EFFECTS OF FERTIGATION SCHEDULING ON WATER RELATIONS OF TOMATO PLANTSGROWN IN PEAT BAGS AND ROCKWOOL SLABS

Greenhouse tomato plants (Lycopersicon esculentum Mill. cv. Capello) w ere grown in peat bags and rockwool slabs to examine the effects of va rying the electrical conductivity (EC) of the nutrient solution and su bstrate matric water potential (Psi(sub)) on plant water relations. EC of the nutrient solution delivered to peat bags varied between 1 to 4 dS.m(-1) with Psi(sub) of either-5 kPa or-9 kPa as the setpoint for s tarting the irrigation. The EC variations were adjusted automatically by a computer system according to potential evapotranspiration (PET), which was calculated from radiation and vapor pressure deficit. Plants in rockwool slabs were treated with three EC levels, 2.5, 4.0 and 5.5 dS.m(-1). Plants in both peat bags and rockwool slabs were overwatere d once a week to leach accumulated salts out of the substrates. Before plants were overwatered, leaf water potential (Psi(L)) was lower in t he plants treated with high EC and low Psi(sub) but turgor potential ( P) was maintained close to the control value. This was attributed to t he osmotic adjustment caused mainly by the active solute accumulation. After plants under high EC or low Psi(sub) were overwatered, Psi(L) r ecovered to the control level, and P became higher because of the lowe r osmotic potential (pi) than in the control plants. At a given Psi(L) , the plants under variable EC, high EC, and low Psi(sub) maintained h igher leaf relative water content (zeta) These leaves had high turgid water content, high symplasmic (osmotically active) water content, low apoplasmic (osmotically inactive) watger content and low rat the poin t of zero turgor (incipient plasmolysis). EC variations in peat bags a ccording to PET caused osmotic adjustment and turgor regulation althou gh the extent was small. Maintenance of P after overwatering was direc tly proportional to stomatal conductance and photsynthetic rate. Our r esults suggested that tomato plants adjusted their internal water stat us and compartment of tissue water in symplasmic and apoplasmic fracti ons in response to high EC and low Psi(sub) and that varying EC togeth er with overwatering plants once a week not only reduced salinity stre ss but also favored turgor maintenance.