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 cm2 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, Pn, of mature leaves fell from 11 ± 1 μmol CO2 m−2 s−1 to less than zero as the leaves wilted. Leaf water potential, Ψ1, 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, Ψ1 returned to −0.9 MPa within 1 day, but Pn took 3 days to return to control levels. A midday Ψ1 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 Ψ1 or Pn. 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.