Stomatal Functioning and Its Influence on Fruit Calcium Accumulation in Northern Highbush Blueberry

Accumulation of calcium (Ca) in fruit is largely caused by transpiration and varies depending on the concentration of Ca in the xylem fluid. The objective of the present study was to evaluate the relationship between fruit stomatal functioning and Ca accumulation during different stages of development in northern highbush blueberry (Vaccinium corymbosum L.). Stomata were scarce on the berries and were concentrated primarily on the distal end near the calyx. The density of the stomata was greatest at petal fall, averaging 5 to 108 stomata/mm2 from the proximal end (pedicel end) to the distal end of the berries. Stomata were wide-open at the early green stage of berry development and had a slight deposit of wax along the guard cells. As the berries expanded during the initial period of rapid growth (stage I), most of the stomata remained near the distal segment of the berries; by the late green stage, almost none was found in the middle and proximal segments. The majority of these stomata were completely covered with wax when the berries began to change color and ripen (stage II and stage III). Stomatal conductance (gS) of the berries averaged 45 mmol·m−2·s−1 at petal fall and rapidly declined as the fruit developed. By the fruit coloring stage, conductance was low and remained less than 15 mmol·m−2·s−1 throughout the ripening period. In four cultivars, including Duke, Bluecrop, Aurora, and Elliott, Ca uptake in the berries increased rapidly during the early green stage; however, it slowed considerably between the late green and fruit coloring stages and stopped completely during fruit ripening. The results of this study strongly suggested that practices used to increase the Ca content of blueberries, such as the application of foliar fertilizers, should be performed early in the season during the first few weeks after flowering.

How Does Water-Stressed Corn Respond to Potassium Nutrition? A Shoot-Root Scale Approach Study under Controlled Conditions

Potassium (K) is generally considered as being closely linked to plant water dynamics. Consequently, reinforcing K nutrition, which theoretically favors root growth and specific surface, extends leaf lifespan, and regulates stomatal functioning, is often used to tackle water stress. We designed a greenhouse pot-scale device to test these interactions on corn (Zea mays L.), and to analyze their links to plant transpiration. Three levels of K nutrition were combined with two water-supply treatments. Shoot and root development and growth were continuously measured during a 60-day-long experiment. Individual plant transpiration was measured by weighing pots and by calculating water mass balances. The results showed that, although K deficiency symptoms resembled those caused by water shortage, there was no advantage to over-fertilizing water-stressed plants. K failed to decrease either the transpiration per unit leaf surface or to improve water use efficiency. The link between K nutrition and plant transpiration appears solely attributable to the effect of K on leaf area. We conclude that K over-fertilization could ultimately jeopardize crops by enhancing early-stage water transpiration to the detriment of later developmental stages.

cGMP signalling in plants: from enigma to main stream

All living organisms communicate with their environment, and part of this dialogue is mediated by secondary messengers such as cyclic guanosine mono phosphate (cGMP). In plants, most of the specific components that allow production and breakdown of cGMP have now been identified apart from cGMP dependent phosphodiesterases, enzymes responsible for cGMP catabolism. Irrespectively, the role of cGMP in plant signal transductions is now firmly established with involvement of this nucleotide in development, stress response, ion homeostasis and hormone function. Within these areas, several consistent themes where cGMP may be particularly relevant are slowly emerging: these include regulation of cation fluxes, for example via cyclic nucleotide gated channels and in stomatal functioning. Many details of signalling pathways that incorporate cGMP remain to be unveiled. These include downstream targets other than a small number of ion channels, in particular cGMP dependent kinases. Improved genomics tools may help in this respect, especially since many proteins involved in cGMP signalling appear to have multiple and often overlapping functional domains which hampers identification on the basis of simple homology searches. Another open question regards the topographical distribution of cGMP signals are they cell limited? Does long distance cGMP signalling occur and if so, by what mechanisms? The advent of non-disruptive fluorescent reporters with high spatial and temporal resolution will provide a tool to accelerate progress in all these areas. Automation can facilitate large scale screens of mutants or the action of effectors that impact on cGMP signalling.

4-Aminobutyrate (GABA): a metabolite and signal with practical significance

We discuss the origin of 4-aminobutyrate (GABA) from glutamate and polyamines, and its subsequent catabolism to succinic semialdehyde and either succinate or 4-hydroxybutyrate. Promiscuous activities of GABA transaminase, glyoxylate/succinic semialdehyde reductases, and aldehyde dehydrogenase 10As appear to be important determinants of cross-talk among metabolic pathways during stress. Imposition of abiotic stress, as well as genetic or chemical disruption of glutamate decarboxylase, GABA transaminase, and tricarboxylic acid cycle reactions, results in non-cyclic carbon flux in the tricarboxylic acid cycle, demonstrating that stress-induced GABA metabolism is strongly linked with respiration. Metabolic generation of 4-hydroxybutyrate is probably linked to the stimulation of succinic semialdehyde reductase activity by an increasing NADPH/NADP+ ratio. We discuss the potential signaling role of GABA in various processes, including pollen tube guidance, interaction with fungal, bacterial, and invertebrate pests, and stomatal functioning, and argue that further research on short-term responses to stress is required to determine whether or not GABA functions by binding to or regulating the activity of GABA receptor molecules. Finally, we describe how emerging information about the metabolic and signaling roles of GABA is being used to improve plant defenses against biotic and abiotic stresses, and benefit human health.