Background: Water deficit is one of the most significant abiotic factors affecting rice and agricultural production worldwide. In hybrid rice, cytoplasmic male sterility (CMS) is an important technique for creating high-yielding crop based on heterosis. The phytohormone kinetin (Kin) regulates cell division in plant during the early stages of grain formation, as well as flow assimilation and osmotic regulation under water stress. The present study performed to estimate the effects of irrigation intervals (irrigation each six days (I₆), nine days (I₉), twelve days (I₁₂) and fifteen days (I₁₅) against continuous flooding (CF, each three days)) and kinetin exogenously application (control, 15 mg L⁻¹ and 30 mg L⁻¹) on hybrid rice (L1, IR69625A; L2, G46A and R, Giza 178 R) seed production. Results: Leaves traits (Chlorophyll content (CHC), relative water content (RWC), stomatal conductance (SC), Leaf temperature (LT) and transpiration rate (TR)), floral traits such as style length (SL) and total stigma length (TSL), in addition to root traits (i.e., root length (RL), root volume (RV), root: shoot ratio (RSR), root thickness (RT), root xylem vessels number (RXVN) and root xylem vessel area (RXVA) were evaluated and a significant enhancement in most traits was observed. Applying 30 mg L⁻¹ kinetin significantly and positively enhanced all growth, floral and roots traits (RV and RXVA recorded the most increased values by 14.8% and 23.9%, respectively) under prolonging irrigation intervals, in comparison to non-treated plants. Conclusions: Subsequently, spraying kinetin exogenously on foliar could be an alternative method to reduce the harmful influences of water deficiency during seed production in hybrid rice.

In rice, cytoplasmic male sterility (CMS) represents an irreplaceable strategy for producing high-yielding hybrid rice based on the commercial exploitation of heterosis. Thereupon, enhancing floral traits and outcrossing rates in CMS lines increase hybrid seed production and ensure global food security. The exogenous application of cyanobacteria could enhance outcrossing rates in CMS lines and, accordingly, hybrid rice seed production. In the present study, we aimed at exploring the impact of cyanobacteria implementation such as Anabaena oryzae, Nostoc muscorum, and their mixture to promote the floral traits, outcrossing rates, and seed production in hybrid rice. The impact of cyanobacteria (Anabaena Oryza (T2), Nostoc muscorum (T3), and their combination (T4) versus the untreated control (T1) was investigated for two years on the growth, floral, and yield traits of five diverse CMS lines, namely IR69625A (L1), IR58025A (L2), IR70368A (L3), G46A (L4), and K17A(L5). The evaluated CMS lines exhibited significant differences in all measured floral traits (days to heading (DTH), total stigma length (TSL), stigma width (SW), duration of spikelet opening (DSO), spikelet opening angle (SOA)). Additionally, L4 displayed the uppermost total stigma length and stigma width, whereas L1 and L5 recorded the best duration of spikelet opening and spikelet opening angle. Notably, these mentioned CMS lines exhibited the highest plant growth and yield traits, particularly under T4 treatment. Strong positive relationships were distinguished between the duration of the spikelet opening, panicle exertion, panicle weight, seed set, grain yield, total stigma length, spikelet opening angle, stigma width, and number of fertile panicles per hill. Cyanobacteria is a potential promising tool to increase floral traits and seed production in hybrid rice.

Hybrid rice seed production predominantly relies on the pollen ability of restorer lines and the stigma properties of the cytoplasmic male sterile (CMS) lines. Improving the pollen ability and agronomic performance of restorer lines could be achieved using mutation to reinforce the outcrossing rate and seed set percentage of CMS lines. Two commercial restorer cultivars (Giza-178 and Giza-179) were treated with three different doses of gamma-ray; 200, 300, and 400 Gy. The derived mutant restorer lines were selfed and constantly evaluated with their corresponding parental cultivars until the M6 generation. Six promising mutant lines were chosen based on their performance to be evaluated with their parents for their pollen and agronomic characteristics. The novel-induced mutant restorer lines exhibited significant differences in all studied pollen and agronomic characteristics. The mutant restorer lines R4 and R7 exhibited superior desirable anther length (2.36 and 2.38 mm, in the same order), anther width (0.47 and 0.45 mm), pollen fertility (97.50 and 97.31%), basal pore length (218.8 and 299.5 mm), apical length (103.6 and 108.1), number of pollen grains per anther (1810 and 1832), plant height (121.8 and 115.9 cm), fertile tillers (23.50 and 18.89), number of spikelets per panicle (203.2 and 202.5), panicle length (25.34 and 24.26 cm), number of filled grains per panicle (195.5 and 191.8), seed set percentage (95.56 and 96.63%), and grain yield (11.01 and 11.36 t/ha). Based on the results of the first two seasons, the two novel promising restorers and their parental cultivars were crossed with four diverse CMS lines. The seed set percentage and hybrid seed production of the crossed CMS lines were evaluated for further two seasons. The results exposed that the combinations derived from restorer lines R4 and R7 with most of the evaluated CMS lines produced the uppermost seed set percentage outcrossing rate. Particularly in combination with CMS2, the restorer lines R4 and R7 displayed the highest seed set percentage (39.57 and 34.80%), number of tillers fertile per hill (25.31 and 23.32), seed yield (2.02 and 1.81 t/ha), and harvest index (23.88 and 22.66%). Conclusively, the derived gamma-ray-induced mutant restorer lines R4 and R7 could be exploited as a new source for ensuring desirable pollen and anther characteristics in order to improve the outcrossing rate of CMS lines and hybrid rice seed production.

