Nighttime gibberellin biosynthesis is influenced by fluctuating environmental conditions and contributes to growth adjustments of Arabidopsis leaves

Optimal plant growth performance requires that the action of growth signals, such as gibberellins (GA), are coordinated with the availability of photo-assimilates. Here, we studied the links between gibberellin biosynthesis and carbon availability, and the subsequent effects on growth. The results presented here show that carbon availability, light and dark cues, and the clock ensure the timing and magnitude of gibberellin biosynthesis and that disruption of these mechanisms results in reduced gibberellin levels and expression of downstream genes. Carbon dependent nighttime induction of GIBBERELLIN 3-BETA-DIOXYGENASE 1 (GA3ox1) was severely hampered when preceded by a day of lowered light availability, leading specifically to reduced bioactive GA4 levels, and coinciding with a decline in leaf expansion rate during the night. We attribute this decline in leaf expansion mostly to reduced photo-assimilates. However, plants where gibberellin limitation was alleviated had significantly improved expansion demonstrating the relevance of gibberellins in growth control under varying carbon availability. Carbon dependent expression of upstream gibberellin biosynthesis genes (KAURENE SYNTHASE, KS and GIBBERELLIN 20 OXIDASE 1, GA20ox1) was not translated into metabolite changes within this short timeframe. We propose a model where the extent of nighttime biosynthesis of bioactive GA4 by GA3ox1 is determined by starch, as the nighttime carbon source, and so provides day-to-day adjustment of gibberellin responses.

Component traits of plant water use are modulated by vapour pressure deficit in pearl millet (Pennisetum glaucum (L.) R.Br.)

Traits influencing plant water use eventually define the fitness of genotypes for specific rainfall environments. We assessed the response of several water use traits to vapour pressure deficit (VPD) in pearl millet (Pennisetum glaucum (L.) R.Br.) genotypes known to differ in drought adaptation mechanisms: PRLT 2/89–33 (terminal drought-adapted parent), H 77/833–2 (terminal drought-sensitive parent) and four near-isogenic lines introgressed with a terminal drought tolerance quantitative trait locus (QTL) from PRLT 2/89–33 (ICMR01029, ICMR01031, ICMR02042, and ICMR02044). Plant water use traits at various levels of plant organisation were evaluated in seven experiments in plants exposed either transiently or over the long term to different VPD regimes: biomass components, transpiration (water usage per time unit) and transpiration rate (TR) upon transient VPD increase (g H2O cm–2 h–1)), transpiration efficiency (g dry biomass per kg H2O transpired), leaf expansion rate (cm per thermal time unit) and root anatomy (endodermis dimensions)). High VPD decreased biomass accumulation by reducing tillering, the leaf expansion rate and the duration of leaf expansion; decreased root endodermis cell size; and increased TR and the rate of TR increase upon gradual short-term VPD increases. Such changes may allow plants to increase their water transport capacity in a high VPD environment and are genotype-specific. Some variation in water use components was associated with terminal drought adaptation QTL. Knowledge of water use traits’ plasticity in growth environments that varied in evaporative demand, and on their genetic determinacy, is necessary to develop trait-based breeding approaches to complex constraints.

Growth Responses to Wind Differ among Papaya Roots, Leaves, and Stems

‘Sunrise’ and ‘Tainung 2’ papaya seedlings were subjected to 3 weeks of ambient winds in Guam during five experiments, and growth responses of roots, leaves, and stems were quantified to compare speed and extent of the plasticity among the organs. The cultivars responded similarly with 1 week eliciting stem growth responses and 2 weeks eliciting root responses. The timeframe of these studies was sufficient to enable adaptive responses in all three organs. Wind reduced stem and leaf expansion rate but not root extension rate, providing one example of how the form of response differed among the organs. A dose–effect was evident among the experiments with magnitude of response increasing with mean ambient wind speed. Asymmetric stem diameter and root tip density were examples of adaptive responses to directional wind load. These data on young papaya plants may be used to inform field experiments aiming to determine how chronic winds influence long-term growth and fitness.

