WATER RELATIONS, CALCIUM AND POTASSIUM CONCENTRATION IN FRUITS AND LEAVES DURING ANNUAL GROWTH IN MATURE KIWIFRUIT PLANTS

Abstract

Actinidia deliciosa fruiting plants trained to “Pergola” system were grown in a semiarid environment located in South Italy (Andriace N 40° 20’, E 16° 48’). During the growing season measurements were carried out on leaf area, fruit growth, leaf and fruit transpiration, Ca and K content. In the first period of fruit development the kinetics of calcium uptake was faster than potassium. In particular, 60 days after fruit set, the calcium content reached about 70% of the amount measured at harvest, whereas potassium only reached 50%. This high value of fruit calcium uptake was correlated with high fruit transpiration observed 60 days after fruit set. After this period the fruit transpiration declined due to epidermis structural changes. Fruit calcium concentration and Ca/K ratio decreased during fruit growth, while they constantly increased in leaves. This behaviour confirms that calcium movement between leaf and fruit through the phloem is limited. INTRODUCTION Fruit mineral element composition is an important factor which has great on fruit quality. Low calcium concentration in kiwifruit, for example, has been found to be involved in premature fruit softening (Prasad and Spiers, 1992). Calcium transport from roots to shoots seems to happen through mass flux in xylem vessels, where ionic exchange processes can also take place (Van de Geijn and Petit, 1979). Transport may occur in non-vascular tissues and also via the phloem, but the importance of this last process has not been completely defined yet (Clark and Smith, 1991). For these reasons, calcium mobility within the plant can be limited and the quantity of the element transferred between leaf and fruit seems to be very small (Jones et al., 1983; Jones and Samuelson, 1983). This paper reports preliminary results concerning the relationships between mineral nutrient partitioning, eco-physiological parameters and anatomical structure of fruits on terminating shoots of kiwifruit. MATERIALS AND METHODS Trials were carried out in Andriace, (N 40° 20' E 16° 48') on mature ownrooted kiwifruit plants (cultivar Hayward), trained to “Pergola” system (625 plants ha) with row orientation E-SE (115° from North). Growth was evaluated on 30 fruits over 5 terminating shoots measuring, every 3 to 10 days, the length and the two diameters. At the same time measurements on 10 fruits from comparable positions on another five trees were taken in order to evaluate their fresh and dry weight and surface area. Fruit surface and leaf area of the terminating shoots were determined using a portable areameter, Model Li-3000, LI-COR. Gas exchange measurements were carried out on leaves and fruits of terminating Proc. IV IS on Mineral Nutrition in Fruit Eds. D. & G. Neilsen, Fallahi & Peryea Acta Hort. 564, ISHS 2001 130 shoots using a portable open system (ADC-LCA4) with a 200 μmol s flow. Measurements on leaves and fruits were carried out at the 2, 3, 4, 5, 7, 8 and 15 week after fruit set (3 June). Daily transpiration values were obtained by integrating five measures from 7AM to 6PM, about every three hours. Light available for fruits ranged from 60 to 160 μmol m s of PAR. Low Temperature Scanning Electron Microscopy (LTSEM) studies were carried out on fruit epidermis sampled at the 4 and at the 17 week (two weeks before harvest) after fruit set. Epidermis specimens were obtained from fresh fruit and immediately cryofixed in liquid propane using a Transfer Freeze Device (Bal-Tec TFD 010). After cryofixation, the frozen-hydrated (FH) epidermis was transferred, under liquid nitrogen vapours, to a dedicated SEM preparation chamber (SEM Cryo Unit Bal-Tec) and freezefractured by a motor-driven fracturing microtome at -120oC. FH samples were surface etched for 5 min at -80oC under high vacuum (P < 2 x 10 Pa) and sputter-coated with 20 nm of gold in an argon atmosphere. Structural and analytical investigations were performed on samples which were kept frozen-hydrated in the cold stage of a Philips SEM 515 at a temperature below -140 °C. Images were digitalised with a 768 x 576 pixel resolution at an acceleration voltage of 8 kV. The calcium and potassium concentration of the terminating shoot tissue was measured on acid digested samples (H2SO4+HNO3) using an AA-30 spectrophotometer (Varian). The leaves and fruits analysed for mineral concentration were sampled respectively at the 4, 6, 9 and 17 and the 4, 5, 6, 9, 11, 14, 17 and 23 week from the fruit set. RESULTS AND DISCUSSION According to Clark and Smith (1988) the kinetics of calcium and potassium uptake in fruit differ. In our experiments, 8 weeks after fruit set (cell division phase) the amount of calcium was 70% of the amount measured at harvest, while the corresponding value for potassium was only 50% (Fig. 1). The concentration of the two elements decreased during fruit development, while the K/Ca ratio increased during the same period (Fig. 2). Calcium concentration of leaves increased during the growing season, whereas K concentration and K/Ca ratio decreased. (Fig. 3). The final Ca and K concentrations were respectively 160 and 60% of the initial values. Fruits and leaves of terminating shoots showed different transpiration behaviours. Fruit transpiration values progressively decreased during the first 6 weeks, reaching negligible values after 8 weeks (1.26 mmol mday) (Fig. 4). Transversal freeze fractures of frozen-hydrated kiwifruit epidermis showed that the external layer of epidermal cells collapsed during fruit development (Fig. 5), producing a bearing-like protection for the fully hydrated parenchymatic cells positioned beneath them (Fig. 5B). These epidermis structural changes are associated with large reduction in the transpiration rate recorded during fruit growth. Leaves always showed high transpiration activity, reaching the highest values the 5 week after fruit set (132.2 mmol m day) (Fig. 4). The high calcium uptake in fruit during the first phase of growth can be correlated with the high fruit and leaves transpiring activity observed in the same period and it is also associated with the decrease of the fruit K/Ca ratio. CONCLUSIONS The relationship between fruit transpiration activity and the kinetics of calcium uptake suggest: a) an appropriate choice of canopy architecture and its correct management; b) the use of summer pruning during the first period of fruit growth, in order to increase light available to fruits; c) a correct management of mineral nutrition and soil, especially in the period immediately in the period following fruit set, in order to improve uptake of calcium from soil.