In this study, artificial growth stimulants (exogenous GA3 and the GA inhibitor - TR) were employed to highlight the effects of growth type habits on the reaction of plants to drought stress. The classification of the studied plants into two subgroups in terms of phenology allowed us to investigate the response of different plants to abiotic stress. In the present study, morphological and phenological observations were confirmed by genotyping data because plant division based on the genetic profiles overlapped with the type of growth habits. For the vast majority of the studied yield-related traits, significant effects were recorded for treatment, group, and T×G. According to previous studies (Wu et al. 2007; Kottmann et al. 2016), plants with different types of growth habits react in different ways to most of the abiotic stresses, which was confirmed in this investigation. For the early- and late-heading plants, the morphology and ability to adapt to conditions varied significantly and were the important reasons contributing to direct stress responses (Shavrukov et al. 2017). In the present study, under drought conditions, the plants developed more tillers compared with control conditions. Interestingly, the early-heading plants were characterized by higher mean values of productive tillers compared with the late-heading plants, which suggests that the development of tillers with fertile spikes was the main goal of the plant’s strategy to survive (to distribute the progeny) under unfavorable conditions. This finding is in line with a previous studies (Mosaad et al. 1995; Xie et al. 2016; Moeller and Rebetzke 2017). The acceleration of plant development (using external GA application) disturbed this strategy: no rapid tiller development was observed under drought conditions for the early-heading plants, which may be associated with the changes in plant growth. Under D conditions, the early-heading genotypes showed rapid development and shaped the tillers only in the secondary tillering process in the rewatering phase. These findings are in line with the nature of tiller development, which is considered a plastic process, being strongly dependent on environmental factors that may promote, or repress, lateral shoot development through a complex network of hormonal and regulatory signals (Kebrom et al. 2012).
ANOVA showed that for a trait linked to spike fertility (FSm), significant effects were recorded for treatments and G×T interaction, which suggests that an appropriate seed development process may be associated with the right growth strategy under stressful conditions. This finding is in agreement with that of a previous study (Begum et al. 2022). The results of the PCA of the yield-related traits of the studied plants revealed that the genotypes clustered in close proximity to each other in terms of the applied treatment, but the locations of the studied plants were disrupted when drought conditions alone and drought conditions combined with foliar growth stimulations were applied. This finding shows that the studied plants exhibited different drought response strategies and, as a consequence, showed different yield performers. In this study, an increase in some yield-related traits may be associated with initiations of sophisticated tools by plants to adapt to unfavorable water conditions. Although the knowledge of plant defense against abiotic stresses like drought has been acquired thanks to many studies conducted recently (e.g. Rehaman et al. 2021; Zhang et al. 2022), the interplay between different signals to generate defense responses still remain elusive (Zhu et al. 2016), mainly due to their complex nature.
In many studies, chlorophyll fluorescence has long been used as a convenient and sensitive indicator of plant stress responses (e.g. Goltev et al. 2005; Kalaji et al. 2016). Fluorescence increase or induction curves, usually called the OJIP test, have been also adapted for screening different varieties of crops subjected to drought stress (Yao et al. 2018), including barley (Daszkowska-Golec et al. 2019). Rosales-Serna et al. (2000) suggested that drought stress is a complicated stressor and that different aspects of plant growth and physiology should be taken into account for the evaluation of plants’ response to drought stress. Therefore, in the present study, the exploration of plants’ response to drought was complemented with both yield performance and physiological analyses. Under stress conditions, significant increases in the parameters linked to RC damage (e.g., ABS_RC) and the parameters associated with heat dissipation (DIo_RC and Φ_Do) were recorded. In the present study, significant differences were also observed for ABS_RC at LFE1 between two subgroups of plants in all types of treatments. It is worth noting that much lower ABS_RC values were recorded for the early heading plants during the first measurement, but over time (LFE3), the mean values observed for this trait were similar for both plant subgroups, which emphasizes the role of stress duration in plants’ response to unfavorable conditions. According to Jedmowski and Brüggemann (2015), inactivation of some