Tolerance to water stress in boron-deficient tea (Camellia sinensis) plants

The effects of boron (B) deficiency and water stress were studied in tea plants (Camellia sinensis [L.] O. Kuntze) grown in growth chambers in perlite irrigated with a nutrient solution. Dry matter production was reduced significantly by both low B supply and water stress. Shoot-root translocation of B declined in water-stressed plants. In addition, the re-translocation of B was impaired under drought, which was reflected in a significantly lower ratio of B content of young to old leaves in both B-deficient and B-sufficient plants. Leaf photochemical parameters were negatively influenced by B deficiency and water stress in the old but not in the young leaves. Although B-deficient plants were more conservative in relation to water loss following elevated stomatal limitation, their water potential was lower than in B-sufficient plants irrespective of the watering regime. Under the combined effects of B deficiency and water stress, the reduction in the CO₂ assimilation rate was more prominent than that under a single stress factor. The reduction of the net assimilation rate (A) in B-deficient plants due to water stress and in water-stressed plants due to low B supply were not accompanied by significant changes in the stomatal conductance, suggesting an involvement of non-stomatal factors. The activity of antioxidant enzymes and proline content increased under B deficiency and water stress conditions. Our results suggested that, in young leaves that have been developed under water stress, an acclimation to water stress conditions occurred that was well reflected in their more stable photochemistry, water relations and an efficient antioxidant defence system compared with the older leaves.