Effects of fertigation scheduling on water relations of tomato plants grown in peat bags and rockwool slabs

Abstract

Greenhouse tomato plants (Lycopersicon esculentum Mill. cv. Capello) were grown in peat bags and rockwool slabs to examine the effects off varying the electrical conductivity (EC) of the nutrient solution and substrate matric water potential (Ψ(sub)) on plant water relations. EC of the nutrient solution delivered to peat bags varied between 1 to 4 dS·m-1 with Ψ(sub) of either - 5 kPa or - 9 kPa as the setpoint for starting 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 overwatered once a week to leach accumulated salts out of the substrates. Before plants were overwatered, leaf water potential (Ψ(L)) was lower in the plants treated with high EC and low Ψ(sub), but turgor potential (P) was maintained close to the control value. This was attributed to the osmotic adjustment caused mainly by the active solute accumulation. After plants under high EC or low Ψ(sub) were overwatered, Ψ(L) recovered to the control level, and P became higher because of the lower osmotic potential (Π) than in the control plants. At a given Ψ(L), the plants under variable EC, high EC, and low Ψ(sub) maintained higher leaf relative water content (ζ). These leaves had high turgid water content, high symplasmic (osmotically active) water content low apoplasmic (osmotically inactive) watger content and low ζ at the point of zero turgor (incipient plasmolysis). EC variations in peat bags according to PET caused osmotic adjustment and turgor regulation although the extent was small. Maintenance of P after overwatering was directly proportional to stomatal conductance and photsynthetic rate. Our results suggested that tomato plants adjusted their internal water status and compartment of tissue water in symplasmic and apoplasmic fractions in response to high EC and low Ψ(sub) and that varying EC together with overwatering plants once a week not only reduced salinity stress but also favored turgor maintenance.