Recently, the occurrences of extreme flooding and drought, often in the same areas, have increased due to climate change. Wetland plant species are known to oxygenate their rhizospheres by releasing oxygen (O2) from their roots. We tested the hypothesis that wetland species could help upland species under flood conditions; that is, O2 released from the wetland crop roots would ameliorate rhizosphere O2-deficient stress and hence facilitate upland crop root function. Flooding tolerance of upland-adapted staple crops-pearl millet (Pennisetum glaucum) and sorghum (Sorghum bicolor) mix-cropped with rice (Oryza spp.) was investigated in glasshouse and laboratory. We found a phenomenon that strengthens the flood tolerance of upland crops when two species-one wetland and one drought tolerant-were grown using the mixed cropping technique that results in close tangling of their root systems. This technique improved the photosynthetic and transpiration rates of upland crops subjected to flood stress (O2-deficient nutrient culture). Shoot relative growth rates during the flooding period (24 days) tended to be higher under mixed cropping compared with single cropping. Radial oxygen loss from the wetland crop roots might be contributed to the phenomenon observed. Mixed cropping of wet and dryland crops is a new concept that has the potential to overcome flood stress under variable environmental conditions.

Ultrafine bubbles (UFBs) exhibit a number of unique physical characteristics; however, reports on plant growth enhancement by UFB application are controversial. Here, we report experiments conducted to test whether UFB technology effectively promotes plant growth. We analyzed the effects of three experimental factors: water (UFB addition), hypoxia (aeration), and fluid movement (stirring) on the growth of soybean seedlings cultured hydroponically under low and high nutrition levels in a controlled environment room under artificial lighting for 8 days. When no nutrients were supplied, positive UFB water effects were evident, but low nutrition reduced UFB water-mediated growth enhancement and high nutrition totally obliterated any growth enhancement by UFB water. We concluded that UFB water-induced growth enhancement was effective and significant under nutrient deficit stress but no growth enhancement was observed, and even negative effects were evident, under favorable plant growth conditions. Graphical abstract

Water movement between cells in a plant body is the basic phenomenon of plant solute transport; however, it has not been well documented due to limitations in observational techniques. This paper reports a visualization technique to observe water movement among plant cells in different tissues using a time of flight-secondary ion mass spectrometry (Tof-SIMS) cryo-system. The specific purpose of this study is to examine the route of water supply from xylem to stem tissues. The maximum resolution of Tof-SIMS imaging was 1.8 μm (defined as the three pixel step length), which allowed detection of water movement at the cellular level. Deuterium-labelled water was found in xylem vessels in the stem 2.5 min after the uptake of labelled water by soybean plants. The water moved from the xylem to the phloem, cambium, and cortex tissues within 30–60 min after water absorption. Deuterium ion counts in the phloem complex were slightly higher than those in the cortex and cambium tissue seen in enlarged images of stem cell tissue during high transpiration. However, deuterium ion counts in the phloem were lower than those in the cambium at night with no evaporative demand. These results indicate that the stem tissues do not receive water directly from the xylem, but rather from the phloem, during high evaporative demand. In contrast, xylem water would be directly supplied to the growing sink during the night without evaporative demand.