Prediction of optimal harvest date for processing tomato based on the accumulation of daily heat units over the fruit ripening period

Abstract

SummaryFor maximum yield of processing tomato by once-over mechanical harvesting, an optimum date for harvest has to be selected at the time when the factory-graded fruit yield is at its maximum. By recording the proportion of green, ripe (i.e. turning and red) and rotten (i.e. over-ripe and damaged) fruit over a period of about 20 d, the optimal harvest date can be identified. Based on observations taken from crops at four sites in the Mediterranean region over three years (1997-1999), the proportional change of fruit types (i.e. percentage total fruit weight) has been found closely related to the accumulation of daily maximum temperature (x, °Cd) during crop maturation. The change of percentage green fruit was y = 6172 l0–8 x2 – 768 l0–4 x + 25 (R2 = 0.882). Over the same period, the change in percentage ripe fruit was described less satisfactorily by y = – 12588 l0–8 x2 + 867 l0–4 x + 65.7 (R2 = 0.568). When heat units were calculated based on the daily maximum and minimum temperatures by six well-known methods, it was found that the reduction in percentage green fruit was significantly related to the sum of heat units calculated by all methods. The maximum percentage ripe fruit, about 80% of the total fruit weight, occurred soon after the percentage green fruit was equal to the percentage rotten fruit, both around 10% of total fruit weight.Therefore, the heat units required to reduce the green fruit from 25% to 10% of total fruit weight, calculated by different methods, can be used to predict accurately the optimal harvest date based on the accumulation of ambient temperature. In a range of likely temperature regimes, 7–9 days prediction for optimal harvest date can be made. However, the simplest method based on daily maximum temperature alone is as accurate as all the more complex methods and is likely to find wider application in the processing tomato industry.