Short term drying of half the root system reduces growth but not water status or photosynthesis in leaves of passionfruit ( Passiflora sp.)

Abstract

The impact of soil water deficits and non-hydraulic root signals on the expansion, functioning and water status of leaves and the opening of flowers of passionfruit was studied in a series of pot experiments. Hydraulic and non-hydraulic signals were separated by withholding water from half the root system. The leaves of well-watered plants expanded in a sigmoid pattern to about 65 cm 2 over 15 to 18 days. Drying the whole root system stopped leaf expansion after 6 days, but did not change the pattern of leaf growth. Growth resumed after rewatering but the final leaf size was halved. As leaf growth slowed, the net photosynthesis, P n , of mature leaves fell from 11 ± 1 μmol CO 2 m −2 s −1 to less than zero as the leaves wilted. Leaf water potential, Ψ 1, of mature leaves at midday ranged from −0.9 MPa in well-watered plants to −3.1 to −3.7 MPa in wilted plants. After rewatering, Ψ 1 returned to −0.9 MPa within 1 day, but P n took 3 days to return to control levels. A midday Ψ 1 of −1.5 MPa was associated with a 50% reduction in relative leaf expansion and net photosynthesis. Drying half of the root system reduced leaf expansion by 26%, compared with well-watered plants, and water use by 21 to 27%, without influencing Ψ 1 or P n . Plants with half the root system dry for 13 days flowered earlier than well-watered plants but had the same number of open flowers 1 month after treatments ended. Non-hydraulic root signals affect leaf expansion and flowering in passionfruit.