Stomatal response to high evaporative demand in irrigated grain sorghum in narrow and wide row spacing

Stomatal activity of leaves can be related to factors under producer control, including row spacing and orientation. In both grain sorghum [Sorghum bicolor (L.) Moench] and peanut (Arachis hypogaea L.), evapotranspiration has been reduced with narrow north-south row. Under high evaporative demand, this row pattern also resulted in a more efficient plant energy budget for peanut. The trigger was stomatal action (i.e., the tendency for stomates in narrow-row plots to close earlier in the day than in wide-row plots), with no effect from leaf water potential or soil water content. To see if high evaporative demand causes stomatal action in narrow-row grain sorghum, two plots were planted at 0.40- and 1.22-m row spacing and used through six growing seasons. Treatments were randomly assigned to plots each year. Stomatal resistance and leaf water potential were measured at half-hour intervals ; soil water content was monitored weekly, prior to irrigation. Sixty-five days of data were classified as to evaporative demand and stomatal activity based on stomatal resistance behavior throughout the day. On 14 d, stomatal resistance was consistently higher in the narrow-row than the wide-row plots. Generally, these 14 d had high evaporative demand. Another 11 d showed intermediate differences, and the 40 remaining days (which included some high-demand days) showed no significant differences. The narrow-row stomatal closure effect was most likely to occur on days that accumulated >6 MJ m -2 of advected energy between 0730 and 1330 h apparent solar time. As with peanut, the stomatal action effect appeared to be insensitive to leaf water potential and soil water content. Narrow north-south planting of these crops may offer some control of evapotranspiration through stomatal action. Given the similarity of stomatal action effects in the dissimilar species of peanut and sorghum, studies of row spacing and orientation vs. stomatal activity in other species seem merited.