Plant growth regulators improve the yield of white lupin (Lupinus albus) by enhancing the plant morpho-physiological functions and photosynthesis under salt stress.

Background: White lupin (Lupinus albus L.) is a multi-purpose, climate resilient, pulse crop with exceptionally high protein content that makes it a suitable alternative of soybean in livestock feed. Although white lupin grows well on marginal sandy soils, previous studies have reported its sensitivity towards salinity stress. This experiment aims to assess the influence of salinity stress and mitigating role of plant growth regulators (PGRs) on performance of white lupin.

Methodology: The white lupin plants were sown in pots maintained at three salinity levels (1, 3 and 4.5 dS m^- 1) throughout the growing season and foliar sprayed with different PGRs, including ascorbic acid, potassium chloride, boric acid, ammonium molybdate and methionine at sowing, four weeks after emergence and at the initiation of flowering. Foliar spray of distilled water and salinity level of 1 dS m^- 1 were maintained as control treatments. Data were recorded for seed germination indices, plant growth, antioxidant enzymes and photosynthetic efficiency variables.

Results: The severe salinity stress (4.5 dS m^- 1) reduced the germination indices by 9-50%, plant growth traits by 26-54%, root nodulation by 12-26%, grain development by 44-53%, antioxidant enzymes activity by 13-153% and photosynthetic attributes by 1-8% compared to control (1 dS m^- 1). Different PGRs improved several morpho-physiological attributes in a varied manner. The application of potassium chloride improved seed vigour index by 53%, while ascorbic acid improved root nodulation by 12% and number of pods per cluster by 75% at the severe salinity level. The foliar application of PGRs also displayed a recovery of 140% in the activity of superoxide dismutase and 70% in catalase. The application of multi zinc displayed an improvement of 37% in plant relative chlorophyll, while ascorbic acid brought an increase of 25% in non-photochemical quenching and 21% in photochemical quenching coefficient at the severe salinity level. On contrary, the application of PGRs brought a relatively modest improvement (8-13%) in quantum yield of photosystem II at slight to moderate (3 dS m^- 1) salinity stress. The correlation analysis confirmed a partial contribution of leaf area and seed vigour index to overall photosynthetic efficiency of white lupin.

Conclusions: Clearly, salinity exerted a negative impact on white lupin through a decline in chlorophyll content, activity of antioxidant enzymes and efficiency of photosynthetic apparatus. However,  PGRs, especially ascorbic acid and potassium chloride considerably improved white lupin growth and development by mitigating the negative effects of salinity stress.