Midday Wilting Under Drip Irrigation in Solapur

Reading the numbers

Predawn leaf water potential is the best available measure of root-zone water status, because transpiration is almost zero overnight and the leaf equilibrates with the soil. A value of minus 0.35 MPa indicates a well-watered root zone. Soil moisture of 24% against a field capacity of 26% and a permanent wilting point of 12% means about (24 minus 12) divided by (26 minus 12), that is roughly 85% of available water remains. The soil is not dry.

What changes by midday is atmospheric demand. High vapour pressure deficit drives transpiration faster than roots can supply water, leaf water potential falls to minus 1.6 MPa, and stomata close to 60 mmol per square metre per second. Closed stomata mean little evaporative cooling, so leaf temperature rises 6 degrees Celsius above air temperature: that is a symptom of stomatal limitation, not of a dry soil. When vapour pressure deficit falls in the evening, transpiration slows, stem and leaf capacitance recharges and the plants recover without irrigation.

Drip irrigation wets only part of the root zone, so the effective root surface may be limiting even when the soil is moist, and root damage by nematodes or vascular pathogens would give the same pattern. Doubling the irrigation time will not raise predawn potential, which is already adequate; it will leach nitrate, waste metered water and risk waterlogging around the emitters. With 45,000 litres per day on 0.4 hectare and a metered cost of 9 rupees per 1000 litres, doubling is also a needless daily expense of about 400 rupees.

Salinity build-up

Flood irrigation in a semi-arid February, with high evaporation and no leaching fraction, leaves salts behind in the root zone. An electrical conductivity rise from 0.8 to 3.4 dS per metre is roughly 1,600–1,700 mg per litre of additional salts, enough to reduce water uptake and to interfere with ion selectivity at the root.

Which ion?

The deciding clue is leaf position, because it reflects phloem mobility:

• Magnesium is mobile. Under shortage the plant withdraws magnesium from older leaves and moves it to growing points, so interveinal chlorosis appears first on lower, older leaves. The pattern described fits magnesium.
• Iron and manganese are immobile. Their deficiency shows on young leaves first, so they are not consistent with the description, even though high pH and bicarbonate from flood irrigation do reduce their availability.
• Sodium competes with potassium and calcium, so potassium deficiency and marginal scorch may also appear, and high salinity can induce iron chlorosis on new growth later.

So the confident identification is magnesium deficiency, with salinity as the underlying cause, and this should be confirmed by leaf analysis rather than by eye alone.

Why flooding is not a model to copy

Waterlogging fills soil pores, roots become hypoxic, ATP for active ion uptake falls, denitrification loses nitrogen, and ethylene accumulates. The neighbour's field is not healthy; it merely avoids wilting because water is always present at the surface.

Decision

Do not double the irrigation. Keep the drip schedule but manage the crop canopy and verify the root zone.

Measurements to make

• Predawn leaf water potential twice a week, on the same leaves; keep it above about minus 0.5 MPa. A fall below minus 0.6 MPa despite irrigation points to root damage rather than dry soil, and the roots should be lifted and checked for galls and vascular browning.
• Soil moisture at two depths and at two distances from the emitter, because drip wets only 30–40% of the root zone; sampling only under the emitter gives a falsely comfortable reading.
• Midday stomatal conductance and leaf temperature, which show whether cooling has failed.

Treatments to compare

Use four replicated plots: (a) current practice, (b) 35% shade net or a kaolin particle film for the three hottest weeks, (c) current irrigation plus a straw mulch over the plastic to cut soil heat load, (d) an extra 30-minute irrigation cycle at 10:30 to test whether a small increase helps. Record fruit set, sunscald, fruit size and total marketable yield.

Expected outcome

If yield in treatment (b) matches or beats the others, the loss is from atmospheric demand, not from a water shortage, and shade or antitranspirant is the cheaper fix. If treatment (d) wins, the emitter wetting pattern is the real constraint. Either way the grower gets an answer from his own field instead of doubling a metered water bill.