We describ an open leaf gas exchange system coupled to a tunable diode laser (TDL) spectroscopy system enabling measurement of the leaf respiratory CO2 flux and its associated carbon isotope composition (d 13 CRl) every 3 min. The precision of d 13 CRl measurement is comparable to that of traditional mass spectrometry techniques. d 13 CRl from castor bean (Ricinus communis L.) leaves tended to be positively related to the ratio of CO2 produced to O2 consumed [respiratory quotient (RQ)] after 24-48 h of prolonged darkness, in support of existing models. Further, the apparent fractionation between respiratory substrates and respired CO2 within 1-8 h after the start of the dark period was similar to previous observations. In subsequent experiments, R. communis plants were grown under variable water availability to provide a range in d 13 C of recently fixed carbohydrate. In leaves exposed to high light levels prior to the start of the dark period, CO2 respired by leaves was up to 11‰ more enriched than phloem sap sugars within the first 10-15 min after plants had been moved from the light into the dark. The 13 C enrichment in respired CO2 then decreased rapidly to within 3-7‰ of phloem sap after 30-60 min in the dark. This strong enrichment was not observed if light levels were low prior to the start of the dark period. Measurements of RQ confirmed that carbohydrates were the likely respiratory substrate for plants (RQ > 0.8) within the first 60 min after illumination. The strong 13 C enrichment that followed a high light-to-dark transition coincided with high respiration rates, suggesting that so-called light-enhanced dark respiration (LEDR) is fed by 13 C-enriched metabolites.

Leaf gas-exchange regulation plays a central role in the ability of trees to survive drought, but forecasting the future response of gas exchange to prolonged drought is hampered by our lack of knowledge regarding potential acclimation. To investigate whether leaf gas-exchange rates and sensitivity to drought acclimate to precipitation regimes, we measured the seasonal variations of leaf gas exchange in a mature piñon-juniper Pinus edulis-Juniperus monosperma woodland after 3 years of precipitation manipulation. We compared trees receiving ambient precipitation with those in an irrigated treatment (+30% of ambient precipitation) and a partial rainfall exclusion (-45%). Treatments significantly affected leaf water potential, stomatal conductance and photosynthesis for both isohydric piñon and anisohydric juniper. Leaf gas exchange acclimated to the precipitation regimes in both species. Maximum gas-exchange rates under well-watered conditions, leaf-specific hydraulic conductance and leaf water potential at zero photosynthetic assimilation all decreased with decreasing precipitation. Despite their distinct drought resistance and stomatal regulation strategies, both species experienced hydraulic limitation on leaf gas exchange when precipitation decreased, leading to an intraspecific trade-off between maximum photosynthetic assimilation and resistance of photosynthesis to drought. This response will be most detrimental to the carbon balance of piñon under predicted increases in aridity in the southwestern USA.Key-words: acclimation; carbon balance; hydraulic limitation; photosynthesis; piñon-juniper woodland; precipitation manipulation; stomatal conductance; water stress.

This review examines how leaf trichomes influence leaf physiological responses to abiotic environmental drivers. Leaf trichomes are known to modulate leaf traits, particularly radiation absorptance, but studies in recent decades have demonstrated that trichomes have a more expansive role in the plant–environment interaction. Although best known as light reflectors, dense trichome canopies modulate leaf heat balance and photon interception, and consequently affect gas exchange traits. Analysis of published studies shows that dense pubescence generally increases reflectance of visible light and near-infrared and infrared radiation. Reflective trichomes are also protective, reducing photoinhibition and UV-B related damage to leaf photochemistry. Little support exists for a strong trichome effect on leaf boundary layer resistance and transpiration, but recent studies indicate they may play a substantive role in leaf water relations affecting leaf wettability, droplet retention and leaf water uptake. Different lines of evidence indicate that adaxial and abaxial trichomes may function quite differently, even within the same leaf. Overall, this review synthesises and re-examines the diverse array of relevant studies from the past 40 years, illustrating our current understanding of how trichomes influence the energy, carbon and water balance of plants, and highlighting promising areas for future research.

We present field observations of carbon isotope discrimination (D) and internal conductance of CO2 (gi) collected using tunable diode laser spectroscopy (TDL). D ranged from 12.0 to 27.4‰ over diurnal periods with daily means from 16.3 Ϯ 0.2‰ during drought to 19.0 Ϯ 0.5‰ during monsoon conditions. We observed a large range in gi, with most estimates between 0.04 and 4.0 mmol m -2 s -1 Pa -1 . We tested the comprehensive Farquhar, O'Leary and Berry model of D (Dcomp), a simplified form of Dcomp (Dsimple) and a recently suggested amendment (Drevised). Sensitivity analyses demonstrated that varying gi had a substantial effect on Dcomp, resulting in mean differences between observed D (Dobs) and Dcomp ranging from 0.04 to 9.6‰. First-order regressions adequately described the relationship between D and the ratio of substomatal to atmospheric CO2 partial pressure (pi/pa) on all 3 d, but second-order models better described the relationship in July and August. The three tested models each best predicted Dobs on different days. In June, Dsimple outperformed Dcomp and Drevised, but incorporating gi and all non-photosynthetic fractionations improved model predictions in July and August.