ACCUMULATION OF CALCIUM, POTASSIUM AND MAGNESIUM IN APPLE FRUITS UNDER VARIOUS CONDITIONS OF HUMIDITY

In each of two experiments done under controlled conditions, starting at bloom, 4 humidity treatments were applied to potted trees of apple (Malus pumila Mill. cv. Cox's Orange Pippin), i.e. in experiment 1: (1 ) high humidity throughout, (2) low humidity throughout, (3) low humid ity for 7 weeks followed by high humidity for 6-7 weeks, and (4) the r everse (first high and then low humidity); in experiment 2:(1) day/nig ht humidity high/high, (2) low/low, (3) low/high, and (4) high/low. In both experiments high humidity favoured shoot growth appreciably. Cha nge from low to high humidity after 7 weeks resulted in some growth st imulation but in the reverse situation growth was markedly reduced. Sh oot growth responded little to different night humidities. In the two experiments fruit growth was little affected by treatments. In experim ent 2 irrespective of night humidity, water consumption was higher at low than at high day humidity. In the high/low humidity regime water u se during the night was high and leaf water potential low, relatively; during the day water potential was little affected by treatments. At any time leaf Ca and Mg were clearly highest at low day humidity; nigh t humidity had no effect. Leaf K did not respond to treatments. Fruit Ca at high humidity throughout was lower than at low humidity througho ut. Increasing humidity later in the season was ineffective but a decr ease at that time tended to reduce fruit Ca slightly. The clearly lowe st Ca values occurred in the high/low day/night treatment. Fruit K and Mg were not or hardly affected by treatments. The findings are discus sed in terms of humidity effects on transpiration, shoot and fruit gro wth, and xylem mineral concentration and ion exchange translocation al ong the xylem walls.