RCs, as already mentioned, increases the ABS/RC under drought stress conditions. Another reason for the increase in ABS/RC is degradation of chlorophyll through early leaf senescence induced by drought stress (Boureima et al. 2012) or regrouping of antennae from inactive PSII RCs to active (Kalaji et al. 2016). Changes in ABS_TR recorded in the present study may suggest that the early-heading genotypes react differently to the initial phase of drought stress, but after a while, damage to RCs occurs in this type of plant. The increases in the parameters linked to heat dissipation (DIo_RC and Φ_Do) were observed in the studied plants, especially for the early-heading plants in the second time of measurement, which, on the one hand, contributed to RC damage and, on the other hand, an increase in heat dissipation was recognized as an effective way for a plant to protect its thylakoid membranes from oxidative damage (Demmig-Adams et al. 2006). In the D + TR treatment, where plant development was inhibited, increases in Φ_Do and Dlo_RC recorded for the early-heading plants were much lower than those noticed for the rest of the stress conditions, which emphasizes the role of earliness in the effective distribution of heat in mitigating the devastating influence of drought on chlorophyll parameters. The decrease in quantum efficiency (Ψ_o, PI_Abs) and the increase in heat dissipation (indicated by DIo_RC and Φ_Do) were observed in the present study. These findings are in line with the studies of Zhu et al. (2021) and Sousaraei et al. (2021), who found a rapid increase in parameters DIo_RC, Φ_Do, and TRo_RC and a decrease in PI_Abs, Fv_Fm, and Eto_RC mean values.
Developmental defects in the tapetum and a lack of starch accumulation are caused by water-deficit stress in pollen grains (Nguyen et al. 2009; Ji et al. 2010), which was confirmed in our investigation as viability monitoring by method 1 decreases significantly under stressed conditions. Stress-tolerant wheat cultivars can maintain starch accumulation and sink strength during the young microspore stage under water stress conditions (Ji et al. 2010), which was not confirmed in our study, as there were no differences in pollen viability evaluated by method 1 (JKJ method) between the early- and late-heading plants under D conditions. The artificial acceleration of the growth of the early-heading plants contributed to the impairment of pollen viability, exacerbating the negative impact of drought on pollen development.
In many plant tissues (barley aleurone, wheat internodes, and anthers), GAMYB expression has been shown to be directly upregulated by the gibberellin GA3 (Gubler et al. 1995). In the present study, HvGAMYB expression was confirmed in the anther tissues of plants subjected to different water conditions. It is interesting to note that depending on the applied growth stimulators, HvGAMYB expression was different for the early- and late-heading plants. Transgenic barley lines with an excess of fourfold levels of endogenous GAMYB protein in their anthers were reported to be male sterile (Murray et al. 2003; Duca et al. 2008). Also, a progressive decrease in anther size was associated with the increase in GAMYB levels, particularly a decrease in anther length (Murray et al. 2003). These findings are in line with results obtained in the present study, where for the early-heading plants with a higher HvGAMYB expression, lower mean values of anther width were recorded. In the present study, HvGAMYB is expressed at a relatively high level under D conditions, which can be linked to the growth process, especially in the early-heading genotypes. Contrary to the expectation, exogenous Ga application did not positively influence HvGAMYB expression, which highlights the complex nature of plant development under stress conditions. On the other hand, the early-heading plants still show a higher level of HvGAMYB expression under D + GA conditions, which confirms the association of the studied gene with early flowering. The use of the GA inhibitor TR under D + TR conditions contributed to the decrease in the HvGAMYB expression level in the early-heading plants, but did not change the HvGAMYB level in the anther tissues of the late-heading genotypes. This phenomenon confirmed the role of Ga in HvGAMYB level regulation under drought conditions and stressed the role of transcription factors like HvGAMYB in the flowering process and anther development under unfavorable environmental conditions. The results of this study show that the HvGAMYB expression level evaluated at time point 2 is correlated positively with traits associated with lateral spike morphology (NGSl and WGSl), which indicate that this gene has an important role in yield performance of plants grown under unfavorable environmental conditions. This finding is in opposite with previous studies (Matsui et al. 2000; Murray et al. 2003), where plants with strong HvGAMYB over-expression turned out to be male sterile. This contrasting revelations highlight the complex nature of pollen development where GA signal transduction pathway may be modify by wide range of internal and external factors.