nKCl. Humus was determined by the Thorin method, and the total nitrogen level by the Kendahl method. The content of physiologically active phosphorus and potassium was determined by the Al-method. The study determined height and diameter increment for the plants studied, and also height and diameter increment for primary branches of each plant. The number of spring and summer shoots with its attending leaves was also determined. Based on the number and surface area of leaves, as well as the projection of crown surface on the ground, index of leaf surface was calculated for each tree. The measurements of photosynthesis intensity and the index of total chlorophyll content were performed early in September. The photosynthesis intensity was measured with the help of the infrared gas analizer portable device LCpro + (ADC BioScientific). While taking photosynthesis intensity measurements, each leaf was subjected to illumination intensity of 1500 µmol m-2 s-1, CO2concentration of 380 ± 10 µmol mol-1and the air temperature of 25 ± 2 °C. The chlorophyll content index was determined by using chlorophyll content meter CCM-200. By using a portable chamber for measuring water potential, the measurement of water potential was taken in the leaves (.) of plants under study. The chemical characteristics of the soil in the test plots were shown in Table 1. There was an extremely acidic reaction in the chlorotic plot, and acidic in the control plot. The chlorotic plot was marked by an extremely low nitrogen supply, while the control plot had a medium to good nitrogen supply. The content of physiologically active phosphorus in the chlorotic plot was extremely low, while the control plot was averagely supplied with physiologically active phosphorus. Both plots under study featured accessible potassium levels on the lower margins of an average supply. The humus content in the studied plots was very low, especially in the chlorotic plot. Statistically significant difference in the measured values of water potential of the plants grown in the plots was determined only during noon measuring throughout July. In August and September, statistically significant differences in the measured values of water potential were not determined (Table 3). All the parameters of the vegetative growth of the plants under study which were grown on the control plot had statistically significant higher values compared to the plants grown on the chlorotic plot, except for the number of primary branches, which showed no statistically significant differences between the two studied plot trials (Table 4). Leaf surface index, value of which was taken in the middle and near the end of the vegetative cycle, was statistically significantly higher in the control plot than in the chlorotic plot (Figure 2). There was statistically significant difference in the intensity of photosynthesis measured on spring leaves of studied plants between the control and the chlorotic plot. The intensity of photosynthesis on the control plot was 28.4 µmol CO2m-2 s-1, while its value on the chlorotic plot was 19.3 µmol CO2m-2 s-1. Content index values of total chlorophyll in the leaves of the studied plants were statistically significantly different regarding the plots under study, no matter whether the measuring of chlorophyll content was conducted on spring or summer leaves (Figure 4). Regression analysis determined a high positive correlation between the content index of total chlorophyll measured on spring leaves of studied plants with the intensity of photosynthesis, leaf surface index, tree’s girth increment, and also length and girth increment in the primary branches (Table 5). The correlation between content index of total chlorophyll and tree’s height increment was very weak. The results of the study indicate that the soil’s chemical characteristics have an effect on the photosynthetic activity and the concentration of chlorophyll in the leaves of pedunculate oak. The higher values of the total chlorophyll’s content index and the intensity of photosynthesis were recorded for plants grown on the soil with a higher concentration of humus, nitrogen, phosphorus and potassium. Positive correlations between the content indexes of total chlorophyll in leaves and the intensity of photosynthesis, as well as most of the parameters of vegetative growth, indicate the possibility of successful application of chlorophyll measuring methods when determining vitality of particular trees and forest stands of pedunculate oak