Drought is one of the main abiotic stresses responsible for reducing crop yields worldwide. In hybrid rice production, cytoplasmic male sterility (CMS) displays an alternative plan for producing high-yielding hybrid rice depending on the hybrid vigor. Kinetin (Kin) has an essential role during the early phase of grain setting by regulating cell division, assimilate flow, and osmotic modification under water deficit. Growth, floral, and yield-related traits were evaluated in two CMS lines under five irrigation intervals and two concentrations of the phytohormone kinetin. Our study was conducted to explore the effects of irrigation intervals (continuous flooding (CF), irrigation every six (I₆), nine (I₉), twelve (I₁₂), and fifteen (I₁₅) days, and kinetin exogenously applied (control, 15 mg L⁻¹, and 30 mg L⁻¹) on growth, floral, and yield-related traits. Growth traits (i.e., number of leaves (NOL), days to complete leaf number (DCLN), days to heading (DTH, 50%), flag leaf area (FLA), and plant height (PH)), floral traits (i.e., duration of spikelet opening (DSO), panicle length (PL), spikelet opening angle (SOA), and panicle exertion (PE)), and seed yield traits (i.e., seed set (SS), panicle weight (PW), seed yield (SY), harvest index (HI), and number of fertile panicles per hill (NFP)) in two CMS lines. Implementation of kinetin displayed the highest significant and positive values for all growth, floral, and yield-related traits when compared to the control (average of applied irrigation intervals). Applying 30 mg L⁻¹ kinetin positively enhanced the growth traits (i.e., NOL, FLA, and PH with 4.1%, 5%, and 3%, respectively), floral traits (i.e., PL, PE, SOA, and DSO with 5.4, 5.7, 5.9, and 5.4%, respectively), and yield-related traits (i.e., PW, SS, SY, HI, and NFP with 22%, 17%, 14%, 14.5%, and 15%, with the same sequence) compared to non-treated plants. Consequently, exogenous foliar spray of kinetin could be an effective process in minimizing the harmful effects (the reduction in PW, SS, SY, HI, and NFP recorded 41%, 61%, 45%, 30%, and 48%, respectively, under I₁₅ conditions when compared to CF) of water deficit in hybrid rice and increasing seed production.

A field trial was performed in Randomized Complete Block Design (RCBD) with three replications at the Experimental Farm of Sakha Agricultural Research Station, Kafr ELSheikh, Egypt, during 2018 and 2019 rice growing seasons, to investigate the effect of three sowing dates (May 10th, June 1st and June 20th) and four levels of nitrogen fertilizer (0, 55, 110 and 165 kg N/ha) on the productivity of Sakha Super 300 variety. The results showed that the sowing date June 1st produced the highest grain yield 10.67 and 10.71 t ha-1 in both seasons. Nitrogen level of 165 kg N ha-1 produced the highest grain yield 10.50 and 10.45 t ha-1 without any significant difference with level of 110 kg N ha-1 in 2018 and 2019 seasons. The interaction between sowing dates and nitrogen levels was highly significant for number of panicles hill-1, panicle length, number of filled grains panicle-1, unfilled grains percentage and 1000-grain weight, as well as grain and straw yield and harvest index in both seasons. Interaction results showed that, the first sowing date (May 10th) with nitrogen level of 110 kg N ha-1 gave the highest grain yield 10.91 and 10.79 t ha-1. On the other hand the second sowing date (June 1st) with nitrogen level of 165 kg N ha-1 gave the highest grain yield 12.03 and 11.92 t ha-1 in 2018 and 2019 seasons. The highest values of accumulated growing degree days (GDD) and photo thermal units (PTU) were recorded when plants sown on May 10th. On the other hand the maximums heat use efficiency (HUE) was achieved when Sakha Super 300 sown on 1st June with application of 165 Kg N ha-1 in both seasons.

Flowering synchronization between the restorer (R) line (Giza 178) and cytoplasmic male sterile (CMS or A) line (IR58025A) in hybrid rice seed production plot is very needed to achieve greater seed yield. Two-year experiment was carried out at the Experimental Farm of Sakha Agricultural Research Station, Kafr El-Sheikh, Egypt in the 2020 and 2021 seasons. This study aims to examine the effect of male to female ratio (2R:10A, 2R:12A and 2R:14A), male direction (one way and two ways) and differences in days in flowering between male and female (five, two days and without differences) on productivity of hybrid rice seed. A split split-plot design in three replications was utilized. The main plots were the male to female ratio, while male direction was arranged in the sub plot sand the differences in days to flowering were located in the sub-sub plots. The obtained results showed that plant height, panicle exsertion percentage, panicle length and flag leaf angle were not significantly affected by the male to female ratio, male direction and differences in days to flowering. On the other hand, the number of panicles hill-1, panicle weight, seed set percentage, seed yield and harvest index (%) were highly significantly affected by the male to female ratio, male direction and differences in days to flowering. The highest values of seed yield (2.432 and 2.521 t ha-1) were obtained when using the male to female ratio 2R:14A, two-way male direction and without differences in flowering.