Seasonal changes in photosynthetic activity and carbohydrate content in leaves and fruit of three fig cultivars (Ficus carica L.)

The fig is amongst oldest fruits and one of the first plants to be cultivated by humans. The good adaptation of the fig tree to a wide range of soils, in particular its tolerance to water deficit and semi-tolerance to salinity (Golombek and Lüdders, 1993), makes it very interesting for cultivation, in particular for dry and semi-dry areas such as the Mediterranean countries. The fig is consequently an important fruit tree for Europe, where its production averages 900,000 tons per year (50% of world production). There has been an increasing interest in figs, particularly for the fresh market, in recent years. In order to increase the income of fig farmers, cultivar selection and improvement are very important.

Since the formation of carbohydrates is the basis of plant growth and production, photosynthetic activity is very important parameter in plant breeding and selection in order to increase plant biomass and productivity. The photosynthetic capacity is known to be influenced by environmental factors (light intensity, air temperature, water availability, CO₂ concentration, soil fertility, relative humidity, etc.). The ability of the plant to absorb and utilize light energy is also affected by endogenous factors related to leaf anatomy and morphology, leaf age, specific leaf weight (SLW), chlorophyll (Chl) content, nitrogen and carbohydrate concentrations in leaves, the presence or absence of sinks, etc. (Jackson, 1980, Proietti and Tombesi, 1990). It is also known that the photosynthetic capacity is strongly affected by the genotype (cultivar) in many fruit trees.

Despite worldwide fig cultivation and the importance of fig fruit, studies on the photosynthetic capacity of fig and the factors that affect the net photosynthesis rate (Pn) are limited in the literature. Higgins et al. (1992), studying seven fruit species of potted glasshouse-grown plants, found fig to have a relatively high Pn. Can et al. (1998) found significant differences in Pn amongst various fig cultivars in Turkey. Pn was found to vary between clones of the same fig cultivar (Kutlu et al., 1998). A study of seasonal changes of Pn from May to September showed there to be significant differences throughout the season, with maximum values in August and September (Can and Aksoy, 2007).

The effects of internal factors, such as leaf anatomy and morphology, SLW, Chl, nitrogen and carbohydrate concentrations in leaves, on Pn in fig have not been studied. To our knowledge, there are no studies on the changes of Pn during fig leaf and fruit development. Several studies in other fruit trees have shown that SLW and Chl content are usually linearly correlated with Pn, especially during leaf development (Bongi et al., 1987, Proietti and Tombesi, 1990, Schaper and Chacko, 1993, Vemmos, 1994). However, the relationship between Chl content and Pn does not always appear to be linearly related (Marini and Marini, 1983). The nitrogen concentration in leaves is also an important factor affecting Pn in various fruit trees (Paul and Foyer, 2001). It has been reported that leaf carbohydrate accumulation and utilization play an important role in Pn regulation in several plant species (Goldschmidt and Huber, 1992, Schechter et al., 1994, Vemmos, 1994, Paul and Foyer, 2001, Zhow and Quebedeaux, 2003, Urban et al., 2004). Moreover, Pn can also be influenced by the presence or absence of a sink, and especially by the presence of inflorescences or fruit (Proietti and Tombesi, 1990, Goldschmidt and Huber, 1992, Schechter et al., 1994, Vemmos, 1994, Urban et al., 2004).

Leaf anatomy and structure, especially that of the palisade layer and spongy parenchyma, are very important for leaf photosynthesis. It has been reported that leaf anatomical differences might affect Pn by changing CO₂ conductance from ambient air to carboxylation sites in chloroplasts (Evans et al., 1994, Syvertsen et al., 1995, Chartzoulakis et al., 1999). Therefore, a study of leaf anatomy of various fig cultivars is important in order to investigate its possible role in regulating Pn.

It is well known that growth and maturation of fig fruit follow a typical double sigmoidal curve, with two periods of rapid growth being separated by a period of slow growth (Tsantili, 1990). However, reports of changes of soluble sugar and starch concentrations during fruit growth and maturation are also limited in the literature. The main sugars found in such studies were fructose and glucose and, in much lower concentrations, sucrose, while sugar concentrations varied among cultivars (Tsantili, 1990, Yahata and Nogata, 1999). However, other researchers found only fructose and glucose in fruit of various fig cultivars (Hakerlerler et al., 1998). These results show that sugar concentrations and accumulation in fig fruit need further investigation.

Study of the photosynthetic capacity and changes in carbohydrate during leaf and fruit development in fig tree is essential for understanding fig leaf physiology and potential tree growth and productivity. In a preliminary study the photosynthetic capacity of two important European fig cultivars, namely ‘Kalamon’ and ‘Fracasana’ was investigated. Subsequently, a third cultivar was added, ‘Mission’. ‘Mission’ and ‘Kalamon’ produce one crop per year, while ‘Fracasana’ produces two.

The purposes of this study were to investigate: (a) the seasonal photosynthetic activity of these three cultivars, especially during leaf and fruit development; (b) the changes in carbohydrate concentrations in leaves and fruit during leaf and fruit development; (c) leaf morphology and anatomy, and the changes in SLW, Chl and nitrogen concentrations during leaf ontogeny; and (d) the possible relationship of the above factors with Pn rates in fig cultivars.