In our previous studies, artificial

In our previous studies, artificial stimulation treatments of short-term mild soil water deficits by restricted irrigations to tomato (Xu et al., 2009c), exposition of hypocotyls of peanut or cloves of garlic (Xu et al., 2009a,b) and low air humidity to tomato (Xuet al., 2007) wereusedto stimulateplants andto induce osmotic adjustment and the consequent improvements in photosynthetic activities, product quality and disease resistance. In the present study, blue light was used as one of the components of strong light, which was in turn as one of the component factors of drought, to induce active osmotic adjustment and the consequent improvement in tomato fruit quality and possibly in fruit yield and pest and disease resistance of tomato crops. As expected, results showed that the solute concentration in the symplasm was increased by blue light stimulation with a lower leaf osmotic potential, the same leaf water potential and a consequent higher leaf turgor potential, which in turn supported a higher leaf photosynthetic activities and the consequent higher fruit yield. The solute concentration in the symplasm was increased and it consequently induced water flow from the apoplasm into the symplasm. This was confirmed by the P–V curve analysis, which showed that the water fraction in the symplasm was larger or the apoplastic water fraction was smaller in leaves of blue light treated plants. Water in the symplasm is directly associated with biochemical metabolism and water in the apoplasm (cell walls) is not directly associated with biochemical metabolism (Auge and Stodola, 1990; Patakas and Noitsakis, 1997). As a consequence of xerophytophysiological response, a higher symplastic water fraction is favorable for biochemical metabolism and plant cells with higher symplastic water fraction possess higher physiological activity, especially under adverse conditions such as drought. According to the P–V curve analysis, both osmotic potential and relative water content at the point of incipient plasmolysis were lower in tomato leaves in blue light treatment, which suggested that these plants were healthier and more resistant to water stress. The incipient plasmolysis is the extreme crisis of the plant cells under drought or salinity conditions. Osmotic adjustment as abovementioned could allow the plant cells to avoid this crisis up to a severer water stress. It was also an indication ofthe crop health. Analysis of transpiration declining curve of the excised leaves also confirmed that less water was lost by transpiration in leaves of blue light treated tomato plants, while stomatal conductance was maintained to a severer water stress during leaf dehydration. Following the blue light, white LED light also showed effect ofinducing osmotic adjustment and the consequent improvement in plant health and fruit quality to an extent less than or similar to blue light. However, red light did not show the same effect of inducing osmotic adjustment and the consequent improvement in plant health and fruit quality although it also showed the same improvement in fruit yield. Some fruit were damaged by H. armigera worms and this damage was more severely in red light treatment, which might be due to the phototaxis of H. armigera adults to the red light.