Plants differ widely in how soil drying affects stomatal conductance ( g s) and leaf water potential ( ψ leaf), and in the underlying physiological controls. Efforts to breed crops for drought resilience would benefit from a better understanding of these mechanisms and their diversity. We grew 12 diverse genotypes of common bean ( Phaseolus vulgaris L.) and four of tepary bean ( P. acutifolius; a highly drought resilient species) in the field under irrigation and terminal drought, and quantified responses of g s and ψ leaf, and their controls (soil water potential [ ψ soil], evaporative demand [Δ w] and plant hydraulic conductance [ K]). We hypothesized that (i) common beans would be more ”isohydric” (i.e., exhibit strong stomatal closure in drought, minimizing ψ leaf decline) than tepary beans, and that genotypes with larger ψ leaf decline (more ”anisohydric”) would exhibit (ii) smaller increases in Δ w, due to less suppression of evaporative cooling by stomatal closure and hence less canopy warming, but (iii) larger K declines due to ψ leaf decline. Contrary to our hypotheses, we found that half of the common bean genotypes were similarly anisohydric to most tepary beans; that isohydric genotypes experienced less canopy warming and hence smaller increases in Δ w in drought, and similar declines in K; and that stomatal closure was similar in isohydric and anisohydric genotypes. g s and ψ leaf were virtually insensitive to drought in one tepary genotype (G40068). Our results highlight the potential importance of non-stomatal mechanisms for leaf cooling, and the variability in drought resilience traits among closely related crop legumes.

Similar to other cropping systems, few walnut cultivars are used as scion in commercial production. Germplasm collections can be used to diversify cultivar options and hold potential for improving crop productivity, disease resistance and stress tolerance. In this study we explored the anatomical and biochemical bases of photosynthetic capacity in 11 J. regia accessions in the USDA-ARS National Clonal Germplasm Repository. Net assimilation rate (An) differed significantly among accessions and was greater in those from lower latitudes coincident with increases in stomatal and mesophyll conductance, leaf thickness, mesophyll porosity and gas-phase diffusion, and leaf nitrogen, and lower leaf mass and stomatal density. High CO2-saturated assimilation rates led to increases in An under limiting conditions. Greater An was found in lower latitude accessions native to climates with more frost-free days, greater precipitation seasonality, and lower temperature seasonality. As expected, water stress consistently impaired photosynthesis with the highest % reductions in three lower latitude accessions (A3, A5, and A9), which had the highest An under well-watered conditions. However, An for A3 and A5 remained amongst the highest under dehydration. J. regia accessions, which have leaf structural traits and biochemistry that enhance photosynthesis, could be used as commercial scions or breeding parents to enhance productivity.

Sap velocity measurements are useful in fields ranging from plant water relations to hydrology at a range of scales. Heat-pulse based techniques are among the most common methods to measure sap velocity, but most lack the ability to measure velocities across a wide range, including very high, very low and negative velocities (reverse flow). We propose a new method, the double-ratio method (DRM), which is robust across an unprecedented range of sap velocities and can provide real-time estimates of the thermal diffusivity of wood. The DRM employs one temperature sensor proximal and two distal to the heat pulse probe and facilitates several theoretical, heat-based approaches to quantifying sap velocity. We tested the DRM using whole-tree lysimetry in Eucalyptus cypellocarpa and found strong agreement across a wide range of velocities.