Lettuce growth under unstressed conditions was compared to growth under
four limiting conditions, i.e. no phosphorus fertilization (0_P), no nitrogen
fertilization (0_N), low light (LR) and water stress (WR) over two different
growing periods. We investigated the adaptive changes in terms of the mor-
phological and physiological leaf traits, identifying stress-specific and ‘stable’
indicators suitable for use in breeding programmes. The plants subjected to the
WR treatments had lower leaf expansion and specific leaf area (SLA), as well
as lower soil–plant analysis development (SPAD) values, stomatal conductance
(POR), water index (WI) and leaf temperature (TIR) compared with plants in
the unstressed CONTROL. Low light increased the leaf area (LA), SLA and leaf
mass ratio (LMR). The 0_N treatment induced a general reduction in the nor-
malised difference vegetation index (NDVI) values, as well as strong changes in
LMR and SLA. In general, 0_P induced less pronounced effects than the other
treatments. Principal component analysis indicated that the stable and suitable
selection indicators of adaptive changes for low nitrogen and low light condi-
tions were LA, SLA, leaf area per unit total plant mass (LAR), LMR, SPAD and
POR, while SPAD, POR, TIR and WI were suitable indicators for drought.