Effects of humidity on the growth and yield of glasshouse tomatoes

Four discrete humidity treatments (0.1,0.2,0.4 and 0.8 kPa vapour pressure deficit) were maintained continuously for 28 days from mid-January and their effect on the growth and yield of a long-season tomato crop was investigated. The average vapour pressure deficits achieved over the 28 day period were 0.15,0.25,0.43 and 0.65 kPa for the four treatments respectively. Temperature differences between the treatments were less than 0.2°C. The rate of plant development was unaffected by humidity. Significant reductions in leaf area at high humidities were associated with low calcium concentrations in the leaf laminae and calcium deficiency symptoms. There were no calcium deficiency symptoms in the fruit. The trusses associated with the smaller leaves resulting from high humidity, produced smaller fruit and thereby a lower yield compared with the low humidity treatments. Fruit yields produced before and after these trusses were picked, were unaffected by the humidity treatments. High humidity also reduced fruit quality. It was concluded that the cost of reducing humidity to vapour pressure deficits greater than 0.3 kPa was likely to exceed any economic gain, because the yield response to lower humidities was very small.