Effects of a seaweed biostimulant on physiology and productivity of mature almonds across tree water status

This study evaluated the effects of soil-applied Ascophyllum nodosum extract (ANE) on the physiology and productivity of a mature almond orchard managed under full and deficit irrigation. ANE consistently enhanced tree water status and stomatal conductance, and reduced leaf osmotic potential and leaf temperature with respect to air temperature. These effects and underlying mechanisms varied across tree water status. Under full irrigation, ANE increased the frequency of very high stomatal conductance (> 0.5 mol m⁻² s⁻¹) at no- to mild- water stress levels (stem water potential, ᴪstem > -1.0 MPa), reducing leaf temperature. In full irrigated trees experiencing short-term stomatal closure, these responses resulted in improved intrinsic water use efficiency, reflecting reduced non-stomatal limitations to photosynthesis. Under deficit irrigation, ANE delayed stomatal closure below 0.37 mol m⁻² s⁻¹ mainly through osmotic adjustment, thereby sustaining gas exchange and postponing the onset of moderate stress (ᴪstem < -1.2 MPa). Collectively, our findings identify a critical ᴪstem range (-1.0 to -1.4 MPa) as a window of maximum ANE efficacy. While these effects did notdirectly increase yield, they promoted slightly heavier kernels. Overall, ANE’s ability to mitigate stress through environmentally sustainable means positions it as a management tool for maintain productivity and enhance orchard resilience under reduced water availability and erratic weather conditions.