India depends more than any other nation on groundwater depletion for food security and must seek ways to increase crop water-use efficiency (WUE). One approach is to increase plant photosynthetic capacity and transpiration ration (TR). Our objective was to measure the effects of ethephon and N on photosynthesis and related physiological traits of Indian mustard (Brassica juncea L. Czern and Coss, cv. Alankar). Field experiments were conducted in Aligarh, India under irrigated and non-irrigated conditions. Four levels of N (0 to 80 kg ha-1) and three levels of ethrel (0 to 200 µl l-1) were used. Ethrel and N nearly always had additive effects on LAI and plant dry matter (PDM) in both irrigated and non-irrigated experiments. Similarly, most gas exchange traits responded linearly to ethrel and N addition. At 80 days after sowing (DAS), photosynthesis increased from 16.5 to 25.7 μmol m-2 s-1 due to N application, and from 19.5 to 24.7 μmol m-2 s-1 due to ethrel application. TR increased from 36 to 45 μmol mol-1 due to N, and from 36 μmol mol-1 to ~43.5 μmol mol-1 due to ethrel. The highest combination of ethrel and N gave TR values of 51 μmol mol-1. Carboxylation efficiency (CE) was nearly constant among treatments at ~0.095 μmol m-2 s-1 per μmol mol-1 increase of [CO2]. However all points above the 0.95 confidence level of the regression curve belonged to the 200 μl l-1 ethrel treatment, and all below to the 0 μl l-1 ethrel treatment, suggesting lower photorespiration and CO2 compensation point for ethrel treated plants. Results are consistent with reports that ethrel affects several cellular processes related to photosynthesis. Ethrel spray, either alone or in combination with N application, may provide a potential management tool for increasing water use efficiency (WUE) in India and other water-limited regions of the world.
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