Water dynamics and productivity in dehydrated watermelon plants as modified by red polyethylene mulch

In order to describe changes in water regime and production parameters caused by red polyethylene (PE) mulch in dehydrated watermelon (Citrullus lanatus (Thunb.) Matsum. Nakai) plants, as well as to define their transpiration efficiency (TE) and water use efficiency (WUE) under different temperature environments, 2 individual pot experiments (summer external experiment and autumnal greenhouse experiment) were conducted. We found that red mulch reduced leaf relative water content (RWC) fall caused by water restriction but osmotic adjustment (OA) was not responsible for this stress alleviation. A general OA was observed in the course of dehydration, possibly associated with developmental changes as amplified by exposition to high air temperatures. Mulched watermelon plants produced significantly more dry biomass (W) and leaf area (A) than plants without mulch protection. They also reached much higher transpiration rates (T) but lower evaporation rates (E). However, due to red PE mulch TE appeared below that of non-mulched plants. On the other hand, WUE corresponded with the results of recent works exclusively on black PE mulch, since it was enhanced by mulch. Cooler autumnal conditions resulted in higher RWC, W, and A, but in lower T, TE, and WUE differences between treatments. These results are discussed in context of effective water use (EUW), a new parameter arising from WUE failure in breeding for drought resistance.