A relook into plant wilting: observational evidence based on unsaturated soil–plant-photosynthesis interaction

Permanent wilting point (PWP) is generally used to ascertain plant resistance against abiotic drought stress and designated as the soil water content (θ) corresponding to soil suction (ψ) at 1500 kPa obtained from the soil water retention curve. Determination of PWP based on only pre-assumed ψ may not represent true wilting condition for soils with contrasting water retention abilities. In addition to ψ, there is a need to explore significance of additional plant parameters (i.e., stomatal conductance and photosynthetic status) in determining PWP. This study introduces a new framework for determining PWP by integrating plant leaf response and ψ during drought. Axonopus compressus were grown in two distinct textured soils (clayey loam and silty sand), after which drought was initiated till wilting. Thereafter, ψ and θ within the root zone were measured along with corresponding leaf stomatal conductance and photosynthetic status. It was found that coarse textured silty sand causes wilting at much lower ψ (≈ 300 kPa) than clayey loam (≈ 1600 kPa). Plant response to drought was dependent on the relative porosity and mineralogy of the soil, which governs the ease at which roots can grow, assimilate soil O2, and uptake water. For clay loam, the held water within the soil matrix does not facilitate easy root water uptake by relatively coarse root morphology. Contrastingly, fine root hair formation in silty sand facilitated higher plant water uptake and doubled the plant survival time.

Biochemical status of in vitro regenerated Lilium bosniacum and Lilium cattaniae plantlets

Lilium cattaniae (Vis.) Vis. and Lilium bosniacum (G. Beck) Beck ex Fritsch, endemic species of Balkan Dinaric Alps, were micropropagated from seeds collected from their natural habitats. The relationship between peroxidase activity, photosynthetic status and differentiation of Lilium cattaniae and L. bosniacum in vitro was investigated. Peroxidase activity recorded for somatic embryos of Lilium cattaniae obtained on Murashige and Skoog (MS) medium containing 9.05 mM 2.4-dichlorophenoxyacetic acid (2.4-D) and 4.44 mM N6-bezyladenin (BA), was about two times higher than for any other treatment. Photosynthetic status of plantlets obtained through regeneration was explant-specific and generally higher for plantlets regenerated from basal leaf explants than from bulb explants. The accumulation of anthocyanin was detected in some regenerated shoots and more often in plantlets obtained through regeneration from bulb explants. High frequency of somatic embryo formation was recorded for L. cattaniae on MS medium containing 9.05 mM 2.4-D and 4.44 mM BA. The peroxidase activity for L. bosniacum plantlets obtained through regeneration showed treatment-specific elevations. We consider that basal leaf parts are suitable for successful regeneration of these two lilies and that elevation in peroxidase activity is a good indicator of somatic embryogenesis in both lilies.

Effects of Simulated Dark Shipping on Photosynthetic Status and Post-shipping Performance in Phalaenopsis Sogo Yukidian ‘V3’

Phalaenopsis plants are routinely shipped long distances in total darkness. To determine how these long dark periods affect photosynthetic status in Phalaenopsis Sogo Yukidian ‘V3’, changes of net CO2 uptake, photosystem II (PS II) efficiency, and abscisic acid (ABA) concentration after a long-term simulated dark shipping were investigated. Net CO2 uptake rate, malate concentration, and titratable acidity in potted Phalaenopsis Sogo Yukidian ‘V3’ decreased after a 21-day simulated dark shipping at 20 °C, but recovered gradually with time after shipping. It took 6 to 9 days to recover to a normal photosynthetic status after shipping. The value of Fv/Fm was little affected by shipping. Therefore, net CO2 uptake rate would be a better indicator for estimating the recovery time after shipping. After shipping, fresh weight loss, leaf ABA concentration, and number of yellowed leaves of bare-root plants were higher than those of potted plants, and increased with longer durations (7, 14, and 21 days) of the simulated dark period. The spiking (the emergence of flowering stems) date was delayed when plants were stored in a bare-root condition. The concentration of ABA in leaves rose in the first 3 days after simulated shipping and then decreased within the next 3 to 8 days. Plants that received photosynthetic photon flux (PPF) at 399 μmol·m−2·s−1 after shipping had lower PS II efficiency and reduced net CO2 uptake rate than those given less PPF levels. We recommend a post-shipping acclimation for 6 to 9 days with gradual light increase (34–72–140–200 μmol·m−2·s−1 PPF) or maintaining a light level of 140 μmol·m−2·s−1 PPF for Phalaenopsis to achieve a better photosynthetic status after prolonged dark storage.

Fluorescence and the Light/Nutrient Interaction in Monochrysis

The observed diminution in photosynthetic efficiency upon vitamin B12 limitation in Monochrysis indicates that nutrient limitation may profoundly influence the photosynthetic status of a natural population and must therefore be taken into account in productivity studies (Droop et al. 1982). It also suggests that photosynthetic efficiency could be used as a criterion between nutrient and light control of growth in natural populations. However, measurement of photosynthetic efficiency is cumbersome under field conditions and it would be advantageous if some other associated variable more quickly measured, such as, e.g. in vivo chlorophyll a fluorescence, could be used as a reliable indicator.