Temperature effects on fruit and shoot growth in the apple (Malus domestica) early in the season

Temperature is a main environmental factor affecting the net carbon exchange (NCE), carbon balance and carbon partitioning in the apple tree. Although shoot and fruit growth and gas exchange have been correlated to temperature in several studies, this research project began with the aim of documenting simultaneously, the effects of temperature on growth early and during the season. Groups of potted 'Royal Empire' apple trees were put in growth chambers every week for a 6-day period for 4 consecutive weeks starting 15 days after full bloom (DAFB) when the average fruit size was about 6 mm in diameter. Fruit growth and shoot growth were measured four times: -3, 0, 3 and 6 days after starting treatments. The day/night temperature regimes were 12/7, 19/14, 26/21 and 33/28degreesC. Relative humidity and light were set to 80 % +/-15 and 14-hour daylength of about 650 mumol PAR m(-2) s(-1) at the top of the trees, respectively. The results indicate that fruit growth is highly sensitive to temperature early in the season, but significantly less responsive later. The highest temperature regimes promoted the highest fruit growth during the first week of study, but later the treatment of cool temperatures (19/14degreesC) induced the highest fruit growth during the next 2 weeks, and, finally, in the last week, when the initial fruit size was about 27 mm (36 DAFB), fruit growth was relatively insensitive to temperature. Shoot growth was higher with the cool temperatures 19/14 and 26/21degreesC. The highest temperature (33/28degreesC) seems to be detrimental for fruit and shoot growth later than 3 weeks after full bloom. It is not clear whether the impact of high air temperature on potted trees was related to temperature only, to temperature-induced water stress and/or high soil temperature.