Mechanisms of enhancing photosynthetic gas exchange in date palm seedlings (Phoenix dactylifera L.) under salinity stress by a 5-aminolevulinic acid-based fertilizer

Photosynthetic gas exchange characteristics, salt uptake, pigment contents, and electrolyte leakage were examined in date palm seedlings (Phoenix dactylifera L.) subject to seawater treatments at 1-, 15-, and 30-mS cm(-1) salinity levels in the presence or absence of 0.08% ALA-based (5-aminolevulinic acid-based) functional fertilizer commercially known as Pentakeep-v. Date palm seedlings accumulated significant amounts of Na+ in the foliage with increasing salinity, about a threefold increase in the accumulated Na+ between the control and 30-mS cm(-1) salinity treatment. Electrolyte leakage indicated a significant reduction in membrane integrity as salinity increased. A strong linear correlation was observed between the chlorophyll (chl) a/b ratio and assimilation rate throughout salinity treatments. The slope (b) and the correlation coefficient between the chl a/b ratio and assimilation suggested that salinity reduced assimilation predominantly via the reduction in chlorophyll a contents (r(2) = 0.885 and b = 1.77, P < 0.05). Plants treated with Pentakeep-v showed a similar response with increasing salinity but at higher levels of both chl a/b ratios and assimilation rates. Mechanistic analysis of A:Ci response curves showed that photosynthetic gas exchange in seedlings of the date palm was significantly reduced with increasing salinity due to gas phase limitation (S (L)) as evident by stomatal conductance (g (s)) values. Salinity did not induce any change in the carboxylation efficiency of the rubisco enzyme (Jmax), or in the rate of electrons supplied by the electron transport system for ribulose 1,5-bisphosphate (RuBP) regeneration (Jmax). Accelerated carbon loss through respiration has significantly contributed to the described reduction in assimilation and increased CO2 compensation point (Gamma). Only at the 30-mS cm(-1) salinity level did treatment with Pentakeep-v reduce Na+ accumulation in the leaves, and caused a reduction in K+ selective uptake, leading to a concomitant reduction in K+/Na+ ratios. Pentakeep-v significantly improved chl a contents in all treatments, which was subsequently reflected in total chlorophyll and chl a/b ratios. The non-gas-phase components of the photosynthetic process (biochemical factors limiting gas exchange) were significantly improved by Pentakeep-v applications. Specifically, Pentakeep-v enhanced the biochemical efficiency of carbon fixation (Jmax) and the rate of electron transport required for RuBP regeneration (Jmax) by 37.4% and 17.8%, respectively, over untreated plants at a salinity level of 15 mS cm(-1). In addition, Pentakeep-v reduced S-L to values similar to those of control plants (9.07%) and lowered CO2 compensation points by reducing respiratory CO2 loss, with increasing salinity to 30 mS cm(-1). We, therefore, conclude that the ALA-based fertilizer Pentakeep-v improves salt tolerance in date palm seedlings by increasing photosynthetic assimilation. The latter is mediated via boosting light-harvesting capabilities of the treated plants by increasing chl a content and by reducing stomatal limitation to photosynthetic gas exchange.