Exploring the Limits of Crop Productivity: Beyond the Limits of Tipburn in Lettuce

The productivity of lettuce in a combination of high light, high temperature, and elevated CO2 has not been commonly studied because rapid growth usually causes a calcium deficiency in meristems called tipburn, which greatly reduces quality and marketability. We eliminated tipburn by blowing air directly onto the meristem, which allowed us to increase the photosynthetic photon flux (PPF) to 1000 μmol·m-2·s-1 (57.6 mol·m-2·d-1); two to three times higher than normally used for lettuce. Eliminating tipburn doubled edible yield at the highest PPF level. In addition to high PPF, CO2 was elevated to 1200 μmol·m-2·mol-1, which increased the temperature optimum from 25 to 30 °C. The higher temperature increased leaf expansion rate, which improved radiation capture and more than doubled yield. Photosynthetic efficiency, measured as canopy quantum yield in a whole-plant gas exchange system, steadily increased up to the highest temperature of 32 °C in high CO2. The highest productivity was 19 g·m-2·d-1 of dry biomass (380 g·d-1 fresh mass) averaged over the 23 days the plants received light. Without the limitation of tipburn, the combination of high PPF, high temperature, and elevated CO2 resulted in a 4-fold increase in growth rate over productivity in conventional environments.

Hydraulic and chemical signals in the control of leaf expansion and stomatal conductance in soybean exposed to drought stress

Both hydraulic and chemical signals are probably important in regulating leaf growth and stomatal conductance of soybean (Glycine max L. Merr.) under drought stress. However, until now they have not been investigated concomitantly in this species. To explore this, a pot experiment in a temperature-regulated greenhouse was conducted, in which plants were subjected to progressive drought during early reproductive stages. Biophysical parameters, viz. relative leaf expansion rate, stomatal conductance, leaf turgor, leaf [ABA], xylem pH and xylem [ABA] were followed in control and stressed plants. Drought stress decreased relative leaf expansion rate, stomatal conductance and leaf turgor, whereas it increased leaf [ABA], xylem pH and xylem [ABA]. As soil dried, significant differences between water treatments for relative leaf expansion rate, stomatal conductance, leaf turgor, leaf [ABA], xylem pH and xylem [ABA] were observed at 14, 9, 14, 14, 14 and 9 d after imposition of stress, respectively. The relationships of relative values for relative leaf expansion rate, stomatal conductance, leaf turgor, leaf [ABA] and xylem pH to the fraction of transpirable soil water (FTSW) were well described by linear-plateau functions that allowed calculation of the soil-water thresholds at which processes in stressed plants began to diverge from well-watered controls. The soil-water threshold for stomatal conductance (0.64) was significantly higher than that for relative leaf expansion rate (0.29), xylem pH (0.28), leaf [ABA] (0.27) and leaf turgor (0.25). Relative xylem [ABA] increased, first linearly (when FTSW > 0.5) and then exponentially (when FTSW < 0.5) with decreasing FTSW. Relative stomatal conductance decreased exponentially with increasing relative xylem [ABA] (r2=0.98). Decreased stomatal conductance coincided with an increase in xylem [ABA] and occurred before any significant change of leaf turgor could be detected, indicating that chemical signals (seemingly root-originated ABA) control stomatal behaviour at moderate soil water deficits. Relative relative leaf expansion rate was linearly correlated with relative leaf turgor (r2=0.93), relative xylem pH (r2=0.97) and relative leaf [ABA] (r2=0.98), implying that both hydraulic and chemical signals were probably involved in regulation of leaf expansion at severe soil water deficits.

The effect of salinity and CO2 enrichment on the growth and anatomy of the second trifoliate leaf of Phaseolus vulgaris

The effect of CO2 and NaCl on the second trifoliate leaf of Phaseolus vulgaris L. was studied. Salt reduced leaf area and volume. Volume density of the palisade mesophyll was increased and that of the intercellular spaces and abaxial epidermis was reduced. Salt increased the numbers of epidermal and palisade cells per unit area and the stomatal density of the abaxial epidermis but reduced the numbers of cells per leaf. Salt reduced stomatal indices of both epidermal surfaces, cell volumes, relative leaf expansion rate, leaf plastochron index, leaf fresh and dry weights, and specific leaf area. Elevated CO2 increased leaf area and volume, reduced the density of epidermal and palisade cells and increased fresh and dry weights. Cell areas and volumes of epidermal and palisade cells, but not stomates, were increased. Elevated CO2 partially overcame some salinity effects such as leaf area, volume, specific leaf area, and relative leaf expansion rate. Leaf fresh and dry weights, leaf volume, palisade and spongy mesophyll tissue volume, and the numbers of palisade and epidermal cells per leaf equalled controls. Under high CO2, epidermal and intercellular space volume, cell areas, stomatal index, and the volume density of intercellular spaces and abaxial epidermis were reduced, and the volume density of the palisade mesophyll increased. Leaf thickness, palisade cell length and volume, volume density of spongy mesophyll, and succulence were greater than controls in salt and high-CO2 leaves. High CO2 had more effect on salt-stressed than unstressed plants in leaf weight, thickness, and cell volume.Key words: CO2 enrichment, leaf growth, leaf anatomy, Phaseolus vulgaris, salinity.