Analysis of humidity effects on growth and production of glasshouse fruit vegetables

Air humidity is a climate factor that can modify final yield and quality of crops through its impact on processes with a short as well as with a long response time. This thesis primarily deals with the long term responses of growth and production of glasshouse cucumber, tomato, sweet pepper and eggplant to humidity in the range of 0.3 to 0.9 kPa Vapour Pressure Deficit. Knowledge of these responses is essential to optimize environmental control for glasshouse crop production.

The influence of humidity on leaf photosynthesis was estimated from its effect on stomatal conductance. Within the range investigated, humidity had limited effects on stomatal density (morphological component) and this did not significantly influence leaf conductance. The relative response of leaf conductance to vapour pressure deficit (dynamic component) was equal for the four species. From simulation it was concluded that the effect of humidity on leaf photosynthesis under normal growing conditions in moderate climates is limited to about 10% which was of the order of actual observations with young tomato plants.

Long term exposure to high humidity significantly increased the leaf area of cucumber through a higher rate of leaf formation whilst with tomato leaf area was reduced due to severe calcium deficiency.

Humidity had no significant effect on dry matter distribution between leaves, stem and fruits but a marginal gain in shoot/root dry weight ratio was observed at high humidity. Dry matter content of leaves and fruits was unaffected by humidity.

Flowering was unaffected by humidity and only limited effects on fruit set were observed. Seed set of tomato was lower at high humidity and closely related to the effects of humidity on pollen dehiscence and adhesion to the stigma. Fruit maturation rate was not influenced by humidity.

Final yield of cucumber was higher at high humidity by day whilst yield of tomato was lower at continuously high humidity. Yield of sweet pepper was unaffected, yield of eggplant was slightly lower at high humidity. Keeping quality was generally lower at high humidity. For each crop practical guidelines for humidity control in glasshouses are presented.

It is concluded that the major effect of high humidity on yield is mediated through its impact on light interception resulting from either the enlargement (through number of leaves and leaf expansion) or the decrease of the LAI (through calcium deficiency) and the (marginal) effect on photosynthesis as such. The results are discussed in the view of current humidity control and the development of enviromnental control strategies.