Leaf greenness as a drought tolerance related trait in potato (Solanum tuberosum L.)

Maintenance of leaf greenness under drought stress is generally considered a positive trait as it indicates reduced chlorophyll degradation. This is particularly relevant at the end of the growth cycle, when delayed senescence a.k.a. stay-green may reflect maintenance of photosynthetic activity during the re-mobilization of carbon products to the harvested plant organs. In the present study, we analyzed the temporal trends of chlorophyll concentration or greenness measured using a SPAD meter (ChlSPAD), maximum net photosynthesis (Amax), stomatal conductance (gs), intrinsic water-use efficiency (Amax/E), nitrogen content per area (Narea), leaf length, plant cover and tuber and aboveground dry biomass in three potato varieties (Désirée, Unica and Sarnav). We compared plants grown at field capacity and under water restriction of 50% of field capacity. Watering treatments were imposed at tuber initiation onset. One week after water restriction initiation, a temporary rise of ChlSPAD was observed in the three genotypes. This increased leaf greenness under water restriction coincided with a decrease in leaf growth. Sarnav, which had the lowest drought susceptibility index, showed the lowest increase of ChlSPAD after water restriction as well as the higher average gs biomass accumulation, and lower Amax/E from tuber initiation onset through the initiation of senescence. We suggest that the highest greenness increase following water restriction in the most drought susceptible varieties is an expression of more conservative strategies under drought, which under moderate water stress (the most common water stresses in agriculture), are not useful from a productive point of view. Under this moderate stress, long growing cycle genotypes showed higher tuber yield driven by the longer period of biomass accumulation. Thus, lower chlorophyll loss rate and later onset of senescence was related to higher yield under water restriction. This study provides more insights on the mechanisms linking ChlSPAD temporal responses to moderate drought tolerance in potato.