Interactive effects of salinity and water stress on growth, leaf water relations, and gas exchange in amaranth (Amaranthus spp.)

Abstract

Amaranth (Amaranthus spp.) is a promising vegetable species often grown under semi‐arid conditions prone to both drought and salinity. This study was initiated to evaluate the effects of water and salinity stresses, both individually and in combination, on plant growth/leaf water relations and gas exchange of two amaranth genotypes— Amaranthus tricolor and A. cruentus. Plants were grown in a greenhouse in plastic pots filled with a sand/vermiculite mixture and exposed to 8 days of drought and/or salinity stress, a recovery period of 8 days, followed by a final 2 weeks of stress. The treatments consisted of: (1) unstressed control; (2) 100 mM Nad (salt stress); (3) PEG (polyethylene glycol Mw 6000) iso‐osmotic to 100 mMNaCl (water stress); and (4) 50 mM NaQ + PEG iso‐osmotic to 50 mM NaCl (salt + water stress). Plant growth (leaf, stem, root dry mass, root: shoot ratio, leaf area) were reduced by stress treatments. The reduction in shoot growth was greater in plants subjected to PEG‐induced water stress (41% in A. tricolor and 44% in A. cruentus) than in salinised plants (37% in A. tricolor and 27% in A. cruentus). Leaf water and osmotic potentials were reduced by stress treatments whereas turgor potential was largely maintained. Photosynthetic rate, stomatal conductance, and water loss were reduced by all stress treatments. Photosynthetic water‐use efficiency was increased by stress and was greater in salinised than in water‐stressed plants. Salinised plants and those subjected to salt + water stress had a greater degree of osmotic adjustment, so that plants were able to continue growth for a longer period before drying, compared to water‐stressed plants. Most parameters recovered when the stress treatments were discontinued. However, photosynthesis in salt‐stressed plants did not recover indicating a toxic effect of salt on the photosynthetic apparatus. The combined effect of salt + water stress is less detrimental to plant growth than the sum of individual stresses.

Key words:

• Amaranthus cruentus
• Amaranthus tricolor
• photosynthetic rate
• osmotic potential