Recent studies have shown that, for greenhouse tomato crops, high levels of humidity and low levels of light depress transpiration and lead to yield losses. These adverse effects could be avoided by properly controlling the greenhouse climate. This work provides the basis for an effective control, by measuring the effect of climate (air speed, vapour pressure deficit, solar radiation and CO2) on tomato transpiration and by assessing five transpiration models. The main results are as follows: transpiration rate increases linearly with solar radiation, air vapour pressure deficit and air speed; air temperature, CO2 concentration and pipe temperature have no significant influence. For a young crop, an increase in solar radiation of 1 MJ m-2 day-1 resulted in an increase in transpiration of 0.09 mm day-1; an increase in vapour pressure deficit of 0.1 kPa (dehumidification of 4% relative humidity at 20-degrees-C) increased transpiration by only 0.013 mm day-1. A forced air movement of 1 m s-1 increased transpiration by 0.13 mm day-1. For a mature crop solar radiation had a slightly higher effect than for a young crop (an increase of 1 MJ m-2 increased transpiration by 0.14 mm day-1), but the vapour pressure deficit effect was much higher (a deficit of +0.1 kPa increased transpiration by 0.24 mm day-1) than for the young crop. Transpiration rates were comparable with most of the experimental results obtained by other researchers. However, large differences between the regression lines of some workers remain. This stresses the limitations of the experimental approach and the need for more general models, which once developed do not involve any adjustment of parameters. Five transpiration models have been checked against measurements. Models using constant values for the stomatal conductance had poor accuracy compared with the measurements (Chalabi -51%, Aikman +62%). A simplified Penman model gave good predictions on average (-2%), but with strong individual variations. Stanghellini's and Jolliet's models were the most accurate (+3% and -8% on average, respectively), and predicted both solar radiation and dehumidification effects well because they take into account the effect of vapour pressure deficit on stomatal conductance. In contrast, air temperature and CO2 influences on stomatal conductance are not significant and do not need to be included in a tomato transpiration model.
