Mechanisms of Enhancing Photosynthetic Gas Exchange in Date Palm Seedlings (Phoenix dactylifera L.) under Salinity Stress by a 5-Aminolevulinic Acid-based Fertilizer

Abstract

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⁻¹salinity 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 (Vc,max), 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 (Γ). 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 (Vc,max) 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.